The INA118 instrumentation amplifier works perfectly from a 5V supply. But the inputs (the patient) must be biased at half the supply voltage which is +2.5V.
The opamps and +2.5V reference voltage in the schematic biases the patient at +2.5V.
I think I have this setup properly now, thanks for the clarification.
The function of pin 5 on the INA118 and INA326 are completely different so you need to modify the original circuit for it to work with the INA118.
Yeah I've looked at the two datasheets now and I can see the problem - unfortunately I was under the impression that the two components were very similar.
My understanding from the datasheet of the INA118 is that pin 5 is the reference that sets the output offset, while pin 5 on the INA326 sets the value of gain for the second internal stage of the Instrumentation amplifier.
However, a couple of things in the 326's datasheet seem to suggest that the two aren't all that different:-
INA326 Datasheet said:Offset voltage and drift for other circuit values can be estimated from the following equations:
VOS = 10μV + (50nA)(R2)/G (3)
dVOS/dT = 0.12μV/°C + (0.16nA/°C)(R2)/G
and
**broken link removed**
Based on this it doesn't seem like I'd need to make a whole load of adjustments to get my circuit working with the INA118 - can you offer any insight into what I might need to do?
Thanks