I am working with a TI INA821 Instrumentation Amplifier to try to amplifier a 0-5mV (delta) analog signal off a strain gauge. To reiterate, every DC analog value between the delta of 5mV represents a specific load (weight).
The delta is the difference between (+) ~2.528 V and (-) ~2.533 V. The circuit is operating off +/-5 V supply.
The circuit I am using is that of the generic recommended configuration in the INA821 Datasheet:
- V+ = 5V
- V- = -5V
- C1 = C2 = 0.1uF
- R1 = R2 = 1M
- Vref = 0V (GND)
- INO+ = INO- = 2.528V @ no load
- INO-' = 2.528V @full load
- INO+' = 2.533V @ full load
- R3 = Varies (see below)
The only configuration that does not result in Vout being pushed to +5V is with R3 DISCONNECTED which yields a gain of G = 1.
Using two 50mVpp, and 60mVpp test sinusoidal signals (same generator, voltage difference via voltage divider) at IN+ and IN- successfully amplifies the delta and reflects change when a pot is used in place of R3.
My only thought is that perhaps the strain gauge, being a DC type signal is causing an offset large enough to send the output to the positive supply rail? However, in theory I did not think this should be an issue given that the amplifier design is of the differential type?