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INA826: Single supply operation, gain doesn't match as expected

ArTzy
Mastermind 19580 points

Part Number: INA826

Hi Experts,

Good day. Seeking assistance on this query:

I'm using it in single supply mode with +28V supply. The gain doesn't match the 1+49.4k/Rg formula. Also in simulation there's a non linear break in the output as the input voltage is increasing. I guess it is not a simple thing to operate it single supply?

Thank you.

Regards,
Archie A.

PN: INA826AIDGKR

  • 0
    Guru 140200 points

    Hi Archie,

    no, you will need a negative supply voltage.

    I see that you permanently connect the -input of INA826 to signal ground and that you don't seem to have any common mode input signal, or by other words, that you don't have any differential signalling at the input. So do you need an instrumentation amplifier at all? Or could a simple standard OPAmp do the job as well?

    Kai

  • +1
    TI__Guru 55466 points

    Hi Archie,

    Yes, in this configuration, you can easily be outside the input common-mode voltage range of the instrumentation amplifier.  The INA826 input and output range is limited when the reference pin is grounded and the negative supply (Vs-) is also grounded.  Calculating the Input and output linear range limitation of the instrumentation amplifier can be a little challenging since the INA826 instrumentation amplifier architecture consists of three amplifiers, and all the amplifiers must be within their linear range.  The input and output range is a function of the supplies, Vref pin voltage and gain.

    However, you can download the "Analog engineer's calculator" to verify the input and output voltage range of the instrumentation amplifier.  The “Vcm vs. Vout” software tool considers the input and output limitation of each internal amplifier and displays a system level graph showing the linear output range, as a function of the voltage supplies, gain, Vref pin voltage and input common-mode.

    https://www.ti.com/tool/ANALOG-ENGINEER-CALC

     In your example, using the “Vcm vs. Vout” software tool, selecting the INA826 with Vs- =0V, Vs+ = 28V, VREF=0V, and Gain =29.39V/V, and assuming the negative input terminal is grounded, the allowed input voltage range at the (+IN) terminal is limited from about +3.5mV to +922mV, or an output of approximately +0.1V to +27.1V.

    If you require to measure negative input signals, you will need to use bipolar supplies.

    Alternatively, as Kai has mentioned, if you are only interested in amplifying a single-ended, positive signal with respect to GND, you could consider using an op-amp on the non-inverting configuration. 

    Thank you and Regards,

    Luis

  • 0 in reply to Luis Chioye
    TI__Guru 55466 points

    Hi Archie,

    Using the INA826 with unipolar supply ((Vs-) =GND., Vref =GND, -IN = 0V, Gain =29.39V/V restricts the +IN non-inverting input to only positive input signals in the range of roughly +3.5mV to +922mV, since the output can only swing ~0.1V to about ~27V.

    In most applications, when VREF is referred to GND, the INA826 will be biased with bipolar supplies to allow full positive and negative range.  Alternatively, the device can be powered with unipolar supply, with VREF biased to a positive around Vs/2, with the input common-mode closer to the middle of the supplies.

    Below is a TI - Precision Labs session discussing the three-amp topology range / topic in detail:

    training.ti.com/three-amp-theory

    If you require a different circuit, please let us know in detail the differential input, input common-mode voltage range and supply requirements.  As we have suggested, if you are only interested on amplifying a positive single-ended signal with respect to ground, one possible alternative is to use an op-amp circuit powered with unipolar supply.  There may be other options.

    Thank you and Regards,

    Luis