Chapter 10 - Focussed Calibrations

1. Chapter 10 - Focussed Calibrations

2. Overview • Traceability to National Standards • National Institute of Standards and Technology • Maintain standards of Volt, Ampere, second, meter, inch, pound etc. • Also chartered to license calibrations to laboratories • maintain “copies” of standards to insure accuracy of automatic testers. • ATE calibration references are used as “golden” standards • removed from the tester regularly and sent to NIST or a calibration lab for recalibration. • Using freshly recalibrated standards, the ATE can maintain general accuracy from day to day.

3. Chain of Accuracy transfer from NIST to DUT

4. Overview • Traceability to National Standards • There are three ways to calibrate a measurement instrument • manual hardware calibration • Potentiometers or variable capacitors • software controlled hardware calibration • Programmable gain amplifiers or timing verniers • Basically a software controlled version of manual calibration • Tester measures its own error and eliminates it using digitally controlled adjustment circuits • software correction algorithms • Leave hardware errors as they are and compensate in software.

5. Overview • Traceability to National Standards • Full ATE calibrations can be initiated manually if needed or executed automatically by the tester • System calibrations occur at periodic intervals or when certain events occur. • Timing initiated (once per week) • Loading a test program • Temperature change • Accuracy drift is most commonly caused by temperature change • Advanced testers perform this function automatically, while less advanced testers require engineer intervention • hardware calibration boards etc.

6. Overview • Why are Focused Calibrations Needed? • Two step process • Focused calibrations are measurements made by a test program on its initial execution, after automatic calibrations are completed, but before DUT testing begins. • Three reasons to perform focused calibrations • transfer of accuracy standard from one instrument to another • transfer of accuracy standard to uncalibrated circuit such as a DIB circuit • store measured values which are used repetitively to reduce test time. • One time measurement of a sine wave to be applied to the input of a circuit (actually a test time reduction)

7. Overview • Why are Focused Calibrations Needed? • ATE instruments are very expensive - why do they need extra calibration??? • ATEs are capable of performing calibrations during the focussed cal process - why not perform a focussed cal during start up for every instrument??? • Each setup has its own errors: offset, gain and phase shift for each different gain setting, input mode setting, sampling rate, filter setting, test tone frequency etc. • The result would lead to an excruciatingly long calibration process to cover all possible combinations required. • Most of these settings may not be used in your test code.

8. Overview • Why are Focused Calibrations Needed? • ATE calibration processes include calibrations at certain points in a device operating range. Interpolation gives a very good approximation of the value to be expected at that setting, but it is not necessarily calibrated at that instrument setting. • When accuracy requirements are not particularly demanding, the standard calibration may be good enough without the need for extra focussed calibrations. • A focused calibration is the calibration of instruments at the exact settings to be used in the test program to insure accuracy without interpolation errors. - We are focussing in on the exact measurement requirements.

9. Overview • Mechanics of a Focused Calibration • Analog measurements are seldom accurate. • Some errors are tolerable - others are not (its up to the test engineer to know the difference for the specific test being performed. • The purpose of focused calibrations is to reduce the errors inherent in the source and measurement signal paths for each test. • Signal path includes: • all mathematical computations • tester instruments • electrical circuits between the idealized mathematical signal representation in the ATE computer and the input node under test

10. Overview • Mechanics of a Focused Calibration • The purpose of focused calibrations is to reduce the errors inherent in the source and measurement signal paths for each test. • Measurement path includes: • all circuits • computations • instruments between the signal under test and the final measurement result. • May include DUT circuits such as buffers etc.

11. Overview • Mechanics of a Focused Calibration • Each combination of instrument configuration is a unique signal path and must be calibrated individually

12. Overview • Mechanics of a Focused Calibration • Basic concept of focussed calibration is the same irregardless of the measurement or source path • Measure non-ideal characteristics of source path • remove these characteristics from the measured result using a software adjustment. • Example: If a DIB voltage follower has a gain of 1.01 V/V at 1kHz, then we must divide the 1 kHz RMS voltage measured by 1.01 to calculate the actual RMS voltage present at its input.

13. Overview • Mechanics of a Focused Calibration • When measuring the DC offset of a cascaded signal path, you can assume that the composite path produces an offset that is the sum of the offsets of each of the circuit elements in the signal path. • Offset could be multiplied by the gain of the following stage which may complicate things. • The total gain is equal to the product of the individual gains. • Gains often vary with frequency • may require a separate calibration at each frequency of interest.

14. Overview • Mechanics of a Focused Calibration • Phase shift through a signal path at a particular frequency is equal to the sum of the phase shift through each of the individual elements • Likewise timing measurements such as delay time are additive. • Measurements such as distortion and noise are neither additive nor multiplicative. • Cascading rules may not be valid in all cases • Test engineer may be required to verify these during characterization.

