阅读说明：本技术 应用于高精度逐次逼近型adc的电容失配和失调电压校正方法 (Capacitor mismatch and offset voltage correction method applied to high-precision successive approximation ADC (analog to digital converter) ) 是由 何乐年 张啸蔚 郝允强 奚剑雄 于 2021-01-07 设计创作，主要内容包括：本发明公开了一种应用于高精度逐次逼近型ADC的电容失配和失调电压校正方法,该方法在传统SAR ADC结构的基础上增加了校正DAC以及对应的逻辑控制电路,在芯片上电一段时间内,通过对校正DAC和主DAC的开关控制,计算、存储电容失配和比较器失调的信息,并在正常工作模式中,将存储器中的偏差信息读入到校正DAC中,以模拟量的形式对偏差进行补偿。本发明方法包含了对每一位电容失配的校正以及比较器失调校正,可以通过改变校正DAC的电容规模,改变校正的精度,使其符合高精度SAR ADC的设计需求,可以广泛应用于高精度SAR ADC的电路设计。(The invention discloses a capacitor mismatch and offset voltage correction method applied to a high-precision successive approximation ADC (analog to digital converter). A correction DAC and a corresponding logic control circuit are added on the basis of the traditional SAR ADC structure, the capacitor mismatch and comparator offset information is calculated and stored by controlling the switches of the correction DAC and a main DAC within a period of time after a chip is electrified, and the offset information in a memory is read into the correction DAC in a normal working mode to compensate the offset in an analog quantity mode. The method comprises the correction of the mismatch of each capacitor and the offset correction of the comparator, and the correction precision can be changed by changing the capacitor scale of the correction DAC, so that the method meets the design requirements of the high-precision SAR ADC and can be widely applied to the circuit design of the high-precision SAR ADC.)

1. A capacitor mismatch and offset voltage correction method applied to a high-precision successive approximation ADC (analog to digital converter) is characterized by comprising the following steps of: firstly, a digital-to-analog converter in a successive approximation ADC is structurally modified, namely, a correction DAC is added on the basis of a main DAC in the digital-to-analog converter, then offset voltage codes of a comparator in the successive approximation ADC are obtained through corresponding operation, correction codes of each capacitor in the main DAC are calculated, and finally mismatch of each capacitor in the main DAC and offset voltage of the comparator are corrected according to the offset voltage codes and the correction codes.

2. The method of claim 1, wherein the step of correcting for capacitance mismatch and offset voltage comprises: the digital-to-analog converter before modification comprises a main DAC, the main DAC consists of two rows of capacitor arrays of a P pole and an N pole of a differential structure, the digital-to-analog converter after modification further comprises a correction DAC, the structure of the correction DAC is the same as that of the main DAC, upper polar plates of the capacitor arrays of the P pole and the N pole in the correction DAC are respectively connected with the upper polar plates of the capacitor arrays of the P pole and the N pole in the main DAC in parallel through bridge capacitors, the number of digits of the capacitor arrays in the main DAC is N, the number of digits of the capacitor arrays in the correction DAC is m, the N is the number of digits of successive approximation type ADCs, and the m is a self-.

3. The method of claim 2, wherein the step of correcting for capacitance mismatch and offset voltage comprises: the specific process of obtaining the offset voltage code of the comparator is as follows:

A1. initializing the states of the main DAC and the correction DAC to ensure that the upper plates and the lower plates of all capacitors in the capacitor array are connected with a common-mode voltage V_CM；

A2. Disconnecting the upper electrode plates of all capacitors in the capacitor array from V_CMThe lower plates of all capacitors of the N-electrode capacitor array in the correction DAC are connected with the reference voltage V_REF；

A3. And carrying out SAR logic conversion operation on the correction DAC, wherein the code obtained by the correction DAC is the offset voltage code of the comparator and is stored, the most significant bit is a sign bit, the sign bit is 0 to indicate that the offset voltage is negative, and the sign bit is 1 to indicate that the offset voltage is positive.

