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INA317: Trying to measure open voltage with INA317

Part Number: INA317
Other Parts Discussed in Thread: INA826

Hi there,

I have made a circuit using the INA317 where I want to measure the open voltage of a load, after it has been charged, but somehow the gain is less than 1. For the Rg, I didn't place any resistor because I wanted a gain of 1. The input+ and input- are disconnected from the circuit and only connected to the load and, when the load is at 1.5V, for example, the output is 0.63V or something like that.

I noticed that with a capacitor placed between input+ and ground, the output gets alot closer. Right now, I have three capacitors, one from Input+ to GND, one from Input- to GND and another from Output to GND and the difference is "just" 100mV.

I need a really high impedance input so as to not disturb the load as I'm measuring the open circuit voltage, that's why I chose this part. The INA317 is being powered by a 3.3V rail.

I have another circuit which I presented here with another issue, where I used the INA826 and that one is working flawlessly but it's being powered by a 10V power rail.

Is there anything I'm missing here? Here's the schematic, I've taken Q1 and Q2 out and am connecting the load directly to their drain.

  • Hi ,

    Your Vin+ = 1.5V and Vin- = Ground (I assume). You need to comply with input Vcm requirements, since this is a singe rail supply configuration. The output is measuring the difference of input delta between Vin+ and Vin-. For a single rail, Vin- has to be above 0.1V. 

    /cfs-file/__key/communityserver-discussions-components-files/14/INA317-Vin-meas-09252020.TSC

    In the simulation, I left Vref = 0. If your Vout is near 2V to 3V range,  you may have to use low impedance reference to elevate the output voltage, where Vout = Vref + G(Delta_Input). Figure 21 is shown single 5V power rail, 3.3V shape looks similar in shape, if you rescale both axes to 3.3V (see Figure 23 in Vs=1.8V). 

    If you have additional questions, please let us know. 

    Best,

    Raymond

     

  • The thing is I'm trying to measure a floating voltage. It's the same as charging a capacitor and then trying to measure it's voltage with it out of the circuit. So your assumption of Vin- = ground is incorrect. Both Vin+ and Vin- are connected to a charged "capacitor" which is not referenced to the ground of the circuit. Essentially, I'm just trying to measure a differential voltage with a very high input impedance and expecting the output to be = Vin+ - Vin-.

  • Hi ,

    I simulated your floating "capacitor"  measurement using INA317. Please let us know if this is what you asking.  

    1.5V represents the floating voltage. Vcap represents the "capacitor" that you want to measure. 100kOhm represents the floating node at Vin-.  Since I am simulating sine wave, I have to use +/-3.3V to meet the input Vcm requirements. If your "capacitor" is in the range of input Vcm, then you can use single supply rail. 

    If the floating voltage measurement at "capacitor" is within the Vcap or VM1 range, the INA317 output will have the 1:1 linear response. 

    /cfs-file/__key/communityserver-discussions-components-files/14/INA317-Vin-meas-09252020A.TSC

    If you have additional questions, please let us know.

    Best,

    Raymond

  • Hi ,

    I have not heard from you for a few days. Hopefully, your issue is resolved. 

    I am going to close the inquiry for now. If you have additional questions, you may reopen the thread or open a new one.

    Best,

    Raymond