Circuit to "zoom in" on mV fluctuations of a DC signal?
Capacitors block DC and pass AC.

You can use a series capacitor into an opamp with whatever gain you need.

Even better might be a simple RC high-pass filter...One capacitor (series) and one resistor (to ground) in front of your amplifier.

Like this:







simulate this circuit &ndash; Schematic created using CircuitLab

R2 and R3 set your gain.  C1 and R1 set your low frequency cut-off. The formula you use to find the cutoff is:

$$F\text{(Hz)} = \frac{1}{2 \pi  R  C}$$