4. The method of claim 2, wherein the step of correcting for capacitance mismatch and offset voltage comprises: the specific process of calculating the capacitance correction code of each bit in the main DAC is as follows:

B1. for the k-th capacitor in the main DAC, k is a natural number and is more than or equal to 1 and less than or equal to N, so that the lower polar plate of the (k + 1) -N) -th capacitor of the P-pole and N-pole capacitor arrays in the main DAC is always connected with the common-mode voltage V_CM；

B2. Initially integrating all of the capacitors in the capacitor arrayThe upper plate of the capacitor is connected with V_CM(ii) a In the main DAC, the lower plate of the kth capacitor of the P-pole capacitor array is connected with a reference voltage V_REFThe lower electrode plate of the kth capacitor of the N-electrode capacitor array is grounded, the lower electrode plates of the 1 st to k-1 st capacitors of the P-electrode capacitor array are grounded, and the lower electrode plates of the 1 st to k-1 st capacitors of the N-electrode capacitor array are connected with V_REF(ii) a In the correction DAC, the lower plate of the m-th capacitor of the P-pole capacitor array is connected with a reference voltage V_REFThe lower polar plates of the capacitors at other positions are grounded, the lower polar plate of the capacitor at the mth position of the N-electrode capacitor array is grounded, and the lower polar plates of the capacitors at other positions are connected with V_REF；

B3. Disconnecting the upper electrode plates of all capacitors in the capacitor array from V_CMThe connection of (1); in the main DAC, the lower plate of the kth capacitor of the P-pole capacitor array is grounded, and the lower plate of the kth capacitor of the N-pole capacitor array is connected with V_REFThe lower polar plate of the 1 st to k-1 th bit capacitors of the P-pole capacitor array is connected with V_REFThe lower polar plate of the 1 st to k-1 th capacitors of the N-electrode capacitor array is grounded; at the moment, the output of the comparator reflects whether the deviation between the kth position capacitor and the 1 st to k-1 th position capacitors of the main DAC is positive or negative, if the output of the comparator is 0, the deviation is negative, the lower plate of the mth position capacitor of the P-electrode capacitor array in the correction DAC is grounded, and the lower plate of the mth position capacitor of the N-electrode capacitor array is connected with V_REF(ii) a If the output of the comparator is 1, the deviation is positive, and the state of the capacitor in the correction DAC is kept unchanged;

B4. carrying out SAR logic conversion operation on the correction DAC, and subtracting the code obtained by the correction DAC from the offset voltage code of the comparator to obtain the result Exp_k；

B5. Calculating the corrected code x of the k-th bit capacitance in the main DAC according to the following formula_k；

Wherein: x is the number of_k+1Coding for correction of the k +1 th bit capacitance in the main DAC, Exp_k+1Is the result of the code obtained from the correction DAC by the steps B1-B4 and the code subtraction of the offset voltage of the comparator for the capacitance of the (k + 1) th bit in the main DAC;

B6. and B1-B5, calculating the correction code of each bit of capacitance in the main DAC from the highest bit.

5. The method of claim 4, wherein the step of correcting for capacitance mismatch and offset voltage comprises: the specific process of correcting the mismatch of each bit capacitor in the main DAC and the offset voltage of the comparator is as follows:

C1. initially connecting the upper plates of all capacitors in the capacitor array with a common-mode voltage V_CMThe lower polar plates of all capacitors in the P electrode capacitor array are connected with a positive phase input voltage V_INpThe lower polar plates of all capacitors in the N-pole capacitor array are connected with an inverted input voltage V_Inn；

C2. In the correction DAC, all capacitor upper plates are disconnected from V_CMThe lower electrode plates of all capacitors are connected with the input voltage, then all the lower electrode plates of the capacitors in the P-electrode capacitor array are in short circuit, and all the lower electrode plates of the capacitors in the N-electrode capacitor array are in short circuit, so that the common mode problem of the input voltage signals is solved;

C3. disconnecting all capacitor upper plates and V in correction DAC_CMAnd the connection of the lower plates of all capacitors to the input voltage, according to the sign code z₀The result added with the offset voltage code is used for carrying out logic control on the correction DAC, namely, the code of a certain bit is 0, the lower plate of the bit capacitor is grounded, and if the code of the certain bit is 1, the lower plate of the bit capacitor is connected with the reference voltage V_REF；

C4. Making the main DAC enter into normal working state, starting SAR logic conversion operation from the highest bit, when the k bit jumps, coding x according to the correction of the bit capacitance_kThe code z is calculated by the following formula_kAccording to the code z_kCarrying out logic control on the correction DAC;

z_k＝z₀+s+x_k+x_k+1+…+x_n

wherein: symbol encoding z₀The bit number of (1) is m, the highest bit is 1, the rest bits are 0, and s is offset voltage coding.

