HP 3457A Service Manual page 193

. Input Coupling
. Attenuators
. Amplifier Circuitry
RMS Converter
. DC Offset Correction
Frequency Comparator
. Auto Calibration
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Figure 8-3. AC Assembly Block Diagram
Refer to schematic A3 for the following discussion.
8-56. Input Coupling
The AC input signal is routed to the AC to DC Converter assembly (A3) from the Input Signal
Conditioning assembly (A2). AC or AC+DC input coupling is selected by relay A3K301. When the AC
function is selected, the input signal is connected to range select relay A3K302 through capacitor
A3C301. Capacitor A3C301 blocks the DC component of the input signal. When the AC+DC function
is selected, the input signal is connected to range select relay A3K302 through relay A3K301 pins 3 - 5
thus bypassing C301. In this configuration, both the AC and DC components of the signal are coupled
to the AC Converter input.
8-57. Attenuators
Two input attenuators are used to condition the input signal. Relay A3K302 switches the input signal
to the selected attenuator and also grounds the input of the unused attenuator to maintain the proper
impedance to the AC feedback circuit. The attenuator composed of resistors A3R302, R303, R304 and
capacitors A3C302 and C303 is selected for the 30mV and 300mV ranges. The output signal from the
30mV/300mV divider is 90% of the input signal. The attenuator composed of resistors A3R305, R307,
R308 and capacitors A3C305, C306, C307, C332 is selected for the 3V. 30V and 300V ranges. This at-
tenuator reduces the input signal to 0.9% of its original value on the 3V and 30V ranges and to .09% on
the 300V range.
HP 3457A Multimeter
8-28