Fluke NetDAQ 2640A, Руководство по обслуживанию

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Theory of Operation
Detailed Circuit Description
2
2-37
AC Volts Measurement Circuitry 2-63.
AC-coupled voltage inputs are scaled by an ac buffer (A3U29), converted to dc by a true
rms ac-to-dc converter (A3U26), filtered by an active ac volt filter, then sent to the
Stallion IC, the Buffer Amplifier, and the A/D Conversion Circuitry (see Table 2-10).
The HI input is switched to the ac buffer through dc blocking capacitor A3C80. The LO
input is sensed through A3L52, A3R146, A3K27, A3R119, and S33 and S37. The gain
or attenuation of the ac buffer is selected by A3U30’s ACR1-ACR4 outputs. 0V turns
JFETS A3Q10 to A3Q16 ON, while -5V (VAC) turns the JFETS OFF. Only one line at a
time is set at 0V.
The ac voltage input signal is routed through and scaled by the buffer to obtain a full
scale buffer output of 0.75V RMS at A3U29-6. A3R120 and A3C76 provides high
frequency compensation on the 300 mV range. The output of the buffer is ac coupled to
the input of the ac-to-dc rms converter. The output of the rms converter (0.75VDC) is
divided by 2.5 by A3Z2 and sent to the acv filter. The filtered output is sent to pin 31
(ACFO) of the Stallion chip via S41. Full scale input to Stallion is 300 mV dc.
Figure 2-10 shows a simplified signal path for the 3V ac range.
Table 2-10. Measurement Matrix for AC Volts
AC Volt
Range
Gain of
AC Volts
Buffer
Amplifier
Full-Scale
Output of
AC Volts
Buffer
Amplifier
Full-Scale
Output of
RMS
Converter
Full-Scale
Input to
Stallion
(dc volts)
Full-Scale
Output of
Stallion
(dc volts)
Gain of DC
Buffer
Amplifier
Full-Scale
DC Volts
Input to
Multislope
A/D
Buffer
Range
Control
Signal
300 mV 2.5 0.75V rms 0.75V 300 mV 300 mV 10 3V BR3
3V 0.25 0.75V rms 0.75V 300 mV 300 mV 10 3V BR3
30V 0.025 0.75V rms 0.75V 300 mV 300 mV 10 3V BR3
150/300V 0.0025 0.75V rms 0.75V 300 mV 300 mV 10 3V BR3
Frequency Measurements 2-64.
The ac input follows the same path as ac volt measurements except the output of the
buffer (A3U29) is sent to the Stallion Chip pin 35 (C+). Internal to the Stallion Chip
switch S38 sends the C+ input to a frequency comparator and counter.
Active Filter (ACV Filter) 2-65.
The active filter is used only for ac volt measurements to filter out the ac ripple and
noise present on the output of the rms converter. The filter uses an op-amp internal to the
Stallion Chip, resistors A3R102, A3R103, and A3R104, capacitors A3C57, A3C58, and
A3C59. A3Q6 turns on to discharge the capacitors between measurements.