5 Theory of Operation
142 Keysight 34970A/34972A Service Guide
AC Circuit
Unless otherwise noted, components in this discussion are located on the A4
circuit assembly (34970-66504).
The multimeter uses a true RMS ac-to-dc converter to measure AC voltages and
currents. 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 the
multimeter’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 HI
terminal or to R121, the current function voltage sense point. Note that the input
to 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 the
AC component of the input signal is measured by the multimeter. The AC circuit
voltage ranging comprises two gain stages U301 and U305/U312. The voltage
gains for each stage are tabulated below.
The 1st stage is a compensated attenuator implementing a gain of x0.2 or x0.002
as selected by U304A and U304D. Each voltage range has a unique 50 kHz
frequency response correction produced by a programmable variable capacitor
connected across R304.
Function Range Shunt Resistor 1st Stage 2nd Stage ADC Input
ACV, Freq, or
Period
ACI
100 mV
1 V
10 V
100 V
300 V
10 mA
100 mA
1 A
5.1 W
0.1 W
0.1 W
x0.2
x0.2
x0.2
x0.002
x0.002
x0.2
x0.2
x0.2
x100
x10
x1
x10
x1
x100
x100
x100
2 VDC
2 VDC
2 VDC
2 VDC
1.4 VDC
0.2 VDC
1 VDC
2VDC
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