INA317: Noise on INA317 output pin

Jagbir

Part Number: INA317
Other Parts Discussed in Thread: TINA-TI

I am getting a 400mV, 23 KHz sine wave output on pin6 which is present irrespective of input signal. In case I get the input signal this noise is offset and rides on the signal.

over 4 years ago

0 Tamara M Alani over 4 years ago

TI__Genius 10850 points

Hi Jagbir,

Thank you for your post, I can help you.

Can you share with me a picture of your schematic including supplies, expected input signal, desired output signal?

-Tamara

0 Jagbir over 4 years ago in reply to Tamara M Alani

Expert 1515 points

Tamara M Alani said:

Hi Jagbir,

Thank you for your post, I can help you.

Can you share with me a picture of your schematic including supplies, expected input signal, desired output signal?

-Tamara

0 Jagbir over 4 years ago in reply to Jagbir

Expert 1515 points

Tamara,

I am wondering what is the source of this baseline noise which is not present if I use INA 317 on a breadboard, just with a gain resistor, without any filtering circuit.

Thanks,

Jagbir

0 Tamara M Alani over 4 years ago in reply to Jagbir

TI__Genius 10850 points

Hi Jagbir,

Thank you for sharing the schematic, this is really helpful.

I have a few tests I'd like you to perform, I suspect the issue is the input filter but let's verify this together:

Can you disconnect the output of the INA317 and test the output? I wonder if anything it loading it to much, causing undesirable behavior. If the issue persists, try this next step >>
Replace the 10k input resistors with a 0-ohm resistor and test the performance. I suspect that the issue will go away. As you know, the INA317 is a zero-drift amplifier. Because it’s a chopper, there are going to be large input bias current spikes due to the internal switching. These spikes in conjunction with the mismatch of large input resistors can be a detriment to precision.

Looking forward to the results.

-Tamara

0 Jagbir over 4 years ago in reply to Tamara M Alani

Expert 1515 points

Step one resolved the noise issue. I disconnected the output (lifted pin 6) from rest of the circuit and got a clean output signal. I believe its either D5 or C7 is loading the output. I suspected that the ESD diode D5 is loading the output but it was not the culprit.

The filter capacitor C7 on the output pin loading the output and hence causing the noise. Wondering why does a capacitor on the INA317 output pin loads the output?

0 Tamara M Alani over 4 years ago in reply to Jagbir

TI__Genius 10850 points

Hi Jagbir,

The cap load drive of the INA317 is 500pF. Anything higher than 500pF will lead to instability. To learn more about amp stability and the effect of a large capacitor at the output of an amp, follow this training video series: https://training.ti.com/ti-precision-labs-op-amps-stability-1?context=1139747-1139745-14685-1138805-13848

Hope that helps.

-Tamara