  We report on a technique for determining electronic transport properties
using commercially available audio cards. Using a typical 24-bit audio card
simultaneously as a sine wave generator and a narrow bandwidth ac voltmeter, we
show the spectral purity of the analog-to-digital and digital-to-analog
conversion stages, including an effective number of bits greater than 16 and
dynamic range better than 110 dB. We present two circuits for transport studies
using audio cards: a basic circuit using the analog input to sense the voltage
generated across a device due to the signal generated simultaneously by the
analog output; and a digitally-compensated bridge to compensate for nonlinear
behavior of low impedance devices. The basic circuit also functions as a high
performance digital lock-in amplifier. We demonstrate the application of an
audio card for studying the transport properties of spin-valve nanopillars, a
two-terminal device that exhibits Giant Magnetoresistance (GMR) and whose
nominal impedance can be switched between two levels by applied magnetic fields
and by currents applied by the audio card.