6. The method of claim 1, wherein the step of correcting for capacitance mismatch and offset voltage comprises: the method belongs to analog foreground correction, each bit of capacitance offset and comparator offset voltage of the SAR ADC are calculated and stored through a correction DAC within a period of time when a chip is electrified, and stored error information is added into the DAC in an analog quantity mode in a normal working mode, so that the influence caused by deviation is reduced.

7. The method of claim 1, wherein the step of correcting for capacitance mismatch and offset voltage comprises: the method can correct the capacitor mismatch of the SAR ADC, can also correct the offset voltage of the comparator, and does not need to additionally add an auxiliary circuit; meanwhile, the method can store and compensate the mismatch information of each capacitor, and the correction effect is better than that of the traditional correction algorithm.

8. The method of claim 1, wherein the step of correcting for capacitance mismatch and offset voltage comprises: the method has low requirements on the capacitor accuracy and the matching degree of the correction DAC, the capacitor mismatch of the correction DAC has little influence on the correction result, so the method is easier to realize, can be applied to the high-accuracy SAR ADC, can change the correction accuracy by changing the capacitor array scale of the correction DAC, is easy to adjust according to the accuracy requirement of the SAR ADC, and has stronger expandability.

Technical Field

The invention belongs to the technical field of Analog-to-Digital converters (ADCs), and particularly relates to a capacitance mismatch and offset voltage correction method applied to a high-precision successive approximation ADC.

Background

The analog-digital converter is a core module for converting an analog signal into a digital signal, and is widely applied to the fields of consumer electronics, war industry, aerospace, industry and the like to finish the A/D conversion. The ADC can be divided into a Flash type (Flash), a Pipeline type (Pipeline), an integral type (Sigma-Delta) and a successive approximation type (SAR) according to different working modes; different ADCs are used in different fields with different performance.

The Flash ADC has a simple structure, high conversion speed and high power consumption, and is suitable for the field of high speed and low precision; the Pipeline ADC is formed by cascading a plurality of ADCs and works in a Pipeline mode, each stage comprises a sampling hold circuit, a low-resolution ADC, a Digital-to-Analog Converter (DAC) and a summing circuit, and the Pipeline ADC has the advantages of good linearity, low offset and the like, but the gain of an integral multiple stage is difficult to control, the power consumption is high, and the Pipeline ADC is applied to the field of high speed and high precision; the precision of the Sigma-Delta ADC is higher than that of other types of ADCs, the speed of the Sigma-Delta ADC is generally lower than that of other ADCs, and the Sigma-Delta ADC has the advantages of high resolution, low noise, low power consumption and the like, and is generally applied to the field of low speed and high precision.

The SAR ADC realizes good balance in the aspects of power consumption, speed and precision, and is widely applied to the fields of high-precision low-power consumption and small size, such as wearable equipment, spectrum analyzers, data collectors and the like. Due to the development of the CMOS process, the sampling rate of the SAR ADC can reach hundreds of MHz at present, the power consumption can be as low as nW level, the precision can reach 14-16 bit, and the SAR ADC has the comprehensive advantages of obvious power consumption and efficiency and is one of ADC frameworks which are most widely applied.

