Fluke 5790B, Руководство по обслуживанию

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5790B
Service Manual
2-8
CPU
CPU
INPUTS
INPUT
SWITCHING
INPUT
ATTENUATORS
AND PROTECTION
MODULE
ULTRA
PRECISE
AMPLIFIERS
FLUKE
THERMAL RMS
SENSOR CIRCUIT
TO A15 A/D
AMPLIFIER
ASSEMBLY
FROM
CHOPPER
ON A15
A/D
AMPLIFIER
TRANSFER
SWITCH
elu007.eps
Figure 2-3. A10 Transfer Assembly Block Diagram
A/D Amplifier Assembly (A15)
The A15 A/D amplifier board contains circuitry for generating the chopped dc
reference for the A10 transfer assembly and circuitry for measuring the output of
the Fluke thermal sensor circuit, also on the transfer assembly. The chopper
circuit provides a symmetrical square wave equal in RMS value to the dc input
voltage (0.7 V to 7 V). A square wave is used instead of dc for making the
transfers for these reasons:
• The square wave passes through the ac-coupled amplifiers on the wideband
board, while dc would be blocked by the coupling caps.
• Errors caused by dc offsets, which add directly to a dc reference, tend to
average out with a dual polarity input.
The chopper circuit contains a 2 V divider/dc reference selector that grounds the
chopper when not in use. The precision switched-capacitor inverter provides -
0.7 V to -7 V from a 0.7 V to 7 V input, which continuously alternates at 400 Hz.
This provides a negative reference voltage with low error. The chopper oscillator
provides a pair of square waves (50 % duty cycle, 180 degrees out of phase) that
clock the chopper switches. The chopper attenuator selects the output level of
the chopper by switching in 20 dB and 40 dB attenuators. See Figure 2-4 for a
block diagram.
.