INA821: Selection and Impact of Input Series Resistor Values for INA821

Yan Wang

Part Number: INA821
Other Parts Discussed in Thread: INA826

The application scenario of INA821 is as follows: the voltage across a resistive temperature sensor is pre-amplified by a preamplifier with a gain of 65 times, and then transmitted over a 30-meter cable to the INA821. The schematic is shown in the figure1.

When using multi-channel sampling, oscillation appears at the input of the INA821 as shown in Figure 2. When the series resistor, originally 4.99 kΩ, is changed to 0 Ω, the oscillation disappears and the sampling returns to normal.

Please help analyze the cause of this phenomenon and suggest the correct solution.

Thanks

Wang Yan

11 天前

0 Eirwen 11 天前

TI__Mastermind 35565 points

Hello, we have received your case and the investigation will take some time. Thank you for your patience.

0 Eirwen 7 天前回复 Eirwen

TI__Mastermind 35565 points

Just to clarify, the oscilloscope is showing the oscillations at the input of the INA821?

Also would you be able to repost the image? It's a little blurry and I want to make sure I have the right values on all the passive components.

0 Yan Wang 7 天前

Prodigy 10 points

Hi Eirwen,

Yes, it is at the input of the INA821....

0 Yan Wang 7 天前回复 Eirwen

Prodigy 10 points

Hi Eirwen,

the oscilloscope is showing the oscillations at the input of the INA821.

Thanks,

Wang Yan

+1 Eirwen 5 天前回复 Yan Wang

TI__Mastermind 35565 points

I think this is something that I can easily test in the lab to verify that it's likely coming from the cables or the AD620 or how it's interacting with it all.

Would you be able to share what the signals on the input look like? I see that the negative input is grounded but what signals should I expect on the positive side?

0 Yan Wang 2 天前回复 Eirwen

Prodigy 10 points

Hi Eirwen,

The abnormal waveform is shown in the figure; if the waveform were normal, it would be a DC level of 0.195 V.

Are there any guidelines for selecting the input series resistors, common-mode capacitors and parallel resistors, and differential-mode capacitors for the INA821?

Wang Yan

0 Eirwen 1 天前回复 Yan Wang

TI__Mastermind 35565 points

I'm going to setup a test on my side to see if I see similar results. I'm guessing the stability issues might be coming from the AD620's interaction with the RC network and INA826 but I'm going to confirm the INA821 is stabile first.

0 Eirwen 1 天前回复 Eirwen

TI__Mastermind 35565 points

Have you tested the output without having the INA821 there? If it shows instability that suggests the AD620 is having issues driving the capacitors?

I ran the tests and found no ringing which leads me to believe its how the AD620 is interreacting with the circuitry on it's output.

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INA821: Selection and Impact of Input Series Resistor Values for INA821