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Stability of LM101A variable-gain instrumentation amp

Jonathan9420
Expert 2056 points
Other Parts Discussed in Thread: TINA-TI, OPA170, PGA2505, PGA2500, OPA376, OPA377

After noticing that an attenuating diff amp followed by variable gain was ruining my SNR, I found this circuit from Application Note 31 that simulates with much better noise performance:

I built it (with different op-amps) and confirmed that the noise matched the simulation.

The example circuit would have overall variable gain from 0 dB (R6=10 kΩ) to +50 dB (R6=3 MΩ). However, when I build it, it oscillates at gains below +17 dB, and I don't understand stability theory enough to fix it.  Does it require the external feedforward compensation of LM101A (Linear Brief 2) to be stable?

LM101A, LM102, and LM107 seem to all be old or obsolete.  If this requires special external compensation op-amps, which modern replacements would you suggest?

  • 0
    TI__Expert 6275 points
    I will make three recommendations:
    1)Download Free TINA-TI SPICE simulator at:
    http://www.ti.com/tool/tina-ti
    2) Download six parts on op amp stability at:
    e2e.ti.com/.../2645.solving-op-amp-stability-issues.aspx
    3) Go through the TIPL learning series at:
    www.ti.com/precisionlabs
    You can build your circuit in SPICE and analyze to see why it oscillates and how to fix it. If you study above and build your circuit in TINA-TI SPICE and still have issues post the schematic and the problem and we can provide analysis help.
  • 0 in reply to Tim Green1
    Expert 2056 points

    Well the LM101A in TINA doesn't have the external compensation pins, and it doesn't oscillate in TINA, either, no matter what op-amp model I use.

    AN-31 variable gain diff amp.TSC

  • 0 in reply to Tim Green1
    Expert 2056 points

    Ok, I went through the Precision Labs tutorials, but it doesn't talk about circuits with op-amps in the feedback loop with their own nested feedback loops. I don't know where to break the loop to measure loop gain, or where to put the "feedback" node.

    Wherever I break it has to include the loading of the op-amp outputs, too, from what I've read.

  • 0 in reply to Jonathan9420
    TI__Guru 62055 points

    Hi Jonathan,

    I verified that the LM101A version of the circuit does not oscillate in SPICE.  This is likely due to the age and simplicity of the model which is preventing the true circuit behavior from coming through.  If you redesign the circuit with a modern op amp with a modern SPICE model, such as the OPA170, the issues become apparent.  Take a look at the image at the bottom of this post that shows the output response of the circuit to a 10mV step input.  The results are a sustained oscillation, indicating significant circuit stability issues.

    We're looking into a solution for your circuit topology that works better with modern amplifiers that don't include the trim pins included with the LM101.

    The LM101 is an older product with modest specifications. Could you provide a little information about your circuit goals? Information such as desired input offset voltage, bandwidth, noise performance, slew rates, and anything else important in your application will be helpful to selecting the best solution.

    Also, have you considered an integrated INA or PGA product such as one of the products featured in the links below?  These devices will out-perform a discretely designed INA, usually for a lower cost and less PCB space.: 

    http://www.ti.com/lsds/ti/amplifiers-linear/dual-supply-products.page?paramCriteria=no

    http://www.ti.com/lsds/ti/amplifiers-linear/programmable-gain-amplifier-products.page?paramCriteria=no

  • 0 in reply to Collin Wells
    TI__Guru 62055 points

    Hi Jonathan,

    I was forwarded some additional information from the local sales team about your application.  We worked together and wanted to suggest the PGA2505 for your application.  Here's some information about the PGA2505.

    • PGA2505:

      • Allows for programmable gain in 3dB steps from 9-60dB with a 0dB option

      • Operates of a ±5V rail, but can also accept a 5V rail and GND rail.

      • -123dBu at 30dB of gain with equivalent input noise of a 150 ohm resistor

      • Several other audio specific features

      • Cheaper than the PGA2500

    If the PGA2505 is not a good fit for your application we can discuss a few options to build a discrete instrumentation stage out of a low-noise amplifier such as the OPA376 / OPA377.