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INA4180-Q1: How to properly configure unused operational amplifiers

Pascual Hilario
Prodigy 30 points
Part Number: INA4180-Q1
Other Parts Discussed in Thread: INA4180

Hi,

I am using the INA4180A1 current sense amplifier from which one of the 4 channels is unused. The device is implemented using a single supply rail.
I was wondering how to properly terminate it and I found the following article from TI: SBOA204A – October 2017. Figure 2 (right) illustrate a proper implementation of unused opamps using a single supply rail. However, in that case it is only applied purely on the opamp itself, whereas the INA4180 mounts internal resistors in a differential arrangement for obvious reasons.

In that sense, and in order to follow the approach of the aforementioned article, I thought about the following solution:

The addition of 475 kOhm resistors would make the series combination with Rint to be 500kOhm, so the gain factor would be unity, but also to have V+/2 at the positive input of the operational amplifier. Would that be an acceptable way to terminate it?

Many thanks in advance.

Best regards,
Pascual

  • 0
    Guru 140200 points

    Hi Pascual,

    I would short the two inputs of unused current sense amplifier together and connect them to a potential lying within the allowed common mode input voltage range of INA4180. GND would be such an allowed potential. So, I would connect the IN+ pin and IN- pin of unused current sense amplifier directly to the GND pin of INA4180, with shortest connection. The unused current sense amplifier would look like a low-side sensing amplifier with a zero Ohm Rsense then. See figure 8-4 of datasheet.

    Kai

  • +1
    TI__Mastermind 32395 points

    Hello Pascual,

    There is an effective 2.5 kOhm differential impedance between IN+ and IN- pins. Although this may not really matter.

    Either way if you really don’t need to use some channels you can either float the input pins (IN+ and IN-) completely, short them and float, or short them and short this to a stable rail (GND or VBUS). There is one advantage to connecting unused input to VBUS and that it is you avoid and channel-to-channel separation offset error. If all inputs have the same VBUS Common mode, then there is not channel-to-channel separation. The trade off is your burning leakage power through the extra input bias currents.

    The main advantage to just shorting them and floating is that you’re not going to be turning on internal amplifier and you save on trace space.

    Sincerely,

    Peter