Zo Tester Collin Wells

Original Zo Tester

Original AOL Circuit

Original AOL Circuit Limitations #1 DUT Zo and feedback resistor form resistor divider causing errors. #2 DUT is not AC Coupled causing errors in the AOL curve. #3 Not capable of 50Ohm Drive

Example of AOL Limitations #1 DUT Zo Causing Errors This region should continue to roll off at -20dB/decade

Example of AOL Limitation #1 DUT Zo Causing Errors

Explanation of AOL Limitation #1 DUT Zo Causing Errors

Solution to AOL Limitation #1 DUT Zo Causing Errors

Solution to AOL Limitation #1 DUT Zo Causing Errors

Solution to AOL Limitation #1 DUT Zo Causing Errors

Solution to AOL Limitation #1 DUT Zo Causing Errors –Actual Implementation

Solution to AOL Limitation #1 DUT Zo Causing Errors –Actual Implementation

Example of AOL Limitation #2 DUT not AC coupled –Even small offsets in the DUT Vcm away from (Vcc(+) - Vcc(-))/2 result in degradation of AOL at low frequencies -0.25V Offset

Example of AOL Limitation #2 DUT not AC coupled –Even small offsets in the DUT Vcm away from (Vcc(+) - Vcc(-))/2 result in degradation of AOL at low frequencies -0.5V Offset

Example of AOL Limitation #2 DUT not AC coupled –Even small offsets in the DUT Vcm away from (Vcc(+) - Vcc(-))/2 result in degradation of AOL at low frequencies -1V Offset

Example of AOL Limitation #2 DUT not AC coupled –Even small offsets in the DUT Vcm away from (Vcc(+) - Vcc(-))/2 result in degradation of AOL at low frequencies -1.5V Offset

Example of AOL Limitation #2 DUT not AC coupled –Even small offsets in the DUT Vcm away from (Vcc(+) - Vcc(-))/2 result in degradation of AOL at low frequencies Railed

Example of AOL Limitation #2 DUT not AC coupled –Although overall AC gain is “1”, DC Gain is 1+((100k+100k))/100 = ~2000V/V –Therefore any part with a Vos of >1.25mV will rail.

Solution to AOL Limitation #2 DUT not AC coupled –AC Couple the DUT with a 100uF capacitor. This will reduce the overall system offset to just the Vos of the DUT. –It will require time when the system initially starts up for the cap to charge and cancel out the offset.

Example of AOL Limitation #3 Not Capable of 50 Ohm Drive High-Z Node

Final AOL Circuit

Final AOL Circuit Results

High-Pass Effects of AC Coupling Cap Fc = 1/2πRC Effects of DUT Cin and Zo of Buffer Circuits

Final AOL Circuit Results

Final AOL Circuit Limitations

Final AOL Circuit Limitation Partial Fix False Summing Junction Gain Changed to 40dB Change to 100k

Final AOL Circuit Limitation Partial Fix

Final AOL Circuit Results

Original Zout Circuit

Original Zout Circuit Limitations Not capable of 50 Ohm Drive DUT is not AC Coupled so DC errors occur Instrument output gets divided down by parallel combination of 50 Ohm termination resistor and RS in series with DUT Zo. System noise floor corrupts low-frequency data

Example of Zout Limitation DUT not AC Coupled –Zout test based on the fact that both FETs in the output stage need to be biased to mid- supply with ½ Iab flowing through them.

Solution to Zout Limitation DUT not AC Coupled –AC Couple the Zout circuit with a 100uF Capacitor

Example of Zout Limitation Divided down Instrument Output –Although this issue does not cause errors in the measurement system it does limit the amount of signal that we inject into the DUT therefore limiting the output of the test.

Example of Zout Limitation Divided down Instrument Output –Although this issue does not cause errors in the measurement system it does limit the amount of signal that we inject into the DUT therefore limiting the output of the test. RS = 80600

Example of Zout Limitation Divided down Instrument Output –Although this issue does not cause errors in the measurement system it does limit the amount of signal that we inject into the DUT therefore limiting the output of the test. RS = 4990

Example of Zout Limitation Divided down Instrument Output –Although this issue does not cause errors in the measurement system it does limit the amount of signal that we inject into the DUT therefore limiting the output of the test. RS = 698

Example of Zout Limitation Divided down Instrument Output –Although this issue does not cause errors in the measurement system it does limit the amount of signal that we inject into the DUT therefore limiting the output of the test. RS = 80.6

Solution to Zout Limitation Divided down Instrument Output –50 Terminate the gain/phase analyzer output and send into the input of a THS4631 high- speed amplifier.

