5 Theory of Operation142 Keysight 34970A/34972A Service GuideAC CircuitUnless otherwise noted, components in this discussion are located on the A4circuit assembly (34970-66504).The multimeter uses a true RMS ac-to-dc converter to measure AC voltages andcurrents. The ac-to-dc converter changes the input AC voltage to a DC voltage.All voltage ranging is performed in the AC circuit so that the input to themultimeter’s DC circuitry (AC_OUT) is nominally 2 VDC for a full scale AC input.The DC amplifier is always configured for x1 gain in AC functions (voltage, current,frequency, and period). Relay K104 connects the AC circuit to either the Input HIterminal or to R121, the current function voltage sense point. Note that the inputto the AC circuit may contain a DC bias from the applied AC signal.Input coupling capacitor C301 blocks the DC portion of the input signal. Only theAC component of the input signal is measured by the multimeter. The AC circuitvoltage ranging comprises two gain stages U301 and U305/U312. The voltagegains for each stage are tabulated below.The 1st stage is a compensated attenuator implementing a gain of x0.2 or x0.002as selected by U304A and U304D. Each voltage range has a unique 50 kHzfrequency response correction produced by a programmable variable capacitorconnected across R304.Function Range Shunt Resistor 1st Stage 2nd Stage ADC InputACV, Freq, orPeriodACI100 mV1 V10 V100 V300 V10 mA100 mA1 A5.1 W0.1 W0.1 Wx0.2x0.2x0.2x0.002x0.002x0.2x0.2x0.2x100x10x1x10x1x100x100x1002 VDC2 VDC2 VDC2 VDC1.4 VDC0.2 VDC1 VDC2VDC
