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We present the design and implementation of a measurement system that enablesparallel drive and detection of small currents and voltages at numerouselectrical contacts to a multi-terminal electrical device. This system, whichwe term a feedback lock-in, combines digital control-loop feedback withsoftware-defined lock-in measurements to dynamically source currents andmeasure small, pre-amplified potentials. The effective input impedance of eachcurrent/voltage probe can be set via software, permitting any given contact tobehave as an open-circuit voltage lead or as a virtually grounded currentsource/sink. This enables programmatic switching of measurement configurationsand permits measurement of currents at multiple drain contacts without the useof current preamplifiers. Our 32-channel implementation relies on commerciallyavailable digital input/output boards, home-built voltage preamplifiers, andcustom open-source software. With our feedback lock-in, we demonstratedifferential measurement sensitivity comparable to a widely used commerciallyavailable lock-in amplifier and perform efficient multi-terminal electricaltransport measurements on twisted bilayer graphene and $SrTiO_3$ quantum pointcontacts. The feedback lock-in also enables a new style of current-biasedmeasurement which we demonstrate on a ballistic graphene device.

Feedback Lock-in: A versatile multi-terminal measurement system for electrical transport devices