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INA3221: Sensing voltage only follow up

Part Number: INA3221
Other Parts Discussed in Thread: INA2331

I understand from this thread that I can simply connect IN+ and IN- for sensing voltage only. My question is, do I connect these 2 pins WITHOUT the Cfilter and Rfilters from page 19 of the datasheet? Do I retain either or both?

Thank you

  • Hi Glenn,

    it depends on your actual application. I guess you have carefully read section 8.4.3 of datasheet?

    Usually, you have a big electrolytic storage cap at the power supply (left side of upper 10R resistor). This big capacitance with its ESR (which is welcome here) will heavily decrease inductive kickbacks by absorbing and dampening the resonance. In any case you should make thorough measurements during the developemt phase (with the INA3221 unconnected) to find out what transients the INA3221 will have to withstand.

    I would mount this recommended 10R resistor in any case, equally what transient are to be expected. So, hook the two inputs together and put a 10R resistor in series to this point. A filtering cap from this point to signal ground can also be very useful, in order to filter ESD and similar fast transient. Take a 100nF cap for the fitering or similar.

    If, on the other hand, you have found out that in your circuit inductive kickbacks will occur which can exceed the absolue maximum ratings, then I would put a suited TVS in parallel to the filtering cap. Keep in mind, that the voltage drop across the TVS can be higher than the specified threshold voltage, if a relevant current flows though the TVS.

    To tell the truth, I would always mount this scheme with big electrolytic storage cap near the INA2331 inputs, the 10Rresistor, the 100n filtering cap and the TVS. I have learned that in industrial circuit design it can be very helpful to be a bit paranoid and to have a bit more low pass filtering than absolutely needed :-)


  • Hello Kai:

    I have carefully read section 8.4.3 and tells me that these filters' (Cfilter and Rfilter) purpose is to filter noise from current. Since my experiment doesn't deal with measuring current, my spidey senses tell me these filters are not needed. However, as acknowledging paranoia as an engineering skill, I still included ceramic Cfilter and Rfilter in my circuit. I didn't include Rshunt as suggested by the expert from the previous thread. See photo for reference:

    Another issue to raise. You mentioned an electrolytic storage cap at the power supply. Is this needed? I didn't see any diagram of an electrolytic cap in the datasheet. In a separate issue, I don't have an inductor in my circuit. Will the electrolytic cap even matter?

    If you see that the connections in my photo are wrong, would it be okay for me to ask you to draw your suggested connections (as mentioned in your 3rd paragraph)? Thanks.

    Please be patient with me as I am simply starting out with circuits :-)



  • Hi Glenn,

    The input RC circuit is optional. It provides a differential low-pass filter to attenuate broadband noise or unwanted higher-frequency content from the input voltage. Please keep in mind that the INA3221 is a voltage-input device, so the filter applies to voltage too. 

    Some customers also choose to set a low cutoff frequency for the filter in order to average down the signal if they care more about average current (or voltage).

    If you are not sensing current in a power supply application and don't have the electrolytic cap or any inductors, the filter can still be helpful. What is the input voltage range you are interested in sensing? Keep in mind that the INA3221 is designed for sensing small voltages of ±163.8mV on the shunt inputs, while it can do from 0 to 26V on the bus inputs.

    Best regards,

    Ian Williams
    Applications Manager
    Current Sensing

  • Hello Ian:

    The power supply will only be 5V. Since I will be connecting IN+ and IN- together, as suggested from the previous thread, the Vshunt should be 0V. The power supply application is actually just a circuit for temperature measurement using a thermistor and a voltage divider circuit:

    R2 is the NTC thermistor.


  • Hi Glenn,

    ok, this looks totally different from what I was thinking about. I thought you would like to measure a low ohmically driven supply voltage. But now you show a resistive voltage divider with much higher resistances.

    Unfortunately, the INA3221 shows rather high input bias currents. Have a look at figure 12 of datasheet. And even more unfortunately, under 5V the input bias currents show a considerable voltage dependency. On the other hand, the input bias currents become smaller and smaller the more the input voltage decreases. By this the error becomes smaller, too. Maybe you can calibrate the error out in a first order approximation by the help of your microcontroller? The input bias currents should not show a too high long term drift.

    And this is the circuit I would use: