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INA317: Output signal behavior: part 2

Intellectual 895 points

Replies: 18

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art Number: INA317

Hi,

I had built the first prototype amplifier circuit with INA317, which was working as expected output (almost  0V  output with no input connected ), see link below for previous discussions.

https://e2e.ti.com/support/amplifiers/f/14/t/903404

When I test the 'new' boards, I am seeing the output voltage drift over time with no input connected. In my Rev 2 prototypes I have removed the output cap(C7),input clamping diodes(D3,D4) as well as well as the 470 Ohm series resistance.  Additionally, I have added couple of 'do not mount' pads in case I need to enable -230mV negative bias generator  circuit or to provide paths for input  bias currents. Is removing the 470 Ohm series resistor causing this instability. 

Thanks

  • Hi Jagbir

    When I test the 'new' boards, I am seeing the output voltage drift over time with no input connected.

    Please check the following. In the new circuit, please short pin2 and pin4 together at J2 (input pins to the INA317), you should get zero voltage output at pin 6 in the new circuit.

    You should not leave the input unconnected at its input. Please let us know the result. 

    Best,

    Raymond

  • In reply to Raymond Zhang1:

    Hi Raymond,

    I do get a zero voltage output at pin 6 in the new circuit. when I pin2 and pin4 together at J2 (input pins to the INA317). Why  the unconnected input pins were giving a zero voltage output with my 'old' circuit.

    Thanks,

    Jagbir

  • In reply to Jagbir Singh:

    Hi Jagbir,

    Kai fixed your old circuit by biasing -0.232V at negative rail. Therefore in your new circuit, 0V or short the input pins are operated within the Vcm common mode voltage requirements. It you let it float, the output voltage can not get the defined voltage differences at input, and it is changing constantly. 

    In your old circuit, your V- is ground, the common-mode voltage range is (V-) + 0.1 and (V+) - 0.1. With unconnected sensor, your input Vcm is outside of Vcm range, That is why that it is shown in zero voltage at output. 

    Best,

    Raymond

  • In reply to Raymond Zhang1:

    Hi Raymond,

     I have not yet connected the V- to -230mV, as this pin is still tied to ground via R5 jumper, meaning my reference is still tied to V-(ground). i have an option to use -230mV rail, but for that I need to populate R2  and obviously remove R5

    Jagbir

  • In reply to Jagbir Singh:

    Hi Jagbir,

    That is what your new schematic is shown, where V- is connected to -0.232V. If V- = GND, then by shorting pin2 and pin4 together at J2 is not valid test for INA317. You have to DC biased up to have proper input within the required Vcm. 

    What are the other difference between old and new circuits now per your initial inquiry? Is INA317 still working or damaged? 

    Best,

    Raymond 

  • In reply to Raymond Zhang1:

    Hi Raymond,

    INA317 is still working.

    In my  new circuit, there are two configurations- one with V- connected to -0.232V and another configuration where V- is connected to GND. My default configuration is  V- tied to GND. What you see in schematic is a  pcb pad without a physical resistor mounted on it (labelled as DNP aka Do Not Populate).

    The list of changes to old schematic include :

      • Removed the 100nF output cap, C7 since it was outside the range of allowable capacitance on the output pin
      • Removed input clamping diodes(D3,D4)
      • Removed the 470 Ohm series resistance.
      • Provided options for input  bias currents (pin3 ,pin3 on IN317) return path to ground via pcb resistor pads but the resistors(R6 and R10)  to complete the circuit are not mounted on these pads.

    • As described above , in new circuit there are options to connect V- to ground as well as -0.232V . In my default configuration is  V-  is tied to GND.
    • Please note that in the new circuit there might be some resistor reference designator which are used in older circuit: R2/R5.

  • In reply to Jagbir Singh:

    Hi Jagbir,

    Your circuit has gain of 1001, which is high. Since your input is floating, Vout is drifting over time should not indicate anything, especially the input voltage is not within the recommended Common-mode voltage range. 

    From the above described circuit. if you short the input and measure output, the following image is a correct way to do it. At Vout = 22.08mV, the input offset voltage is approx. 22.08mV/1001 = 22 uV.

    If your V- = -0.232V is implemented, then you can short the input as I replied in the early email. The input common mode voltage has to be within the range below in order to get linear output response. 

    If the output is drifting vs. time with unconnected input, it is not linear output response that is prescribed in the datasheet. I am unable to explain it. 

    If you have other questions, please let me know. 

    Best,

    Raymond

  • In reply to Raymond Zhang1:

    Raymond,

    Couple of questions:

    • When I connect a 1000 Ohm strain gauge to the new circuit at J2, the output voltage drift vanishes. However, I am seeing a stable/constant 0.2V offset at the output voltage, even in absence of an input strain. What is is the reason for it.
    • Why Vcm is not a problem with my old circuit and suddenly it's an issue with new circuit.

    Thanks,

    Jagbir

  • In reply to Jagbir Singh:

    Hi Jagbir,

    If you have configured the strain gauge in the following manner, you should get near zero output as shown below. 

    You told me that the INA317 is working. That is why I am unable to explain your issue (see the prior reply). 

    If your test circuit is configured same as the above, and Vout = 0.2V, then something is wrong with your INA317 or circuit. However, you need to verify that Vin+ and Vin- inputs have the same dc voltage of 3.3V/2 = 1.65Vdc, 

    Best,

    Raymond

  • In reply to Raymond Zhang1:

    I believe the issue is due to bias current path, as without a bias current path, the inputs float to a potential that exceeds the common-mode range of the INA317 device, and the input amplifiers saturate.

    After I soldered R6 and R10 =10K  into my new circuit, I am getting 0V offset.