  We have built and demonstrated a digital four-arm bridge for the comparison
of resistance with capacitance. The digital bridge mimics the classical quad
bridge in the digital domain with three balances: the source balance, the
detector balance, as well as the main balance. Due to correlation, the required
precision of the source voltages is only of the order of the square root of the
ultimate bridge precision. For the comparison of a 100 k$\Omega$ resistor with
a 1 nF capacitor near 1592 Hz the combined standard uncertainty (k = 1) is
$5\times 10^{-9}$.
