Calibrating a differential measurement circuit

Abstract

Disclosed are example circuits comprising a first circuit to provide a low signal, a second circuit to provide a high signal, wherein the high signal has a greater voltage amplitude than the low signal, and a differential amplifier configured to receive the low signal from the first circuit and the high signal from the second circuit. The differential amplifier is to produce an output voltage based on the high signal and the low signal. The example circuits comprise a first measurement circuit to measure the output voltage, a second measurement circuit to measure the low signal at the first circuit, and a processing logic portion to determine a differential measurement based on the output voltage measured by the first measurement circuit, the low signal measured by the second measurement circuit, and a calibration value obtained for the circuit.

Description

TECHNICAL FIELD

[0001] This specification relates generally to example processes for calibrating a differential measurement circuit. BACKGROUND

[0002] An example differential measurement circuit generates an output voltage based on a difference between two input signals. An ideal differential measurement circuit changes its output only in response to a change in the difference between the two input signals. Thus, for example, if the voltages of the two input signals increase or decrease by equal amounts, the output voltage of an ideal differential measurement will not change. However, if one of the input signals changes and the other does not, or the changes in the two input signals are different, the output voltage of an ideal differential measurement circuit will change. SUMMARY

[0003] An example circuit is disclosed that includes a first circuit to provide a low signal, a second circuit to provide a high signal, where the high signal has a greater voltage magnitude than the low signal, and a differential amplifier configured to receive the low signal from the first circuit and the high signal from the second circuit. The differential amplifier is to produce an output voltage based on the high signal and the low signal. The example circuit includes a first measurement circuit to measure the output voltage, a second measurement circuit to measure the low signal at the first circuit, and a processing logic portion to determine a differential measurement based on the output voltage measured by the first measurement circuit, the low signal measured by the second measurement circuit, and calibration values obtained for the circuit. An example circuit can include one or more of the following features, either alone or in combination.

[0004] The processing logic portion can be configured to determine the differential measurement (hs-ls) as follows

[0005]

[0006] where Vo is the output voltage measured by the first measurement circuit, ls(m) is the low signal measured by the second measurement circuit, GH and GL are the calibration values, and Ofs is an offset value produced when the low signal and the high signal are at or near zero volts (0V).

[0007] The first measurement circuit can include a first analog-to-digital converter (ADC), the second measurement circuit can include a second ADC, and the second ADC can have a lower precision than a final measurement specification for the circuit. The first measurement circuit can include a first ADC, the second measurement circuit can include a second ADC, and the first ADC can be more precise than the second ADC.

[0008] In the foregoing equations, GH can be a positive value and GL can be a negative value different from GH, where the difference between the absolute values of GH and GL can be 10% or less. The difference between the absolute values of GH and GL can be 5% or less. The absolute value of ls(m) can be 200 mV or less. GH can be a value determined by connecting the first circuit to electrical ground and connecting the second circuit to a known voltage. GH + GL can be a value determinable by connecting the first circuit and the second circuit together and applying a known voltage to the first circuit and the second circuit connected together. GH and GL can be determined separately from each other.

[0009] The differential amplifier can include resistors on a feedback path and a feedforward path. The output voltage can be based at least in part on a value of the resistors. The processing logic portion can include one or more microprocessors. The processing logic portion can include programmable logic. The circuit can be part of a ground re-reference circuit.

[0010] An example automatic test equipment (ATE) includes a circuit board connected to a device under test (DUT), where the DUT is connected to a first electrical reference on the circuit board; and a test circuit connected to a second electrical reference at a different voltage than the first electrical reference, which causes a pair of signals output from the DUT to the test circuit to change in voltage value while maintaining a constant voltage difference. The test circuit can include a first circuit to provide a low signal of the pair of signals, a second circuit to provide a high signal of the pair of signals having a greater voltage amplitude than the low signal, and a differential amplifier configured to receive the low signal from the first circuit and the high signal from the second circuit. The differential amplifier can be used to produce an output voltage based on a difference between the high signal and the low signal. The test circuit can also include a first measurement circuit to measure the output voltage, a second measurement circuit to measure the low signal at the first circuit, and a processing logic portion to determine a differential measurement based on the output voltage measured by the first measurement circuit, the low signal measured by the second measurement circuit, and a calibration value obtained for the test circuit. An example ATE can include one or more of the following features, either alone or in combination.

