Section 4
4-9
To understand how the 6675A makes a resistance measurement it is best to go back and look at the
Model 9975 (manually operated version of the bridge). See Figure 4-3 for a block diagram of the
9975.
The measurement principle is based on a DC transformer and flux detector. Electronic circuitry
maintains the flux balance and the operator adjusts the voltage balance by changing the turn dials
while watching the Galvanometer balance dial. The photocell galvanometer has a coil resistance of
20 . This isn't a problem until we get to the higher Rs, R
x
values when the 60/50 Hz "line
frequency" (induced pickup) current is a significant portion of the DC signal current and the
galvanometer becomes an AC short.
The core of the 9975 is a 27-bit current digital to analog converter (dac) where the upper 13 ½ bits
consists of 0 to 11,111 real turns and the lower 13 ½ bits consists of a (4 10 turn ) current splitting
network based on a 53 manganin wire-wound resistor and hence stability and accuracy of the
lower 13 ½ bits depends on the stability and accuracy of a resistor network.
To assist in the nulling of the flux in the core the upper 13 ½ bits of the flux on the "B" side (N
x
turns, I
x
current) is fed over to the "A" side by means of a multiplying digital to analog converter
(mdac) so that the servo circuitry on the A" side only has to null out the lower 10-13 bits of the total
flux. There is a resistor network on the real turns dials whose value is changed by 1 /turn and that
is the dac portion of the mdac.
(Note that there is more than 1 link electrically between the "A" and "B" sides.)
4.2.1. Theory II 6675A
See Figure 4-4 for a block diagram of the 6675A. The measurement principle is again, based
on a DC transformer and flux detector and also includes a linear voltage detector and an
interpolation algorithim described in Figure 4-9.
In the 6675A, electronic circuitry maintains the flux balance in the toroid and the 6675A
controller measures the voltage imbalance while changing turns and current directions. The
"I
x
-side", "I
s
-side" and nanovoltmeter are all isolated from each other and the 6675A
controller by fibre optic communications for maximum isolation.
The core of the 6675A is a 13 bit current dac based on 0to 8747 real turns which is done in a
base 3 switching arrangement to minimize the number of relays used.
The voltage imbalance between the "X" and "S" sides is measured by an autoranging, 5
range, uncalibrated nanovoltmeter. It has a resolution of 22 bits and more than 14 bits
linearity. On the mV ranges of the nanovolt detector where we are (generally) measuring 3
k on its input, the nanovolt detector bias current is 50 pA and on its "volts ranges", for Rs
>3 k
, its input bias current <0.1 pA. When these bias currents are compared to the currents
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