As shown in fig. 1, the SAR ADC includes four parts: a sample-and-hold circuit, a digital-to-analog converter, a comparator, and SAR logic. The sample-and-hold circuit samples the analog input voltage and the initial value V of the DAC_FS/2(V_FSFull scale voltage) is compared through a comparator, if the analog input voltage is greater than the output voltage of the DAC, the code of the current bit is 1, and the switch state of the DAC is kept unchanged; if the analog input voltage is smaller than the output voltage of the DAC, the code of the current bit is 0, and the switch state of the DAC returns to the state before action; the SAR logic control then moves to the next bit, changing the switching state of the DAC again, then comparing with the analog input voltage, and so on,until the comparison of the last bit is completed, all A/D conversion codes are obtained. The core idea of the working principle of the SAR ADC is dichotomy, and the change of the DAC output voltage is 1/2ⁿThe analog input voltage is successively approximated to realize the A/D conversion.

At present, a widely used SAR ADC is realized based on a charge redistribution principle, a DAC module of the SAR ADC has no static power consumption, the switched dynamic power consumption is smaller, the structure of the SAR ADC is compatible with a switched capacitor structure of a sample-and-hold circuit, and the circuit design can be simplified; however, the capacitance mismatch caused by the process deviation affects the linearity of the ADC, and the effect is more obvious in the design of high precision and advanced process, so that the error caused by the capacitance mismatch needs to be corrected by using a correction algorithm.

There are generally two types of correction, one is analog correction and digital correction, and the other is foreground correction and background correction. Analog correction refers to adjusting the capacitance weight of the current bit by changing the capacitance value of the capacitor array to realize coding correction; digital correction refers to changing the output encoding by changing the digital weight of the current bit; foreground correction refers to correcting the weight of the capacitor within a period of time after the chip is powered on, and normal conversion work is carried out after the correction is finished; background correction refers to synchronous correction when the chip works normally; the two classification methods generally form four correction schemes: analog foreground correction, analog background correction, digital foreground correction and digital background correction.

The DAC for analog foreground correction comprises a main DAC and a correction DAC, wherein the correction DAC is used for storing capacitance deviation of a main array and is connected into the main DAC in series through a proper capacitor. The chip is electrified for a period of time, a correction mode is entered, and the correction DAC calculates and stores the deviation of each bit capacitor of the main DAC; and then entering a normal working mode, correcting the capacitance deviation information of the current bit released by the DAC, compensating the main DAC, eliminating the capacitance mismatch of the main DAC, and normally quantizing the main DAC in the secondary process.

The analog background correction working principle is similar to the analog foreground correction, and the analog background correction working principle also comprises a main DAC and a correction DAC, wherein the correction DAC stores deviation information, but the analog background correction does not have a separate correction link, the system directly enters a working mode, and the mismatch of the capacitors is measured, stored and compensated through an algorithm in the quantization process.

The digital foreground correction corrects the mismatch of each capacitor of the ADC within a period of time after the chip is electrified, stores the weights of the capacitors after correction, then enters a normal working mode, and sums the output code of each SAR with the weights to obtain the final output code; compared with the analog correction technology, the digital foreground correction omits a correction DAC, thereby saving the area and the power consumption; compared with digital background correction, the method has no convergence problem and ensures the stability of the system.

The digital background correction refers to correction in a digital mode in a normal working mode, an ideal value is obtained through an accurate reference ADC, the ideal value is compared with an actual DAC quantization result, and an obtained error is used for modifying the weight of a current bit and reducing errors caused by capacitor mismatch.

The process deviation not only causes the capacitor mismatch of the SAR ADC, but also causes the offset voltage of the comparator to be overlarge, thereby influencing the comparison result and causing the error code. Therefore, when the high-precision SAR ADC is designed, the offset voltage of the comparator also needs to be corrected, and the error rate is reduced.

Disclosure of Invention

In view of the above, the present invention provides a method for correcting capacitor mismatch and offset voltage applied to a high-precision successive approximation ADC, so as to improve the precision of the SAR ADC; the method belongs to analog foreground correction, each bit of capacitance offset and comparator offset voltage of the SAR ADC are calculated and stored through a correction DAC within a period of time when a chip is electrified, and stored error information is added into the DAC in an analog quantity mode in a normal working mode, so that the influence caused by deviation is reduced.

