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INA126 with single power supply problems

Matt Smit
Prodigy 10 points
Other Parts Discussed in Thread: INA326, INA126

I am using an INA126PA to monitor the voltage difference between two photodiodes. The one photodiode is used as a light reference. I hooked up two INA126PA instrumentation amplifiers in the way indicated in the data sheet (http://www.ti.com/lit/ds/symlink/ina126.pdf)

V+ = 2.85V, V- = Vref = 0V. Rg = 20kohm to start such that G = 9. Vin+ and Vin- are expected to receive a signal between 0V and 0.55V depending on lighting conditions. I was expecting that when Vin+ = Vin- that the output signal Vo would equal 0. However, no matter what I put as an input signal in Vin+ and Vin- the output is always 2.11V. This is the case even if I tie Vin+ to Vin- to Vref.

For example, if Vin+ = 0.09V and Vin- = 0.06V the output should be be 0.27V, however, Vo is still 2.11V.

Do I require two power supplies to run this chip correctly?

  • 0
    TI__Genius 12325 points

    Matt,

    The input common-mode voltage range of the INA126 does not extend to the negative power supply because this device was meant to be used with dual power supplies. Because your input signals are near the negative power supply voltage of the INA126, the internal amplifiers are saturated and will not provide a valid output voltage. If you only have a single supply in your system, you might consider using the INA326. This instrumentation amplifier uses a unique topology intended specifically for single-supply applications.