INA128: Low Frequency Ringing?

Hi Nicholas, 

Could you share the schematic with us? A hand drawing schematic will work, if you do not have the electronic schematic available in a file.

I have several questions.

1. What is INA128 driving? What are the load capacitance, inductance and resistance?

2. What are your supply voltage rails? single or dual supply rails.

3. When input audio frequency > 5kHz presented in front of INA128's input, you do not hear ringing. If input f <5kHz, INA128's output can hear the ringing.

4. Please also specify the input signal amplitude with Gain = 1k V/V

I will attempt to simulate the scenario and see if I am able to find possible issues. 

Best,

Raymond

1. The INA128 is driving an OPA4277 to provide additional gain, the signal goes into a 10k load resistor to ground, then continues through a 1nF capacitor into a 100 Ohm resistor then into the first OPA4277 gain stage. 

The input is a transformer coming off the microphone for impedance matching.

2. Supply rails are +-12V

3. That is correct

4. Input amplitude is in the 0 to 100 microvolt range.

Thanks for taking a look. Below is the schematic:

Hi Nicholas, 

Here is what I will try. Please short C1 or 1nf capacitor between two op amps or use 1uf coupling capacitor or larger (10uf).

The INA128 stage is feed forwarded to the second op amp stage, there should not have loop stability issues, However, 1nF at 5kHz or greater has impedance of <31.8kΩ, while 1nf at 1kHz will have impedance of ~159kΩ. My guess is that the AC impedance is doing some interference or impedance mismatching (create higher poles in the loop). 

You may short out C1 or increase its value from 1nf to 1uf. My guess is that the ringing should be gone below 5kHz of input frequency. 

INA128 Microphone E2E 06232021.TSC

Please let me know if this resolves your issue. 

Best,

Raymond

Hi Nicholas,

R44 is way too small. 100R looks like a short-circuit to the output of INA128. Increase (R44 and R42) by at least a factor of 10...20. Better you wire U7A as a non-inverting amplifier.

Also, as already stated by Raymond, heavily increase C13, if this is a typical audio application.

Audio transformers can behave weird when being terminated improperly at the input and output. And they don't like bias currents either. More, all too cheap audio transformers can sound trashy and ring like a bell. Can you tell more about your transformer section?

Kai

It definitely seems like shorting my C13 (your C1) helped, it made the ringing higher frequency and seemingly less intense. As kai mentioned I also bumped up the resistances for the gain resistors for the first stage into the OPA so that I wouldn't be running it into overcurrent range.

Hi Nicolas,

Thanks for Kai's suggestions. 

If you are able to place approx. 5pf capacitor above RF of 10kΩ resistor, it may help to attenuate higher frequency noise based on the AC analysis. 

Best,

Raymond