[0011] The example ATE can include a test instrument to perform one or more tests on the DUT. The test circuit can be part of the test instrument. The first electrical reference can be a first electrical ground for the DUT, and the second electrical reference is a second electrical ground for the test circuit.

[0012] The processing logic portion can be configured to determine the difference measurement (hs-ls) as follows

[0013]

[0014] where Vo is the output voltage measured by the first measurement circuit, ls(m) is the low signal measured by the second measurement circuit, GH and GL are calibration values, and Ofs is an offset value that results when the low signal and the high signal are at or near zero volts (0V).

[0015] The first measurement circuit can include a first ADC, the second measurement circuit can include a second ADC, and the second ADC can have a lower precision than a final measurement specification for the test circuit. The first measurement circuit can include a first ADC, the second measurement circuit can include a second ADC, and the first ADC can be more precise than the second ADC.

[0016] In the foregoing equation, GH can have a positive value, and GL can have a negative value that is different from GH. The difference between the absolute values of GH and GL can be 5% or less.

[0017] Any two or more of the features described in this specification, including this summary section, can be combined to form specific embodiments of the application.

[0018] The systems and techniques described herein, or portions thereof, can be implemented as a computer program product that includes instructions stored on one or more non-transitory machine-readable storage media (e.g., in the form of a bundle of instructions executable by one or more processing devices and to control, e.g., coordinate, the operations described herein). The systems and techniques described herein, or portions thereof, can be implemented as an apparatus, method, or electronic system that can include one or more processing devices and memory storing executable instructions for implementing various operations.

[0019] The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings, and from the claims. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is a circuit diagram illustrating an example differential measurement circuit.

[0021] Figure 2 is a block diagram illustrating components of an example automatic test equipment that includes a differential measurement circuit.

[0022] Like reference numbers in different drawings indicate like elements. DETAILED DESCRIPTION

[0023] An example differential measurement circuit generates an output voltage based on a difference between two input signals. For example, a differential gain is applied to the difference between the two input signals to generate the output voltage. Real-world differential measurement circuits can behave differently than the ideal differential measurement circuit described previously. For example, a real-world differential measurement circuit can generate a different output voltage even if the voltages of the two input signals increase or decrease by equal amounts. This phenomenon can be due to errors introduced by the differential amplifier used to generate the output voltage during the measurement process. Accordingly, the circuits and processes described herein are used to generate a calibration value that can be used to correct some or all of the errors introduced by the differential amplifier.

[0024] In this regard, example circuits described herein can include a differential measurement circuit for measuring a difference between two signals. The circuit can be used in a test system to determine whether a device under test (DUT) passes a test. In a particular implementation, the circuit includes a circuit for providing two signals. In this case, the circuit for providing two signals includes a first circuit for providing a low signal (LS) and a second circuit for providing a high signal (HS). The high signal has a greater voltage amplitude than the low signal. A differential amplifier is configured to receive the low signal from the first circuit and the high signal from the second circuit and to produce an output voltage based on the high signal and the low signal. In this example, the high signal amplification is different than the low signal amplification, which results in an error in the output voltage as described previously.

[0025] The first measurement circuit is configured to measure the output voltage; and the second measurement circuit is configured to measure the low signal at the first circuit. Processing logic, such as a microprocessor or one or more other suitable processing devices, is configured to determine a differential measurement based on the output voltage measured by the first measurement circuit, the low signal measured by the second measurement circuit, and a calibration value obtained for the circuit. The differential measurement can be based on or can represent a difference between the high signal and the low signal. The calibration value can be used to correct for errors introduced by the differential amplifier into the measurement process.

