Combining the well-known true rms-to-dc circuit with a simple S/H circuit eliminates offset errors, which increases accuracy and temperature stability. The circuit in Fig 1 uses a low-cost analog multiplier, IC₁; an integrator, IC_3A, R₅, and C₁; and an analog inverter, IC₂, R₃, R₄, and D₁ to convert the analog V_IN into a true-rms dc V_OUT. The input signal can be ac, dc, or a combination. With an input-signal range of 0 to 2V, the circuit can handle ac signals with crest factors up to 5.

Unfortunately, offsets in IC₁ and IC_3A contribute significant error to the circuit. Moreover, these offsets drift with temperature. To remove these offset errors, the circuit performs self-calibration with the help of one logic-control input (CNTRL) and three DG419 SPDT switches, SW₁, SW₂, and SW₃. During calibration, when CNTRL is low, the analog inverter connects the multiplier pins X₁, X₂, Y₁, and Y₂ to the signal ground. The circuit disconnects D₁ and C₁ from the output. Also during calibration, the feedback loop includes the S/H circuit, IC_3B, C₂, R₆, and the attenuator comprising R₁ and R₂. When conditions settle, the voltage on IC_3B's output compensates the combined offsets of IC₁ and IC_3A. The R₁-R₂ attenuator increases the compensation voltage level on C₂ from millivolts to volts. The offset of S/H amplifier IC_3B doesn't play any role, because this amplifier is inside the feedback loop.

During rms-to-dc conversion, when CNTRL is high, V_IN connects to multiplier pins X₂ and Y₁. V_OUT connects to Y₂, to X₁ through the analog inverter, and to D₁ and C₁. Also during conversion, the S/H circuit disconnects from IC_3A's output. Now, the output voltage of the S/H circuit compensates for the offsets of IC₁ and IC_3A. This output voltage changes slowly because of the leakage current. Thus, you must periodically calibrate the circuit. The circuit requires about 0.1 sec of calibration time. The conversion time is much longer, limited only by the circuit's time and temperature drift. Periodic calibration compensates for the drift, so you can use the circuit in extreme climate conditions.