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OPA187: The loop test of the dual - operational - amplifier is abnormal.

c cc
Prodigy 60 points
Part Number: OPA187
Other Parts Discussed in Thread: TLV6001

Tool/software:

Hello,

I recently came across an informative post on EDN that discusses methods for testing the characteristics of operational amplifiers. It was very insightful, and I decided to try out some of the circuits mentioned in the post.

While testing one of the circuits using a dual operational amplifier, I noticed that the output of the circuit becomes unstable when the device under test (TLV6001) is powered by a single supply. However, when I use a dual supply (±2.5V), the output becomes stable. I’ve tried adding a loop feedback capacitor (22pF), but the output is still unstable when using a single supply.But he is stable in the simulation, and I think a single power supply should have the same effect as a dual power supply.

Could you please provide me with some suggestions or guidance on how to address this issue? I’m quite new to this, so I would really appreciate your help.

Thank you in advance!

  • 0
    TI__Guru* 76021 points

    Hi cc, 

    c cc said:
    Could you please provide me with some suggestions or guidance on how to address this issue?

    This is TLV6001's the input common mode range (Vcm), and you need to make sure that the op amp is operating in a linear range. 

    In theory, your circuit is met the TLV6001's Vcm requirements. In practice, it does not have the safety margins due to spread of different IC lots. The input needs to provide some bias voltage, say V3 = 200mV, 500mV or similar Vcm above GND for single supply rail.  GND maybe too low from certain TLV6001 op amps. 

    For dual supply rail, the Vcm = GND meets the datasheet's requirements.

    Op_amp_Test_Basics_from_TI Servo Method.pdf 

    If you have other questions, please let us know. 

    Best,

    Raymond

    HVA Application Team

  • 0 in reply to Raymond Zhang1
    Prodigy 60 points

    Hi Raymond,

    Thank you for your reply and for sharing the document. I found that it seems to be the same content as what I had seen before. Here is the link to the article I read on EDN. I think it’s exceptionally well-written. After carefully reading through it, I believe I’ve found a solution. I tried increasing the CCOM capacitor (C2) to a value greater than 22pF, and the issue of loop output instability disappeared. The output has now stabilized to a DC value.

    https://www.edn.com/the-basics-of-testing-op-amps-part-3-configurable-circuit-tests-op-amps/  

    https://www.edn.com/the-basics-of-testing-op-amps-part-4testing-op-amps-requires-stable-test-loops/

    However, I still have some questions that I haven’t fully understood and would appreciate your guidance on:

    1. When testing the loop op-amp, what principles should be followed when selecting the LOOP AMP? Should it meet the following criteria:

      • GBW > DUT GBW?
      • AOL > DUT AOL?
      • VOS > DUT VOS?

    2. Could you provide a formula or method for quickly estimating the range of values for the loop compensation capacitor (C2)?

  • 0 in reply to c cc
    TI__Guru* 76021 points

    Hi cc, 

    c cc said:
    When testing the loop op-amp, what principles should be followed when selecting the LOOP AMP?

    The DUT op amp in this case has GBW = 1MHz. Since gain is configured at 51k/102 +1 = 501 V/V, its usable BW = 1MHz/501 = 1.996 kHz. In order to increase the BW and measurement speed, it adds the loop amplifier for the purpose.  

    c cc said:
    • GBW > DUT GBW?
    • AOL > DUT AOL?
    • VOS > DUT VOS?

    I do not believe that the above criteria has to be true in order to select the loop gain amplifier, but generally the loop amplifier is a high grade op amp (power amplifier) that is capable to drive ATE's output load. So the loop amplifier's selection criteria should have low open loop output impedance and capable to delivery higher current than a standard op amp to drive a measured instrument (ADC, DMM etc.). 

    For linear operation, total Aol in open loop, A_ol_total = A_ol_dut x A_ol_loop_amplifier and the gain bandwidth product is still applied in the linear operation. The output will follow the DUT's Vos behavior, if the circuit is operating in a linear range in DUT and Loop Amplifier. Notice that the Loop Amplifier is using the dual supply rails per the suggested configuration, and the loop amplifier is operating in open loop topology on its own, and the overall feedback loop is fed back to DUT op amp.   

    c cc said:
    Could you provide a formula or method for quickly estimating the range of values for the loop compensation capacitor (C2)?

    There is no quick method to calculate the C2 value. You have to perform open loop AC stability analysis or measured the loop stability via Bode 100, 500 or other gain and phase analysis via Network analyzer. The article is shown the Bode plot and phase margin plots for the Type II compensation technique. 

    If you are interested the op amp's different compensation method, you can check out the video series below. 

    https://www.ti.com/video/series/precision-labs/ti-precision-labs-op-amps.html?keyMatch=op%20amp%20lab%20video%20series&tisearch=universal_search

    https://www.ti.com/document-viewer/lit/html/sszt999?keyMatch=op%20amp%20stability%20video%20series&tisearch=universal_search

    If you have other questions, please let me know. 

    Best,

    Raymond