Fluke 2680A, Service Manual

Page: 83 / 254

Theory of Operation
A/D Theory of Operations
2
2-35
Table 2-9. Channel Bits
Channels Enabled CH3 CH2 CH1 CH0
1, 11 0 0 0 0
2, 12 0 0 0 1
3, 13 0 0 1 0
4, 14 0 0 1 1
5, 15 0 1 0 0
6, 16 0 1 0 1
7, 17 0 1 1 0
8, 18 0 1 1 1
9, 19 1 0 0 0
10, 20 1 0 0 1
The time required for the channel switches to settle is given in Table 2-10. Note that for
both the FAI and the PAI, the switches are guaranteed to have a select time that is longer
than their deselect time. This means that you can select a new channel at the same time as
you deselect the previous channel, without worrying about shorting together the two
channels.
Table 2-10. Tree and Channel Switch Settling Times
Description PAI FAI
Select 1 ms 150
µ
s
Deselect 1 ms 120
µ
s
Function Relays
There are three relays (K25, K26, and K27) that route the signal to different portions of
signal-conditioning circuitry on the A/D board. These are relatively slow relays, requiring
6 ms to change position. Each relay has a SET and RESET position, which are
configured by pulsing the SET and RESET coils, respectively. Each change of state of
the function relays requires two writes by the A3U5 A/D microprocessor: one to set the
appropriate bits and energize the relays and another to reset all the bits and de-energize
the coils once the relays have switched (after 6 ms).
Table 2-11gives the required relay states and bit patterns for the various measurement
functions. Note that after the indicated bit pattern is written and 6 ms have elapsed, a
pattern of 000000 should be written. Also note that the two bits associated with any given
relay, corresponding to SET and RST, are never set to 1 at the same time. Table 2-12
gives the required time for the relays to settle for a given function.