Solution to Zout Limitation Divided down Instrument Output –50Ohm Terminate the gain/phase analyzer output and send into the input of a THS4631 high-speed amplifier. RS = 80600

Solution to Zout Limitation Divided down Instrument Output –50Ohm Terminate the gain/phase analyzer output and send into the input of a THS4631 high-speed amplifier. RS = 4990

Solution to Zout Limitation Divided down Instrument Output –50Ohm Terminate the gain/phase analyzer output and send into the input of a THS4631 high-speed amplifier. RS = 698

Solution to Zout Limitation Divided down Instrument Output –50Ohm Terminate the gain/phase analyzer output and send into the input of a THS4631 high-speed amplifier. RS = 80.6

Example of Zout Limitation Zo Circuit not capable of 50 Ohm Drive –50Ohm termination on Vreference re-creates the previous problem of dividing down the input signal amplitude. –50Ohm on Vtest adds DC load to the DUT, which will affect Zo.

Solution to Zout Limitation Zo Circuit not capable of 50 Ohm Drive –Add THS4631 buffers to both Vreference and Vtest nodes.

Example of Zout Limitation Noise corrupts low-frequency data –Zout = Zo/(1+AOLB) so at low frequencies when AOLB is still large Zout gets very small and when we measure using Vtest = Ibackdrive*Zout the signal is smaller than the system noise. –Although this issue affects every part measured, there are two types of op-amps that cause the most problems, both cause issues due not being able to create a large enough voltage to measure (V=IR): 1 – Small I – Ultra-Low-Power Op-Amps. Since we have to backdrive with Iab/2 we are very limited on our backdrive current when the quiescent current of the part is in the <100uA. 2 – Small R - Power Op-Amps with very low Zo. After dividing down by Loop Gain the Zout is in the sub 1-Ohm range and we can still only backdrive with Iab/2.

Example of Zout Limitation Noise corrupts low-frequency data OPA333 Data hit noise floor on Zout Measurement Data invalid for Zo calculation below 1kHz

Example of Zout Limitation Noise corrupts low-frequency data OPA333 Zo does not go capacitive Until ~10Hz

Example of Zout Limitation Noise corrupts low-frequency data OPA564 Data hit noise floor on Zout Measurement Data invalid for Zo calculation

Example of Zout Limitation Noise corrupts low-frequency data OPA564 Zo does not go capacitive Until ~60Hz

Other Zout Issues Data has a “hump” at the frequency where the part runs out of Loop Gain. OPA564 Gain = 55dB F = 20kHz Hump in data at 20kHz

Other Zout Issues Data has a “hump” at the frequency where the part runs out of Loop Gain. OPA564 Gain = 40dB F = 110kHz Hump in data at 110kHz

Other Zout Issues Data has a “hump” at the frequency where the part runs out of Loop Gain. OPA552 Gain = 42dB F = 90kHz Hump in data at 90kHz

Other Zout Issues Data has a “hump” at the frequency where the part runs out of Loop Gain. OPA344 Gain = 40dB F = 10kHz Hump in data At 10kHz

Other Zout Issues Data has a “hump” at the frequency where the part runs out of Loop Gain. OPA364 Gain = 46dB F = 40kHz Hump in data At 40kHz

Areas for Future Research Increasing Back-Drive Current Measuring “DC Zo” – Marek Lis

Areas for Future Research Increasing Back-Drive Current –Still struggling to get enough “V” in the V=IR equation, so increase I and test the results. OPA376 RS – 4990 I = Iab/2 Hi-F Zo = 200Ohms Data Valid until 500Hz

Areas for Future Research Increasing Back-Drive Current –Still struggling to get enough “V” in the V=IR equation, so increase I and test the results. OPA376 RS – 1150 I ~ Iab/2*5 Hi-F Zo = 190Ohms Data Valid until 300Hz

Areas for Future Research Increasing Back-Drive Current –Still struggling to get enough “V” in the V=IR equation, so increase I and test the results. OPA376 RS – 576 I ~ Iab/2*10 Hi-F Zo = 150Ohms Data Valid until 100Hz

Areas for Future Research Increasing Back-Drive Current –Still struggling to get enough “V” in the V=IR equation, so increase I and test the results. OPA364 RS – 3570 I = Iab/2 Hi-F Zo = 200Ohms

Areas for Future Research Increasing Back-Drive Current –Still struggling to get enough “V” in the V=IR equation, so increase I and test the results. Hi-F Zo = 180Ohms OPA364 RS – 825 I ~ Iab/2*5

Areas for Future Research Increasing Back-Drive Current –Still struggling to get enough “V” in the V=IR equation, so increase I and test the results. Hi-F Zo = 120Ohms OPA364 RS – 412 I ~ Iab/2*10

Areas for Future Research Issues with Increasing Back-Drive Current –The reduction in the Zo values when the back-drive current is increased is not predictable. Unlike adding a DC load, the “AC Loaded” Zo results do not decrease at 1/(gm*I) for BJT or 1/sqrt(k*Id) for MOS. –There also does not appear to be any correlation between the decrease in Zo value from one part to the next.

Areas for Future Research Measuring “DC Zo” –Would be a method to get a low-frequency intercept point so we could interpolate between the areas where the current Zo tester runs into noise issues. –Method entails measuring AOL with and without a DC load and then calculating the Zo based on the difference between the two AOL values.