13. In the 1 microvolt to 1 millivolt ranges, a 10-megohm resistor is
connected directly across the input of the instrument. The input resistance
may be increased on these ranges by disconnecting the 10-megohm resistor
where it attaches to the RANGE switch. However, the input resistance will
no longer be well defined.
Typical input resistances with the 10-megohm
resistor removed are as follows:
RANGE
INPUT RESISTANCE
V
1
300
megohms
V
3
1,000
megohms
V
10
3,000
megohms
30 V to 1
mV
10,000
megohms
Learning Event 3:
DESCRIBE THEORY OF OPERATION OF THE 845AR NULL DETECTOR
1.
As shown in Figure 4-6, the Model 845AR is a photochopper stabilized
amplifier with the overall gain of the amplifier being precisely controlled
The instrument's main circuits are an input range
divider, a photocell modulator, an ac amplifier, a synchronous demodulator,
a dc amplifier, a meter, an isolation converter, a neon drive, an 84 Hz
multivibrator, a supply rectifier, and a rectifier filter.
a. The input range divider provides a fixed input impedance to signals
of less than 1 millivolt while allowing reduction of input signals about 1
millivolt. Photochoppers modulate the input signal to the ac amplifier at
84 Hz.
The drive signal for the photo modulator is provided by the neon
drive which is composed of neon lamps driven alternately at 84 Hz by the 84
Hz multivibrator. Eighty-four Hz is used to provide the Model 845AR with an
operating frequency asynchronous with the power line frequency and its
harmonics. The 84 Hz multivibrator also drives the following circuits:
(1) The supply rectifiers which provide operating voltages for the
(2) The synchronous demodulator which demodulates the amplified ac
signal.
(3) The isolation converter which produces the meter and isolated
recorder output.
The entire amplifier and the secondaries of both
transformers are surrounded by a guard shield which permits the use of
external guard voltages.
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