A capacitor mismatch and offset voltage correction method applied to a high-precision successive approximation ADC (analog-to-digital converter) comprises the steps of firstly carrying out structural modification on a digital-to-analog converter in the successive approximation ADC, namely adding a correction DAC on the basis of a main DAC in the digital-to-analog converter, then obtaining offset voltage codes of a comparator in the successive approximation ADC through corresponding operation, calculating correction codes of each bit of capacitor in the main DAC, and finally correcting the mismatch of each bit of capacitor in the main DAC and the offset voltage of the comparator according to the offset voltage codes and the correction codes.

The improved digital-to-analog converter comprises a main DAC (digital-to-analog converter) and a correction DAC, wherein the main DAC comprises two rows of capacitor arrays of a P pole and an N pole of a differential structure, the structure of the correction DAC is the same as that of the main DAC, upper polar plates of the capacitor arrays of the P pole and the N pole in the correction DAC are respectively connected with upper polar plates of the capacitor arrays of the P pole and the N pole in the main DAC in parallel through bridge capacitors, the number of bits of the capacitor arrays in the main DAC is N, the number of bits of the capacitor arrays in the correction DAC is m, the N is the number of bits of a successive approximation type ADC, and the m is a self-set natural number.

Further, the specific process of obtaining the offset voltage code of the comparator is as follows:

A1. initializing the states of the main DAC and the correction DAC to ensure that the upper plates and the lower plates of all capacitors in the capacitor array are connected with a common-mode voltage V_CM；

A2. Disconnecting the upper electrode plates of all capacitors in the capacitor array from V_CMThe lower plates of all capacitors of the N-electrode capacitor array in the correction DAC are connected with the reference voltage V_REF；

A3. And carrying out SAR logic conversion operation on the correction DAC, wherein the code obtained by the correction DAC is the offset voltage code of the comparator and is stored, the most significant bit is a sign bit, the sign bit is 0 to indicate that the offset voltage is negative, and the sign bit is 1 to indicate that the offset voltage is positive.

The common mode voltage V_CMAnd a reference voltage V_REFAre given values.

Further, the specific process of calculating the capacitance correction code of each bit in the main DAC is as follows:

B1. for the k-th capacitor in the main DAC, k is a natural number and is more than or equal to 1 and less than or equal to N, so that the lower polar plate of the (k + 1) -N) -th capacitor of the P-pole and N-pole capacitor arrays in the main DAC is always connected with the common-mode voltage V_CM；

B2. Initially combining all capacitors in the capacitor arrayUpper polar plate is connected with V_CM(ii) a In the main DAC, the lower plate of the kth capacitor of the P-pole capacitor array is connected with a reference voltage V_REFThe lower electrode plate of the kth capacitor of the N-electrode capacitor array is grounded, the lower electrode plates of the 1 st to k-1 st capacitors of the P-electrode capacitor array are grounded, and the lower electrode plates of the 1 st to k-1 st capacitors of the N-electrode capacitor array are connected with V_REF(ii) a In the correction DAC, the lower plate of the m-th capacitor of the P-pole capacitor array is connected with a reference voltage V_REFThe lower polar plates of the capacitors at other positions are grounded, the lower polar plate of the capacitor at the mth position of the N-electrode capacitor array is grounded, and the lower polar plates of the capacitors at other positions are connected with V_REF；

B3. Disconnecting the upper electrode plates of all capacitors in the capacitor array from V_CMThe connection of (1); in the main DAC, the lower plate of the kth capacitor of the P-pole capacitor array is grounded, and the lower plate of the kth capacitor of the N-pole capacitor array is connected with V_REFThe lower polar plate of the 1 st to k-1 th bit capacitors of the P-pole capacitor array is connected with V_REFThe lower polar plate of the 1 st to k-1 th capacitors of the N-electrode capacitor array is grounded; at the moment, the output of the comparator reflects whether the deviation between the kth position capacitor and the 1 st to k-1 th position capacitors of the main DAC is positive or negative, if the output of the comparator is 0, the deviation is negative, the lower plate of the mth position capacitor of the P-electrode capacitor array in the correction DAC is grounded, and the lower plate of the mth position capacitor of the N-electrode capacitor array is connected with V_REF(ii) a If the output of the comparator is 1, the deviation is positive, and the state of the capacitor in the correction DAC is kept unchanged;