[0026] Reference Figure 1Exemplary differential measurement circuit 10 includes exemplary differential amplifier 20. The differential amplifier includes first input circuit 12 in a path with resistor 13 for providing a low signal (LS), and second input circuit 14 in a path with resistor 15 for providing a high signal (LS). In some implementations, the first and second input circuits can be merely conductors. In some implementations, the first and second input circuits can include passive electronic devices, active electronic devices, or a combination of passive and active electronic devices. Resistor 13 has a resistance Rl, and resistor 15 has a resistance R2. "High" and "low" do not require or imply any particular numerical values. In this example, the high signal has a greater voltage amplitude than the low signal. Analog-to-digital converter (ADC) 18 is connected to first input circuit 12 and is configured to measure LS on the first input circuit. Amplifier 19, such as an operational amplifier, is configured to receive HS and LS and to produce an output voltage (Vo) based on the difference between HS and LS. ADC 22 is connected at the output of differential amplifier 20 along circuit path 24 and is configured to measure output voltage Vo. In differential amplifier 20, resistor 25 connects circuit path 24 to low signal input circuit 12 along feedback path 27. Resistor 29 connects high signal input circuit 14 to ADC 22 along feedforward path 30. The resistances of resistor 25 and resistor 29 are represented as the product of K1*R1 and K2*R2, respectively, where K1 and K2 are constants that represent or affect low-side amplification and high-side amplification of differential amplifier 20, respectively. Processing logic portion 32, which can be implemented using any appropriate circuit or processing device such as those described herein, uses calibration values to correct errors in output voltage Vo. For example, processing logic portion can use calibration values to determine the differential voltage (hs-ls) according to the following equation (1). Arrow 35 conceptually represents the transfer of values measured by ADC 18 and ADC 22.

[0027] In this regard, if K1≠K2, errors in differential amplifier 20 can be introduced into the output voltage. Although the absolute values of K1 and K2 can vary, in some implementations the absolute values of K1 and K2 are close. For example, K1 and K2 can be within 10% of each other, within 5% of each other, within 3% of each other, or within 1% of each other. The circuits and processes described herein can be used to determine a differential measurement, (hs-ls), in which errors introduced into the measurement process by the differential amplifier have been reduced, minimized, or eliminated. In other words, (hs-ls) can be the difference between the high signal (HS) and the low signal (LS) in which any errors introduced by the differential amplifier or other circuit of Figure 1 have been reduced, minimized, or eliminated. In one example, this differential voltage (hs-ls) is represented as follows:

[0028]

[0029] where Vo is the output voltage measured by ADC 22, ls(m) is the low signal (LS) measured by ADC 18, GH and GL are the gain values of the amplifiers, and Ofs is the offset value that occurs when the low and high signals are at or near zero volts (0V).

[0030] In equation (1), the differential voltage (hs-ls) is expressed in terms of two measurements, namely the output voltage Vo measured by ADC 22 and ls(m) measured by ADC 18. ls(m) is a separate term in equation (1), multiplied by a small number value, In this regard, because the absolute value of K1 is close to K2 in this example, GL is almost equal to -GH, resulting in a value of is relatively small. So the value of GL and GH added together is close to zero. In some examples, the absolute values of GL and GH can be within 10% of each other, within 5% of each other, within 3% of each other, or within 1% of each other. Because the contribution of the error will be relatively small, ADC 18 does not need to have high precision. That is, even if ADC 18 has low precision (e.g., ADC 18 is 10% inaccurate, 20% inaccurate, 30% inaccurate, etc.), the error term of equation (1) including ls(m) will be small and thus will hardly affect the value of (hs-ls).

[0031] In some implementations, ADC 18 can have lower precision than the final measurement specification for circuit 10. In one example, ADC 22 can be more precise than ADC 18. Thus, ADC 18 can be less expensive than ADC 22. In some implementations, the absolute value of ls(m) (the value measured by ADC 18) is relatively low. For example, ls(m) can be 500 mV (millivolts) or less, 400 mV or less, 300 mV or less, 200 mV or less, or 100 mV or less. In some implementations, the low voltage LS is 50% of the high voltage HS; the low voltage LS is 40% of the high voltage HS; the low voltage LS is 30% of the high voltage HS; the low voltage LS is 20% of the high voltage HS; or the low voltage LS is 10% of the high voltage HS.

[0032] In some implementations, the calibration values are gains or "gain factors" GH and GL. In some implementations, the calibration values are based on these gains. The calibration values can be applied according to equation (1) in order to calibrate the errors introduced by the differential amplifier. For example, the processing logic can use the calibration factors to determine the differential voltage (hs - ls) using the measurements of ls(m) and Vo by ADC 18 and ADC 22 and equation (1) above. The following operations can be performed to obtain the gains GH and GL of equation (1).

