The switches connected to the arms of the potentiometers in this voltage divider are other
sections of the range relays that close simultaneously with those in the feedback circuit of U9.
The output of U11 is scaled down through the divider to 0.6 volts for the 600 volt range, 0.3
volts for the 30 and 300 volt range and 1.5 volts for the 150 volt range, with the potentiometers
RV15 through RV17 providing the line calibration adjustments.
Figure 7-2 | Simplified Voltage Scaler/Amplifier
7.3.3 Current Amplifiers and Scaling
Schematic 2105-070 sheet 2 & 4
The source of the signal for the current meter is a 0.01 ohm shunt resistor, R200, through which
the load current flows. The resistance of R200 is very low so there is minimal effect on the
voltage applied to the load.
The signal developed across R200 must be amplified, especially in the lowest current range. The
current signal amplifiers are shown in the schematic 2105-070 sheet 2. A simplified diagram of
the input current amplifier is shown in Figure 7-3.
The minimal full-scale voltage developed across R200 is only 2mV. Therefore, the input
amplifier must remain stable at DC and from 40 Hz through the bandwidth of the instrument.
Operational amplifier U7-1 and chopper-stabilized amplifier U6 operate in concert to achieve
the required stability. Any offset appearing at the output of U7-1 is reduced by a factor of 25 at
the arm of RV2 where it is applied to the non-inverting input of U6. A differential exists
between the two inputs of U6. An offset is produced at the output of U6 that places a charge on
C13 with a polarity that drives the output of U7-1 toward zero. The gain of U6 is 200 and its
output is integrated by R11 and C13 which eliminates any tendency toward oscillation. The
system finds a point of equilibrium where the offset at U7-1 has been reduced to a negligible
level.
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