I am designing a simple resistance measurement circuit. Resistance range is from \$10m\Omega\$ to \$10k\Omega\$ and desired accuracy is 0.1%.
The circuit provides constant current to the resistance using a transconductance amplifier. An instrumentation amplifier is used to measure the voltage drop across the resistor. The current through the unknown resistance has to be sinusoidal, as per the requirements of my assignment.
When I connect both the instrumentation amplifier inputs to ground, I get a voltage at the output of \$2.22mV\$:
I've found that this is due to the offset input voltage. To overcome this, I have included a voltage source on the inverting input (pins 1 and 5 - the offset null pins - are not enabled in the spice model). This minimizes the output voltage offset:
I tested the circuit using a sinusoidal signal of \$1\mu V\$ RMS (the lowest I'll be working with), and the output is perfect:
Now the problem:
When I connect the inputs across the unknown resistance, with the constant current source (transconductance amplifier) supplying current to that resistance, even though the voltage drop across it is still \$1\mu V\$ RMS, there is a mysterious increase in the output of the instrumentation amplifier:
As the simulation shows, this increase happens in final op-amp of the instrumentation amplifier. I'm completely confused by this. Why has the output increased all of a sudden, even though the voltage drop being measured is the same? How can I get the accurate output?