[0033] In a first exemplary procedure for obtaining GH, the first input circuit 12 is connected to electrical ground in order to provide a ground signal at LS; the second input circuit 14 is connected to a known voltage level in order to provide a known voltage signal at HS; and Vo is measured using ADC 22. Vo here is referred to as Vo(hs) because it is based only on the value of the high signal. The output voltage Vo(hs) is equal to HS*GH, where GH is the gain from the second input circuit 14 to ADC 22. Because the voltage signal HS is known, and the output voltage Vo(hs) is known, GH can be determined by Vo(hs) / HS.

[0034] In a first exemplary procedure for obtaining GL, the second input circuit 14 is connected to electrical ground in order to provide a ground signal at HS; the first input circuit 12 is connected to a known voltage level in order to provide a known voltage signal at LS; and Vo is measured using ADC 22. Vo here is referred to as Vo(ls) because it is based only on the value of the low signal. The output voltage Vo(ls) is equal to LS*GL, where GL is the gain from the first input circuit 12 to path 24. Because the output voltage Vo(ls) is known, and the voltage signal LS is known and can be measured by ADC 18, GL can be determined by Vo(ls) / LS. In this example, LS will be a negative value.

[0035] Alternatively, the following operations can be performed in a second exemplary procedure to obtain gain values for GH and GH+GL. Knowing GH and GH+GL, the value for GL can be determined by subtracting GH from GH+GL. These operations can be used, for example, when it is difficult to connect to the first input circuit 12 to produce a known voltage at LS or when it is not possible to connect.

[0036] In a second exemplary procedure for obtaining GH, the first input circuit 12 is connected to electrical ground so as to provide a ground signal at LS; and the second input circuit 14 is connected to a known voltage level. Thus, the output voltage Vo(hs) is equal to HS*GH, where GH is the gain from the second input circuit 14 to the ADC 22. Since the voltage signal at HS is known, and the output voltage Vo(hs) is known, GH can be determined. That is, because the voltage signal at HS is known, and the output voltage Vo(hs) is known, GH can be determined by Vo(hs) / HS.

[0037] The second exemplary procedure obtains a single value for GH+GL. That is, a single value for GH+GL is obtained; GL is not obtained separately. To obtain GH+GL, the first input circuit 12 and the second circuit input 14 are connected together, e.g., HS is electrically shorted with LS. The resulting shorted HS-LS connection is connected to a known voltage level V(hl), which can be measured by the ADC 18. In this example, the output voltage Vo is a function of V(hl) and is denoted Vo(hl). Vo(hl) is determined as follows: Vo(hl) = V(hl)*GHL, where GHL is the gain from the electrically shorted HS-LS connection to the ADC 22. Because V(hl) is known and the output voltage Vo(hl) is known, GHL can be determined. That is, GHL is defined as Vo(hl) / V(hl).

[0038] Due to linearity, the contributions of the electrically shorted HS and LS can be represented as a superposition of HS and LS and their respective gains, as follows:

[0039] (2) V(hl)*GHL = V(hl)*GH + V(hl)*GL

[0040] To obtain GL, both sides of equation (2) are divided by V(hl), which yields the following:

[0041] GHL = GH + GL

[0042] GL = GHL - GH

[0043] The output voltage Vo can be represented as follows:

[0044] Vo = hs*GH + ls(m)*GL

[0045] = hs*GH + ls(m)(GHL - GH)

[0046] = hs*GH - ls(m)*GH + ls*GHL

[0047] (3) = (hs - ls)GH + ls(m)(GHL)

[0048] Solving equation (3) for (HS - LS) gives:

[0049]

[0050] Equation (4) is identical to the next term of equation (1).

[0051]

[0052] Note that equation (5) assumes a zero offset voltage value. A non-zero offset voltage (Ofs) value is accounted for in equation (1) and is calibrated by setting HS and LS to zero and measuring the output voltage, which is added to equation (5).

[0053] An exemplary differential measurement circuit of the type described herein can be used in a ground re-reference circuit. The ground re-reference circuit can be included in test circuitry that is part of an automatic test equipment (ATE). In an exemplary ATE, a circuit board, such as a device interface board (DIB), is connected to a DUT. The DUT is connected to a first electrical reference, e.g., a first electrical ground ("ground"), on the DIB. Test circuitry, which can be included in a test instrument, is connected to a second electrical reference, e.g., a second electrical ground ("ground"). The second ground is at a different voltage than the first ground, which causes a pair of signals output from the DUT to the test circuitry to change in voltage value, even though the pair of signals maintains a constant voltage difference. For example, the first and second electrical grounds can have a voltage difference of the order of millivolts. In one example, the first and second electrical grounds can have a difference of 200 mV.

