Ac Circuit - Agilent Technologies 34970A Service Manual

Chapter 5 Theory of Operation
Internal DMM

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
ACV, Freq, 100 mV
or Period 1 V
10 V
100 V
300 V
ACI 10 mA
100 mA
1 A
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.
134
Shunt
1st Stage
Resistor
x0.2
x0.2
x0.2
x0.002
x0.002
5.1 x0.2
Ω
0.1 x0.2
Ω
x0.2
0.1
Ω
2nd Stage ADC Input
x100 2 Vdc
x10 2 Vdc
x1 2 Vdc
x10 2 Vdc
x1 1.4 Vdc
x100 0.2 Vdc
x100 1 Vdc
x100 2 Vdc