B4. carrying out SAR logic conversion operation on the correction DAC, and subtracting the code obtained by the correction DAC from the offset voltage code of the comparator to obtain the result Exp_k；

B5. Calculating the corrected code x of the k-th bit capacitance in the main DAC according to the following formula_k；

Wherein: x is the number of_k+1Coding for correction of the k +1 th bit capacitance in the main DAC, Exp_k+1Is the result of the code obtained from the correction DAC by the steps B1-B4 and the code subtraction of the offset voltage of the comparator for the capacitance of the (k + 1) th bit in the main DAC;

B6. and B1-B5, calculating the correction code of each bit of capacitance in the main DAC from the highest bit.

Further, the specific process of correcting the mismatch of each bit capacitor in the main DAC and the offset voltage of the comparator is as follows:

C1. initially connecting the upper plates of all capacitors in the capacitor array with a common-mode voltage V_CMThe lower polar plates of all capacitors in the P electrode capacitor array are connected with a positive phase input voltage V_INpThe lower polar plates of all capacitors in the N-pole capacitor array are connected with an inverted input voltage V_Inn；

C2. In the correction DAC, all capacitor upper plates are disconnected from V_CMThe lower electrode plates of all capacitors are connected with the input voltage, then all the lower electrode plates of the capacitors in the P-electrode capacitor array are in short circuit, and all the lower electrode plates of the capacitors in the N-electrode capacitor array are in short circuit, so that the common mode problem of the input voltage signals is solved;

C3. disconnecting all capacitor upper plates and V in correction DAC_CMAnd the connection of the lower plates of all capacitors to the input voltage, according to the sign code z₀The result added with the offset voltage code is used for carrying out logic control on the correction DAC, namely, the code of a certain bit is 0, the lower plate of the bit capacitor is grounded, and if the code of the certain bit is 1, the lower plate of the bit capacitor is connected with the reference voltage V_REF；

C4. Making the main DAC enter into normal working state, starting SAR logic conversion operation from the highest bit, when the k bit jumps, coding x according to the correction of the bit capacitance_kThe code z is calculated by the following formula_kAccording to the code z_kCarrying out logic control on the correction DAC;

z_k＝z₀+s+x_k+x_k+1+…+x_n

wherein: symbol encoding z₀The bit number of (1) is m, the highest bit is 1, the rest bits are 0, and s is offset voltage coding.

Based on the technical scheme, the invention has the following beneficial technical effects:

1. the invention can correct the capacitor mismatch of the SAR ADC, can correct the offset voltage of the comparator and does not need to additionally add an auxiliary circuit.

2. The correction method can store and compensate the mismatch information of each capacitor, and the correction effect is better than that of the traditional correction algorithm.

3. The correction method has low requirements on the capacitance precision and the matching degree of the correction DAC, and the capacitance mismatch of the correction DAC has little influence on the correction result, so the correction method is easier to realize.

4. The correction method can be applied to the high-precision SAR ADC, can change the correction precision by changing the scale of the capacitor array of the correction DAC, is easy to adjust according to the precision requirement of the SAR ADC, and has stronger expandability.

Drawings

Fig. 1 is a schematic diagram of the structure of a SAR ADC.

FIG. 2 is a flow chart illustrating a calibration method according to the present invention.

Fig. 3 is a schematic diagram of a capacitor array structure of a digital-to-analog converter in the SAR ADC of the present invention.

Detailed Description

In order to more specifically describe the present invention, the following detailed description is provided for the technical solution of the present invention with reference to the accompanying drawings and the specific embodiments.

According to the method for correcting the capacitor mismatch and the offset voltage of the comparator of the high-precision SAR ADC, a correction DAC is added on the basis of a main DAC of the SAR ADC and is connected to the main DAC through a bridge capacitor. Since the function of the correction DAC is to calculate and store the offset information of the capacitance mismatch and the offset of the comparator, and the offset amount is added to the main DAC in the normal operation mode, the influence of the capacitance mismatch of the correction DAC on the quantization process is not significant, so in the method of the present invention, it is assumed that the capacitance of the correction DAC is accurate. When the offset voltage of the comparator is corrected, the input voltages of the positive end and the negative end of the main DAC are equal by switching the correction DAC, and the switching state of the correction DAC is information of the offset voltage of the comparator.

