Large dynamic range current sense 1uA - 100mA

Question

I am looking at large dynamic circuits and came across an interesting project by ETH Zurich called the rocket logger - https://rocketlogger.ethz.ch/

I am a little confused by their current sensing principle for which they implement a low and high range current sensing application. Please see this picture of the circuit below - full circuit @ https://gitlab.ethz.ch/tec/public/rocketlogger/blob/master/hardware/pcb/Cape/Cape_v1_1.PDF

What is the point of/what is U303B doing? I tried simulating the circuit to understand but as I expected because there is no feedback it drives to it's negative rail to try to get it's negative input to be at ground, which is connected at it's positive terminal.

In short - what is the purpose of it? and why does the resistor divider coming into it's negative input have a capacitor on it? Any help would be greatly appreciated!

Answer 1

Referring to your second schematic, the output voltage of U1 is \$V_O=V_{R1}\cdot G + V_{REF}\$, and \$V_{REF}\$ is generated by U2.

Assuming U2 has some negative feedback, it works so that its negative input is at 0 V, so basically \$V_{REF} = -V_{O}\$.

Putting the two equations together you get \$V_O=\frac{V_{R1}\cdot G}{2}\$.

But is there any negative feedback? Let's try to "prove" it. Assume \$V_O = 0, V_{REF} = 0\$, this is a valid state for the circuit.

Some small variation happens at \$V_O\$, so it increases slightly. The negative input of U2 increases, the output decreases until the negative input is back to ground.

What if I redraw U2 circuit like this:

^{simulate this circuit – Schematic created using CircuitLab}