Created by Created February 26, 2013 Last modified February 26, 2013 Tags

Summary

Measuring or matching high value resistors using a Wheatstone Bridge and an opamp transimpedance (Vout/Iin; i.e. a current to voltage converter) amplifier.

Simulate > DC Sweep > Run DC Sweep

Description

Measuring or matching high value resistors using a Wheatstone Bridge and an opamp transimpedance (Vout/Iin; i.e. a current to voltage converter) amplifier.

OA1 is the transimpedance amplifier. At balance it's output, V(out), sits at V(y). For R1 < R2, V(out) < V(y). For R1 > R2, V(out) > V(y).

OA2 simply buffers V(y) so that V(out) - V(ybuf) gives a voltage that is zero at balance and swing positive or negative as R1 varies.

At balance, with nothing connected between the two nodes; V(x) = V(y). Therefore if the input offset and input bias current of the opamp is zero, no current flows through R5.

http://en.wikipedia.org/wiki/Wheatstone_bridge

There is no need for a high impedance voltmeter to measure the difference between the two nodes. It is often simpler to connect the two nodes through sensitive ammeter or galvanometer:

http://en.wikipedia.org/wiki/Galvanometer

R1 is swept from 9.99M to 10.01M, i.e. +/- 0.1%.

R5 sets the transimpedance of OA1 and is stepped at 10M and 100M.

OA1 & OA2 need to be low input bias current, low offset voltage opamps.

e.g:

http://www.ti.com/product/lmp7721#technicaldocuments

Simulate > DC Sweep > Run DC Sweep

http://signality.co.uk