In the capacitance mismatch correction, assuming that the ADC has N bits in total, the ADC actually includes N +1 bit equivalent capacitance (existence of redundant bit), w_n(n＝1^*1, …, N) represents the weight of the nth bit, 1^*Represents a redundant bit measured by an n-th equivalent capacitance C_nAs a proportion of the equivalent capacitance of the total DAC. Ideally, w_n＝2^n-1However, in practical cases, the weight of each bit is deviated, and the unit equivalent capacitance is defined as:

the actual weight of each bit capacitance is then obtained:

the weight deviation of the nth bit is:

w_εn＝w_n-2^n-1(n＝1^*,1,…,N)

taking the Nth bit capacitance as an example, consider w_NAnd w_1*～w_N-1The deviation of the sum is denoted Exp by the capacitive array of the correction DAC_NTotal weight of w_T. According to the previous analysis, it is required to ensure that the change of the capacitor array at the N-th bit jump is w_TAnd/2, the capacitor array of the correction DAC is required to be matched with the Nth bit for jumping, and the code value of the corresponding bypass array is assumed to be x at the moment_NThen, three relations can be obtained as follows:

w_N+k·x_N＝w_T/2

according to the above relation:

it should be noted that the weights represented by the capacitor array codes of the correction DAC and the weights w of the main capacitor array are due to the presence of the bridge capacitors connecting the correction DAC with the main DAC_nCompared with a fixed coefficient k, since the addition and subtraction operations of correction coding are all performed on the bypass array, the accuracy of subsequent correction is not affected by the existence of the coefficient k.

Similarly, analysis of the N-1 bit capacitance results in three similar relationships:

w_N-1+k·x_N-1＝w_T/4

according to the above relation:

by analogy, a capacitance deviation formula of each bit can be obtained:

preset Exp considering iterative circuit design convenience_N+1＝0，x_N+1When 0, the above formula can be rewritten as:

according to the calculation process, the mismatch information of each bit of the capacitor of the main DAC can be obtained and stored in the register, and when the correction DAC works normally, the capacitor mismatch information in the corresponding register is read out and placed in the correction DAC through each switching, so that the corresponding deviation is compensated, and the whole correction process is completed. It should be noted that the capacitance mismatch information obtained in this process includes the offset voltage of the comparator, so the capacitance mismatch information actually stored in the register needs to be subtracted by the offset voltage information of the comparator. When the correction capacitor array works normally, offset voltage information of the comparator is added into the correction capacitor array, and the offset voltage information is added only when a complete A/D conversion starts; and then, every time the capacitor array switch of the main DAC is switched, the deviation information corresponding to the related capacitor is added on the basis of the current correction DAC, and the current capacitor of the main DAC is corrected.

Fig. 2 is a correction flow of the SAR ADC chip of the present invention, after the chip is powered on, the offset voltage of the comparator is corrected first to obtain the offset voltage information of the comparator, then the capacitor mismatch correction of the main DAC is performed to store the capacitor mismatch information of each bit, after the correction information storage is completed, the chip enters a normal working state, the offset information is released in the quantization process, and the accuracy is improved.

The capacitor array in the following embodiment is composed of a main DAC and a correction DAC as shown in fig. 3. The main DAC adopts a bridge capacitor structure and is formed by connecting a high 7-bit capacitor array and a low 8-bit capacitor array in series through a bridge capacitor; the correction DAC also adopts a bridge capacitor structure and is formed by connecting a high 7-bit capacitor array and a low 6-bit capacitor array in series through a bridge capacitor, and the correction DAC is also connected with the main DAC in series through the bridge capacitor.

The offset voltage correction operation of the comparator comprises the following steps, and the capacitor of the main DAC is connected into the capacitor array in the whole process, but does not participate in the operation.

