Section 4
4-19
4.2.5. 6675A Measurement Cycle - 4-Terminal "Normal-"
See Figure 4-7 for a graphical representation of the 6675A measurement cycle.
The first thing to notice is that it is an 8 phase measurement cycle, where on every turns
switch we've added in 2 intermediate states. These 2 intermediate states guarantee that the
current through the N
x
windings of the toroid is zero (and hence the flux in the toroid) before
the turns are switched. This means that any relay chatter on a turns switch has no effect on
the "0" flux in the toroid and the current is switched back on after the relays have settled
down. The net result is that the flux detector sees two small glitches on a turns switch
instead of one rather large transient.
The 6675A uses the algorithm described in section 4.2 and the reasons for doing so was that
the algorithm described in section 4.1 and the next section (the original 6675A measurement
algorithm) was based on the original 9975 measurement principle which didn't correctly
account for the Primary Current offsets. The difference between the 2 algorithms are : for Rs
1 k, difference 0.01 ppm: for Rs = 10 k difference 0.3 ppm: and for Rs = 100 k
difference 4 ppm
To Next Page
Loading...