Stabilizing a SQUID Current Amplifier in High Magnetic Fields

SQUID-based galvanometers are unstable when connected to sources exposed to strong magnetic fields. We present a SQUID-based current amplifier with a 17 fA (root Hz)(-1) current noise, which is unconditionally stable with a source exposed to magnetic fields up to 10 T. Stability is achieved by inserting a four-pole low-pass filter between the source and the current amplifier. With this circuit, we demonstrate Johnson noise measurements below 1 K for source impedances in the k Omega range subjected to strong magnetic fields.