I'm currently using the MP111 hybrid op-amp set up as a voltage controlled current source with a gain of 1. But what I'm finding is that at low input voltages the gain is much higher than it should be – up to 8 A/V. As we increase the input voltage the gain decreases sharply and then becomes 1.
Why is the gain so off at low input voltages? Is there a way to fix this?
I'm calculating the gain by dividing the current seen passing through the shunt resistor (Rs) by the input voltage.
MP111 datasheet: https://www.apexanalog.com/resources/products/mp111u.pdf
MP111 configuration as current source (figure 8): https://www.apexanalog.com/resources/appnotes/an07u.pdf
Rs = 0.01 Ohms, Rload = 0.05 Ohms, Rin = 101 kOhm, Rf = 1 kOhm.
The positive and negative rails are at +15V, -15V respectively. Boost functionality is not being used therefore Vboost pins are connected directly to the power rails.
Best Answer
Thermal feedback, of unknown magnitude and polarity, will either buck or boost your desired control-loop signal.
To test, abruptly change the Rload by 50% (simply hold a similar Resistor in parallel), and observe the behavior.