(1) Initializing the states of the main DAC and the correction DAC, wherein the upper electrode plates of the capacitors of the main DAC and the correction DAC are connected with a common-mode voltage V_CMCorrecting the lower plate of DAC capacitor to be connected with V_CM。

(2) Disconnect all capacitor upper plates from V_CMCorrecting the lower plate of the N-end capacitor array of the DAC to be connected with the reference voltage V_REF。

(3) And carrying out SAR logical operation on the correction DAC, wherein the code obtained by the correction DAC is offset voltage information of the comparator, and storing the offset voltage information to a corresponding register. Wherein the most significant bit is the sign bit, "0" indicates that the offset voltage is negative, and "1" indicates that the offset voltage is positive.

The capacitive mismatch correction operation steps are as follows, taking the k-th bit capacitive correction of the main DAC as an example.

(1) K + 1-15 bit P end and N end lower electrode plate initial termination V of main DAC capacitor array_CM。

(2) Initially, the upper plate of the capacitor is connected to V_CMThe lower polar plate of the P end of the k-bit capacitor of the main DAC is connected with V_REFThe lower polar plate at the N end is grounded, the lower polar plate at the P end with k-1 to 1 bits is grounded, and the lower polar plate at the N end is connected with V_REF. The lower electrode plate of the capacitor P end for correcting the highest bit of the DAC is connected with V_REFThe lower polar plate at the N end is grounded, the lower polar plates at the P ends of the capacitors at other positions are grounded, and the lower polar plate at the N end is connected with the V_REF。

(3) Disconnect all capacitor upper plates from V_CMThe lower polar plate at the P end of the kth capacitor is grounded, and the lower polar plate at the N end is connected with the V_REFK-1 to 1 bit P end lower polar plate connecting V_REFAnd the lower polar plate at the N end is grounded. At this time, the output of the comparator reflects whether the deviation between the k-th bit and the k-1 to 1 bits is positive or negative. If the output is '0', the deviation is negative, the lower polar plate at the P end of the highest bit of the correction DAC is grounded, and the lower polar plate at the N end is connected with V_REF(ii) a If the output is "1", indicating that the deviation is positive, the correction DAC array state remains unchanged.

(4) And (4) next, correcting the DAC capacitance from the second highest bit to the lowest bit to perform the similar SAR logic operation of the step (3), and subtracting the obtained code from the code of the offset voltage of the comparator to obtain Exp_k。

(5) Finally according to the formulaAnd obtaining the correction code of the current bit capacitor for correction in normal work.

Under the normal working mode, the steps of adding correction codes and SAR logic conversion are as follows:

(1) all the upper electrode plates of the capacitors are connected with V_CMAll the P-end capacitor lower polar plates are connected with V_INpN end capacitor lower polar plate connected to V_INn。

(2) Disconnecting the upper plate of the correction DAC capacitor from V_CMThe connection between the lower plate of the capacitor of the correction DAC and the input voltage is disconnected, and then the lower plates of the capacitors at the P end and the N end of the correction DAC are in short circuit, so that the common mode problem of the input signal is solved.

(3) Disconnecting the upper polar plate of the main DAC capacitor from V_CMDisconnecting the lower plate of the main DAC from the input voltage.

(4) The correction DAC is first put into "100 … 0" + "the coding of the offset voltage of the comparator, and the main array enters the SAR logic switching initial state.

(5) And carrying out SAR logic conversion from the highest bit, and putting the corresponding code into a correction DAC for addition and subtraction operation when each bit jumps.

Under the correction algorithm, through 15-bit SAR ADC capacitance mismatch and comparator offset voltage correction, the simulation and test result shows that the output deviation can be controlled within 1LSB, and the target requirement is met.

The embodiments described above are presented to enable a person having ordinary skill in the art to make and use the invention. It will be readily apparent to those skilled in the art that various modifications to the above-described embodiments may be made, and the generic principles defined herein may be applied to other embodiments without the use of inventive faculty. Therefore, the present invention is not limited to the above embodiments, and those skilled in the art should make improvements and modifications to the present invention based on the disclosure of the present invention within the protection scope of the present invention.