16. Problem: • A signal path consists of two cascaded DIB circuits and a medium accuracy voltmeter. The DIB circuits have a DC gain of G1 = 1.002 and G2 = 2.102 and an offset of O1 = 10mV and O2 = 20 mV respectively. The voltmeter has an offset of O3 = -1 mV and a DC gain of G3 = 0.997. • These offsets and gains are measured and stored as cal factors in the first execution of the program. • A DUT output is applied to the input of the first DIB circuit and the voltmeter reads the output of the second DIB circuit. The voltmeter produces a reading of 2.523V. What is the actual output voltage of the DUT?

17. Calculate the DC Gain and Offset of the composite path: • DC Gain = G1*G2*G3 = 2.09989 • Offset = (O1*G2*G3) + (O2*G3) + O3 = 39.9 mV • Vout = Vin * DC Gain + Offset • 2.523V = Vin * 2.09989 +39.9mV • therefore Vin = 1.1825V

18. Overview • Mechanics of a Focused Calibration • The test engineer has a choice of generating a composite calibration factor or creating a calibration factor for each individual block. • Individual block calibration allows you to reorient the blocks or reuse them in subsequent tests without recalibration. • Composite calibration reduces test time • Composite calibrations don’t make assumptions about the interactions between circuit blocks • Sometimes the calibration errors just “wash out” as in the calculation of the gain of an amplifier, since we are looking for a ratio of output over input - thus a focussed calibration is unnecessary.

19. Overview • Program Structure • Integral part of most test programs • Test code performs focussed calibrations during first run and stores data as global variables • subsequent executions of the program retrieve cal. factors • Focused calibrations should be repeated on a periodic basis since tester drift may cause errors which are not accounted for. • Focussed calibrations should be regenerated when test conditions change • Also, any time the ATE performs an auto-calibration, the focused calibration needs to be rerun, since the reference calibration of the instrument will change.

20. Overview • Program Structure - flow chart

21. DC Calibrations • DC Offset Calibration • Measured by setting the input of the instrument or circuit to mid-scale and observing the offset from its ideal output level. • DC voltage sources, DC voltmeters, AWGs, Digitizers, DIB circuits • buffer amplifiers • filters • Mid-scale definition depends upon circuit application • DC offset of tester instruments (Voltmeter, AWG etc.) can be measured to generate offset calibration factors • This allows a more accurate measurement using these devices.

22. DC Calibrations • Cascading DC Offset Calibration • When cascading offset calibrations you need to take the DC gain of each stage into account. • DC model of an offset path is given by a standard linear model: • Vout = Vin * Gain + Offset • Cascade of two circuits gives: • Vout = ( Vin * Gain1 + Offset1 ) * Gain2 + Offset2

23. DC Calibrations • DC Gain Calibration • Apply two known DC input levels and measure two output levels. • The DC Gain is defined as the change in output divided by the change in input • Thus the total DC transfer characteristic (transfer function) is given by: • Output = Input * Gain + Offset • Inversely Gain and Offset values can be used to calculate the actual value by: • Input = ( Output - Offset ) / Gain

25. AC Amplitude Calibrations • Calibrating AWGs and Digitizers • Calibration must be confirmed at each frequency of interest • Several common techniques • Calibrate the digitizer first using DC calibration, anti-aliasing filter response calibration, then use the digitizer to calibrate the AWG • Calibrate AWG using highly accurate RMS voltmeter and then use the AWG to calibrate the digitizer • Use highly accurate AC signal source to calibrate the digitizer and then calibrate the AWG with the digitizer.

26. AC Amplitude Calibrations • Calibrating AWGs and Digitizers • Several common techniques • Calibrate the digitizer first using DC calibration, anti-aliasing filter response calibration, then use the digitizer to calibrate the AWG • assumes the digitizer gain is flat across frequency band of interest - bad assumption due to anti aliasing filter response. • If you can bypass the filter you can pass a calibration signal directly to the digitizers ADC which we assume has a flat frequency response. • A multi-tone signal is generated and digitized with the filter still bypassed and then the filter is inserted. The filter response function can be calculated from the data.

27. AC Amplitude Calibrations • Calibrating AWGs and Digitizers • Several common techniques • Calibrate the gain of the AWG using a highly accurate RMS voltmeter, then use the AWG to calibrate the digitizer • calibrate the gain of the AWG at each test tone. • Simultaneously measure the tones with the digitizer (anti-aliasing filter enabled) to get the filter response function • care should be taken due to the accuracy of the RMS voltmeter at high frequencies versus low frequencies. • Use a highly accurate AC signal source to calibrate the digitizer, then calibrate the AWG with the digitizer • Option three is not used very often due to availability of an accurate signal source.

28. AC Amplitude Calibrations • Low Level AWG and Digitizer Amplitude Calibrations • Used in gain tracking, signal to distortion, crosstalk, CMRR and PSRR tests. • Want to calibrate test tones not noise components - RMS voltmeter measures both. • DSP based testing provides a solution to the problem. • First generate a high amplitude sine wave from the AWG to calibrate the gain of the digitizer at each frequency • Once the cal factors have been identified, a low amplitude signal can be generated and digitized. The combination of digitizer and FFT can differentiate between noise and signal, giving a much more accurate measurement.

