The circuit in Figure 1 converts an input digital code into electrical impedance. The figure shows the equivalent circuit, for which you can digitally program C_EQ and R_EQ with 12-bit resolution. Possible applications for this circuit include programmable filters and signal generators, digital measuring bridges, impedance standards, and particularly, dynamic skin-impedance simulators for biomedical applications.

The first stage of the circuit comprises a simple high-impedance amplifier, IC₁. This amplifier's output drives multiplying DACs (MDACs) A and B, which, together with IC₂ and IC₃, perform digitally-programmable-gain conversion. If you apply a constant ac voltage to the input terminals, you can prove that the current consumption from this voltage source, namely its active and reactive parts, is proportional to the input digital code of the respective MDACs. Thus, the circuit can simulate the programmable impedance, for which you can independently control the resistive and capacitive parts. The parameters of the equivalent passive circuit are as follows:

where A_i and B_i are the input digital binary coefficients of MDACs A and B, respectively. R₁ and R₂ perform the scaling. These resistors are optional. If you omit them, the input amplifier becomes a simple voltage follower. The LF353 op amp suits this circuit because of its good dynamic performance, but any other fast op amp is also applicable. IC₃ has to fit the additional demand of driving a significant capacitive load. The LM6361M can drive an output capacitance of 10 nF without performance degradation. This circuit uses standard dual MDACs, but other MDACs are also applicable. (DI #1866)