Agilent Technologies 34970A, Service Manual, Page: 136 / 273

AC Circuit
Unless otherwise noted, components in this discussion are located
on the A4 circuit assembly (34970-66504). The schematics are
included in Chapter 8 starting on page 234.
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.
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
Ω
0.1
Ω
0.1
Ω
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
2 Vdc
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.
Chapter 5 Theory of Operation
Internal DMM
134