[0054] In Figure 2 , an exemplary ATE 40 includes a DIB 42 that holds a DUT 44 connected to a first ground 45. The exemplary ATE 40 also includes a test instrument 46, such as a pin electronics (PE) or parameter measurement unit (PMU), that includes test circuitry 47 connected to a second ground 49 that is at a different voltage value than the first ground 45. The test circuitry 47 includes a differential measurement circuit, which can be of the type shown. The differential measurement circuit includes a differential amplifier 50 of the type described herein. High and low signals 51, 52 are sent from the DUT 44 to the test circuitry 47. These signals can be in response to test signals output by the test circuitry 47, or these signals can be independent of test signals output by the test circuitry 47. The differential amplifier 50 compares the high and low signals 51, 52 to produce an output voltage, which can be indicative of whether the DUT 44 passed a test, or which can be used for further processing to determine whether the DUT 44 passed a test. Figure 1

[0055] ​The voltage level difference between the first ground 45 and the second ground 49 introduces errors of the type described herein. For example, in the event that the voltage of the high signal HS 51 and the low signal 52 sent from the DUT 44 increases or decreases by an equal amount, the differential amplifier 50 can generate different output voltages (Vo). This can occur, for example, due to voltage induced by different current loops for the ground 45 and the ground 49. The processes described herein can be used to determine calibration values to correct for these errors, store the calibration values in computer memory, and use the calibration values to produce corrected output voltages during operation of the circuit.

[0056] Testing using the example systems described herein can be implemented using hardware or a combination of hardware and software. For example, a system similar to the systems described herein can include various controllers and / or processing devices positioned at various points in the system to control the operation of automated elements. A central computer can coordinate the operation in the various controllers or processing devices. The central computer, controllers, and processing devices can execute various software routines to implement control and coordination of the various automated elements.

[0057] The processes described herein can be performed by a system or any other suitable computing device. The processes can be controlled, at least in part, using one or more computer program products, e.g., one or more information carriers, such as one or more non-transitory machine-readable media, having tangibly embodied thereon one or more computer programs for execution by, or to control the operation of, one or more data processing apparatus, e.g., a programmable processor, a computer, multiple computers, and / or a programmable logic component.

[0058] A computer program can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a network.

[0059] Actions associated with implementing all or part of the testing can be performed by one or more programmable processors executing one or more computer programs to perform some of the functions described herein. All or part of the testing can be implemented using special purpose logic circuitry, e.g., an FPGA (field programmable gate array) and / or an ASIC (application-specific integrated circuit).

[0060] Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read-only memory or a random access memory or both. The elements of a computer (including a server) include one or more processors for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more machine-readable storage media, such as a mass storage device for storing data like a magnetic, magneto-optical or optical disk. Machine-readable storage media suitable for embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks.

[0061] Any "electrical connection" as used herein can comprise a direct physical connection, or an indirect connection via intervening components that allow electrical signals to be passed from one connected component to another. Unless otherwise stated, any "connection" to a circuit through which electrical signals flow as referred to herein is an electrical connection, whether or not the term "connection" is modified by "electrical." It is not necessarily a direct physical connection.

[0062] The elements of different implementations described herein can be combined in whole or in part with each other to form additional implementations not specifically described herein. Individual elements can be excluded from some of the implementations described herein without adversely affecting the operation of the systems. In addition, groups of elements can be combined, dissolved into a single element for performing the functions described herein and the like.

Claims

1. Circuitry comprising: a first circuit for providing a low signal; a second circuit for providing a high signal, the high signal having a greater voltage amplitude than the low signal; a differential amplifier configured to receive the low signal from the first circuit and the high signal from the second circuit, the differential amplifier for producing an output voltage based on the high signal and the low signal; a first measurement circuit for measuring the output voltage; a second measurement circuit for measuring the low signal at the first circuit; and a processing logic portion for determining a differential measurement based on the output voltage measured by the first measurement circuit, the low signal measured by the second measurement circuit, and calibration values obtained for the circuitry.

