Why designing a instrumentation amplifier in a discrete way?
The schematic shows a discrete designed instrumentation amplifier including a power supply. Non of the common ICs like the famous INA-line is used. It is only built-on single operational amplifiers. This gives us the advantage of setting every parameter of the circuit for the individual application.
The power supply is built with common linear power supply technology and provides a high precision of the output voltage. Linear regulators (LT1085 and LT1033) from Linear Technologies are used to provide a very low output voltage error of 0.0015%. Furthermore a line filter should be used to block high frequency interferences on the primary side of the transformer. The power supply of the Wheatstone bridge is realized by a separate LT1086-5CT, which could be used as a voltage reference for further digital processing.
The amplifier is built upon the ultra low offset operational amplifier OP07, which provides a secure operation even without a zero voltage adjustment. The whole amplifier is separated in five stages. Stage one and two (instrumentation amplifier circuit) have fixed gain (Vu=24.57), the third stage is able to set a gain between 1-56 with a potentiometer, the fourth stage provides a offset and the fifth and last stage includes a low-pass filter (fg=250Hz). With this formula you are able to set the gain for stages one and two: Vu=(1+(2*R13/R12))*R17/R15.
It is recommended to assemble the resistors of stage one and two with 0.1% resistors.
For a offset adjustment SW1 should be switch to ground.