Channel Selection Circuitry; Dc Volts And Thermocouples Measurement Circuitry - Fluke 2640A Service Manual

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Channel Selection Circuitry

Channel selection is done using reed relays on the 2640A and by optically coupled
solid-state relay on the 2645A. Channel selection is done by a set of 24 relays organized
in a tree structure. Relays A3K1 through K20 select the specific channel 1-20. The
selection of relays A3K21 through K24 (Treeing Relays) depends on which bank of 10
channels is being used (both banks are selected for four-wire ohms) and the channel
function and range being used.

DC Volts and Thermocouples Measurement Circuitry

For 3 volts and lower ranges, the input to Stallion (A3U30) are as follows for signal HI
and signal LO inputs:
•
•
For the 30 and 300 volt range, the input to Stallion (A3U30) are as follows for the HI
and LO signal inputs:
•
•
The outputs from Stallion (A3U30) are as follows:
•
•
The ranges for the buffer amplifier are shown in Table 2-7 and measurement matrix in
Table 2-8. Figure 2-8 shows a simplified signal path for the 300V dc range.
2-34
HI is a direct input via the HI SENSE line A3R11, A3K26, A3R130, and pin 50
(HI1) input of A3U30.
LO is an input to LO SENSE via A3R132 to pin 80 (LO2) of A3U30.
The HI signal is scaled by A3Z7. The input is applied to pin 1 of A3Z7 and a 101:1
divider is formed by the 10 MΩ 100 kΩ resistors when switches S3 and S13 are
closed. The attenuated HI input is then sent via S24, S64, and S44 to the Buffer
Amplifier and then to A/D Converter.
The LO signal is sensed through A3L52, A3R146, A3K27, A3R119, and S33 and
S37.
HI (pin 20) is to Buffer Amplifier circuitry (A3U27 and A3U28).
LO (pin 100) is to Buffer Amplifier circuitry (A3U27 and A3U28).
Table 2-7. Range of Buffer Amplifier
Range
90mV Range
300mV Range
750mV Range
3V Range
30V Range
300V Range
Buffer Range Control Signals (Gain)
BR1 (x32.168 gain)
BR3 (x10 gain)
BR2 (x4.021 gain)
BR4 (x1 gain)
BR3 (x10 gain)
BR4 (x1 gain)
2-60.
2-61.