Original question:
Tool/software: TINA-TI or Spice Models
Hi，
I am Daniel. I want to use INA317 and INA333 to measure the unblance of resistance bridge. I know the two instrumentation amplifiers need approximately 0.1 V below the positive
supply voltage and 0.1 V above the negative supply. But I don't know how to set the Vcm for the amplifiers.
Thank you for help. I am looking forward to an answer.
Daniel,
The input common-mode voltage range of INA333 is a fuction of supply voltage, gain, Vref and it will NOT work in your application for Vcm=0.1V - see below.
In order for the circuit to have maximum output from 50mV to 3.1V range, you must lift Vcm to Vs/2, which in your application means Vcm=1.65V - see below.
This may be accomplished simply by changing the values of the resistive bridge - see below.
Daniel,
According to INA333 datasheet the maximum output swing is within 50mV or grater above negative rail so if your PR100 variation represent 80ohm to 157ohm R6 variation, the circuit will not work linearly for 157ohm - see below.
In fact, you need to lower R6 to around 148ohm to lift the Vout above 50mV - see below.
Of course, you could simply increase the value of R5 to 160ohm and by doing so bring the output within its linear range - see below.
Since the variation in Vout is about 2.84V for 77 degree change in temperature, 2 degree precision represents ~2.6% or 73.8mV, thus as long as you use higher precision resistors, or calibrate the system, you should have no trouble meeting your accuracy requirement.