29. Other AC Calibrations • Phase Shifts • In the same way signal paths modify the amplitude of a signal, they also modify the phase shift of the signal. • Where cascaded gains are multiplicative in nature, the phase shifts are additive throughout a system. • Phase measurements are implemented by simply digitizing the output of a circuit and then digitizing its input. • The phase at each frequency is simply: • Phase = arctan(Im/Re) • where Im is the imaginary part of the FFT and Re is the real part of the FFT. • Phase shift is simply Output phase - Input phase

30. Other AC Calibrations • Digitizer and AWG Synchronization • It is critical to synchronize the timing of both the AWG and the digitizer, so that the phase shift of the digitized signal is the same every time a test is run. • May find that the phase of the digitized waveform is different every time you execute the exact same measurement. • This is not important in gain calibration, since we are interested in the ratio of the signals not the timing. In phase tests, the timing is the critical factor. • Synchronization is tester dependent • Should be covered in training class

31. Other AC Calibrations • DAC and ADC Phase Shifts • To measure the phase shift through an ADC or DAC, we have to know the phase of each test tone relative to the digital samples sent to or captured from the converter circuit. • Mixed-signal testers don’t usually align analog waveforms and digital signals precisely. The output minus input phase calculation does not work. • The solution is to input a square wave from the digital pattern generator at the frequency of interest and capture it using the digitizer. • Since a square wave has known amplitude and phase characteristics, this technique can be used to get the digitizer’s phase shift relative to the digital pattern generator.

32. Other AC Calibrations • Distortion Tests • Distortions are neither additive nor multiplicative in nature, therefore they are very hard to extract from a signal path. • However, distortion can be compensated by characterizing the output versus input transfer characteristic of the circuit. • A software table of cal factors allows us to compensate for non-linearities of voltmeters, digitizers, AWGs and DC sources. • Distortion varies with frequency, so there are many variations of distortion calibrations. • Fortunately, most of the distortion calibration is already performed by the ATE.

33. Other AC Calibrations • Noise Tests • Occasionally a test engineer wants to subtract the noise generated by the measurement path from the total noise measured. • Noise is not entirely additive in cascaded circuits • It is a good assumption that the noise from the tester will not cancel the noise from the DUT - uncorrelated noise adds constructively not destructively • Usually the noise floor of the tester is the limiting factor in the accuracy with which a measurement can be made.

34. Error Cancellation Techniques • Gain and Phase Matching • Obviously it is desirable to avoid any calibrations that are not needed!! • Gain and Phase matching of two identical channels can be performed in two ways: • Use two AWGs and two digitizers to measure the gain and phase of both channels individually and compare the two. This requires calibration of two AWGs and two digitizers. • Use a single AWG connected to both channels simultaneously - measure the output using two channels of the same digitizer simultaneously, therefore the distortions and noise of the instruments cancels out, thus no calibration is required.

35. Error Cancellation Techniques • Gain and Phase Matching • Gain and Phase matching of two identical channels can be performed in two ways: • Use three UTPs to measure the output. The first UTP is for measuring channel 1, the second UTP is junk where you are switching from channel 1 to 2 and is discarded, the third UTP is for measuring channel 2. Thus the input signal remains the same (also the same AWG) and the digitizer remains the same. • The cost is extra test time because one UTP is wasted in the transition of channels.

36. Three UTP Technique for AWG/Digitizer Alignment

37. Error Cancellation Techniques • Differential Gain and Differential Phase • Focussed calibrations can sometimes be eliminated using DIB circuits • Differential Gain is defined as the change in AC amplitude with varying DC offset. • Differential Phase is defined as the change in AC phase with varying DC offset. • It is obvious we want to digitize two signals - same AC with different DC offsets and calculate the Differential gain and phase - digitizer non-ideal??? • The solution is to de-couple the DC offset from the AC response using an RC high pass filter, therefore the digitizer sees the same DC offset (0 volts) regardless of the circuit’s DC offset.

38. Problem: • An AWG needs to produce a single-ended 1.0V peak to zero sine wave with 2.5 V DC offset. The AWG has an offset specification of +/- 10 mV, but we need an input offset accuracy of +/- 1 mV for this measurement. The tester has a high accuracy DC voltmeter with a total error (gain plus offset) of +/- 100 uV when set to its 5.0V range. Determine the focussed calibration process that will achieve the offset accuracy of +/- 1 mV DC from the AWG. • Assume superposition - We can assume that the AWG has the same DC offset whether there is a sine wave at its output or not. • You can set the AWG to a DC level of 2.5 V by simply creating a short waveform containing only the value of 2.5 in each sample.

39. Use of the high accuracy voltmeter allows you to calculate a global variable whose value remains unchanged between one execution of your test code to another. To produce a calibrated waveform from the AWG, we subtract this offset from the desired signal when we calculate the actual DUT signal we want. • The resulting waveform should have an offset (relative to 2.5 V) very near 0V. • Accuracy of this calibration relies on the fact that we set the AWG to the exact same conditions during the calibration as we plan to use during the actual DUT test.