Fluke 2680A, Service Manual

Page: 70 / 254

268XA
Service Manual
2-22
AC Volts Measurement Circuitry.
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 2-6). 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. 0 V turns
JFETS A3Q10 to A3Q16 ON, while –5 V (V ac) turns the JFETS OFF. Only one line at a
time is set at 0 V.
Table 2-6. 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 0.75 V rms 0.75 V 300 mV 300 mV 10 3 V BR3
3 V 0.25 0.75 V rms 0.75 V 300 mV 300 mV 10 3 V BR3
30 V 0.025 0.75 V rms 0.75 V 300 mV 300 mV 10 3 V BR3
150/300 V 0.0025 0.75 V rms 0.75 V 300 mV 300 mV 10 3 V BR3
The ac voltage input signal is routed through and scaled by the buffer to obtain a full
scale buffer output of 0.75 V rms at A3U29-6. A3R120 and A3C76 provide 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 (0.75 V dc) 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 2-8
shows a simplified signal path for the 3 V ac range.