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# INA322: voltage divider to step down the voltage

Part Number: INA322

Hi Team,

Good day. I am posting this inquiry on behalf of the customer.

"I'm looking to use the INA322 to measure a DC voltage. Basically using a voltage divider to step down the voltage. The INA322 + and - terminals are across the lower leg of the voltage divider.

The - input is connected to GND

Do I need to deal with bias in this situation?

For some additional information. The ground reference for the voltage being measured is weakly separated from the INA322 power supply. There is a 499K resistor between those two ground references."

Kind regards,

Marvin

• Hi Marvin,

It is very difficult to interpret what the customer means here without a schematic (i.e. what do they mean by "bias", where is the "ground reference" connected, etc.).  Can you ask for a schematic and we can evaluate?

Regards,
Mike

• Hi Michael,

Thank you for your prompt response. Please see the attached schematic from the customer.

"Here is a snippet.  There are several of these circuits monitoring different voltages.

A_MON can range from 0-34V.  I have 17.3V-34.0V listed as that is the valid range.  FCC and CTRL_PWR_RTN need to have at least 100k between them.  I realize that the INA322 needs a return path so I added the 499k resistor between the two grounds."

Kind regards,

Marvin

• Hi Marvin,

Ok, I don't understand the 499 kOhm resistor between the grounds - Is there going to be a voltage difference between the 2 grounds (FCC and CTRL_PWR_RTN)?  Is there a low impedance connection for both of them?

The common mode input range of the amplifier will depend on the voltage between FCC and CNTRL_PWR_RTN.  If they are both close to the same voltage, then this circuit should be OK.  But, if it is high, then the device may be outside the input common mode range.  How much difference would you expect between FCC and CTRL_PWR_RTN?

Regards,
Mike

• Hi Michael,

Thank you for your response. Please see the feedback from the customer.

"We do not expect there to be much difference in potential between FCC and CTRL_PWR_RTN.  However, by our spec, they need to be “isolated” from each other by >100kohms.  We also need to be able to monitor the DC voltage of several supplies referenced to FCC.  I originally had optically isolated amplifiers in there, but availability issues have made it so we pivoted to this solution.  Assuming that we don’t have a common mode problem, do I need to include any other circuitry?  And, by extension, do you think this solution will work?  Thanks."

Kind regards,

Marvin

• Hi Marvin,

this looks ok in theory. But in a real world circuit many problems can occur. What if from one ground to the other ESD or even Surge is travelling? In this case I would strongly recommend to add series resistors at the input of INA322 to limit the input currents in an overvoltage event.

If stronger overvoltage events can occur, you can add a more robust protection to the inputs of INA322, like two sets of BAV99 input protection diodes from the inputs to the rails (behind the current limiting resistors).

But don't forget to also mount protection measures from the "A_MON" driver output to its supply rails.

And it can be helpful to mount a small 1...10nF Y-cap between the two grounds, to at least provide a common potential at HF and ESD.

Kai

• Hi Marvin,

Are you sure there are no other paths connecting the two grounds that could be lower impedance than the 499kΩ resistor? Including parasitic paths? For example, if you have the specified 100kΩ connecting the two grounds elsewhere in the circuit than the parallel combination would be less than your specified requirements.

I agree with Kai's recommendation to include current limiting resistors in series with both input pins. 100kΩ will limit the current to <10mA for surges up to 1000V.

Regards,

Zach