HP 3400B, User'S And Service Manual

Page: 45 / 77

Chapter
3
Theory of
Operation
Chopper Op Amp I Thermocouple Pair
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Chopper Op Amp
I
Thermocouple Pair
The chopper op amp and the thermocouple pair form a servo loop which
functions to position the direct reading meter (Ml) to the RMS value of
the ac input signal. The wideband amplifier output is applied to the
heater of thermocouple pair (TC30l). The ac voltage causes a dc voltage
to be generated in the resistive portion ofTC30l which is proportional to
the heating effect (RMS value) of the ac input The dc voltage is applied
to the chopper op amp.
The chopper op amp circuitry consists of a chopper op amp (U303),
filter, and output amplifier (U304C). The chopper op amp is a high-gain,
low·drift op amp and is used to modulate the applied dc voltage,
amplify the voltage, and then demodulate the voltage back to a dc
voltage (the output voltage of the amplifier).
The
output of the amplifier
is the ac stabilizing and summing feedback for the thermocouple pair
(TC30l), and is also applied to the filter. The ac stabilizing feedback
(using R323, R320, and C309) is used to prevent the circuitry from
oscilla ting .
The output from the chopper op amp (summing feedback) is applied to
the heating element of thermocouple pair (rC30l). The dc voltage
developed in the resistive portion ofTC30l is effectively subtracted from
the voltage developed by TC301. The input signal to the chopper op amp
then becomes the difference in the dc outputs of the two thermocouples.
When the difference between the two thermocouples becomes zero, the
dc output from the chopper op amp
will be equal to the ac output from
the wideband amplifier.
The low pass filter, consisting ofR325, R326, C3l2, C3l3, and U304C,
is used to further reduoe the noise on the output of the chopper op amp.
This is to make the meter reading quiet.
The output of the low pass filter and output amplifier (U304C) drives
the meter (Ml) and the DC Out connector on the rear panel. The rear
terminaYs output impedance is approximately
10000.
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