2. The circuitry of claim 1, wherein the processing logic portion is configured to determine the differential measurement (hs-ls) as follows where Vo is the output voltage measured by the first measurement circuit, ls(m) is the low signal measured by the second measurement circuit, GH and GL are the calibration values, and Ofs is an offset value produced when the low signal and the high signal are at or near zero volts (0V).

3. The circuitry of claim 1, wherein the first measurement circuit comprises a first analog-to-digital converter (ADC), the second measurement circuit comprises a second ADC, and the second ADC has a lower precision than a final measurement specification for the circuitry.

4. The circuitry of claim 1, wherein the first measurement circuit comprises a first analog-to-digital converter (ADC), the second measurement circuit comprises a second ADC, and the first ADC is more precise than the second ADC.

5. The circuitry of claim 2, wherein GH is a positive value and GL is a negative value different from GH, wherein the difference between the absolute values of GH and GL is 10% or less.

6. The circuitry of claim 2, wherein GH is a positive value and GL is a negative value different from GH, wherein the difference between the absolute values of GH and GL is 5% or less.

7. The circuitry of claim 2, wherein the absolute value of ls(m) is 200 mV or less.

8. The circuitry of claim 2, wherein GH is a value determined by connecting the first circuit to electrical ground and connecting the second circuit to a known voltage.

9. The circuitry of claim 2, wherein GH+GL is a value determined by connecting the first circuit and the second circuit together and applying a known voltage to the first circuit and the second circuit connected together.

10. The circuitry of claim 2, wherein GH and GL are determined separately from each other.

11. The circuitry of claim 1, wherein the differential amplifier comprises resistors on a feedback path and a feedforward path, the output voltage being based at least in part on the values of the resistors. ​ 12. The circuit of claim 1, wherein the processing logic portion comprises one or more microprocessors.

13. The circuit of claim 1, wherein the processing logic portion comprises programmable logic.

14. The circuit of claim 1, wherein the circuit is part of a ground re-reference circuit.

15. Automatic test equipment (ATE), the ATE comprising: a circuit board connected to a device under test (DUT), the DUT connected to a first electrical reference on the circuit board; and a test circuit connected to a second electrical reference, the second electrical reference at a different voltage than the first electrical reference, which causes a pair of signals output from the DUT to the test circuit to change in voltage value while maintaining a constant voltage difference; wherein the test circuit comprises: a first circuit to provide a low signal of the pair of signals; a second circuit to provide a high signal of the pair of signals, the high signal having a greater voltage magnitude than the low signal; a differential amplifier configured to receive the low signal from the first circuit and the high signal from the second circuit, the differential amplifier to produce an output voltage based on a difference between the high signal and the low signal; a first measurement circuit to measure the output voltage; a second measurement circuit to measure the low signal at the first circuit; and a processing logic portion to determine a differential measurement based on the output voltage measured by the first measurement circuit, the low signal measured by the second measurement circuit, and calibration values obtained for the test circuit.

16. The ATE of claim 15, the ATE further comprising a test instrument to perform one or more tests on the DUT, the test circuit being part of the test instrument.

17. The ATE of claim 16, wherein the first electrical reference is a first electrical ground for the DUT, and the second electrical reference is a second electrical ground for the test circuit.

18. The ATE of claim 15, wherein the processing logic portion is configured to determine the differential measurement (hs-ls) as follows wherein Vo is the output voltage measured by the first measurement circuit, ls(m) is the low signal measured by the second measurement circuit, GH and GL are calibration values, and Ofs is an offset value produced when the low signal and the high signal are at or near zero volts (0V).

19. The ATE of claim 18, wherein the first measurement circuit comprises a first analog-to-digital converter (ADC), the second measurement circuit comprises a second ADC, and the second ADC has a lower precision than a final measurement specification for the measurement circuit. ​ 20. The ATE of claim 18, wherein the first measurement circuit comprises a first analog-to-digital converter (ADC), the second measurement circuit comprises a second ADC, and the first ADC is more precise than the second ADC.

21. The ATE of claim 18, wherein GH is a positive value and GL is a negative value different from GH, wherein the difference between the absolute values of GH and GL is 5% or less.

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