Scion Instruments 436-GC Service Manual page 95

436-GC/456-GC
Bridge balance DAC and signal amplifier
Bridge signals down to one microvolt must be detected by the instrument, so it is necessary to have the
bridge balanced to within 1mV or less for efficient digitization of the signal. Balance is adjusted by DAC U4
and the associated circuitry. Four sections of R21, a precision-tracking network, provide an effective shunt
resistance of 1K across the bridge element connected between J 1-3 and J1-4. The reference voltage to
the DAC is just exactly the voltage across this resistance. The DAC inverts this voltage, and multiplies it by
the ratio of the DAC setting to 4096. The DAC output voltage is applied to a 500Q resistance, consisting of
two paralleled sections of R21, which sources current into the bridge at J1-3 (which is also Ground 3).
Since this current can be as high as 28mA, Q3 is used to boost the current output capability of amplifier
AR5 (pin 1). When the DAC is set to mid-scale, the variable current through the 5000 resistance exactly
cancels the current through the fixed 1K shunt, and there is no effect on the bridge balance. Setting the
DAC either above or below 2048 allows the bridge to be offset in either direction.
For the DAC to work properly, pin 2 (AGND) must be held at the same voltage as pin 1 (IOUT), which is at
Ground 3. However, the current from this pin must be returned directly to the floating supply, so the current
does not have to flow back through the bridge. AR5 (pins 5-7) buffers Ground 3 for this purpose. C17 and
C21 reduce the effect of digital noise on the balance circuit.
The bridge signal at J1-2 is amplified by AR6. Gain is set to 2, 20, or 200, by closing one of the switches of
U7. Only one switch should be closed at a time. C23 sets the time constant of the amplifier to 50ms for
noise reduction. The capacitors connected to the inputs of AR6 all reduce digital noise interference, and
R30 and C27 also attenuate noise from the switching power converter noise. R28 and C31 reduce the
disturbance caused by switching the loads connected to the output of AR6.
Signals from the bridge balance circuit appear at the signal amplifier only if a bridge is connected. In order
to be able to test the circuit board without a detector, a high-resistance bridge (R17) is always present on
the board, in parallel with the normal detector connection. All of the board functions are operational in this
state, but the balance control is about 40 times more sensitive, due to the greater bridge resistance. As
noted in the preceding section, the temperature limit function must be defeated for the circuit to operate
without an external bridge.
SCION Instruments
436-GC/456-GC Service Manual Revision B February 2019
610 Hardware description
Page: 94