switch is gated in synchronization with the chopper since the gating pulse
comes from the voltage that is used to drive the chopper. During the other
portion of the chopper cycle, the output of the amplifier is filtered by R-C
filter R225-C219 to provide a dc current for the meter. A portion of the
current that flows through the meter is shunted back to the 200 ohm feedback
resistor R236, thus completing the feedback loop. The null detector has a
basic sensitivity of 1 millivolt except in the most sensitive null mode for
the 1- and 100-volt range where the sensitivity is 100 microvolts. For the
two most sensitive null modes in the 100- and 1000-volt range, the
sensitivity is boosted 10% to compensate for the loading effect of the TVM
attenuator on the dc input attenuator.
The output resistance of the dc
input attenuator is 100K and the input resistance of the TVM dc attenuator
is 1 megaohm. Thus, when monitoring voltage excursions, much more accurate
off null readings are obtained due to the 10% boost in null sensitivity.
d. In the dc TVM mode, two positions on the TVM attenuator selected by
range switch section S2H (fig 5-15 (fo)) provide the necessary reduction of
the 1- and 10-volt ranges for proper null detector input.
The same two
positions on the TVM attenuator are used for the 100- and 1000-volt ranges
because the input attenuator divides the input signal by 100 and thus
reduces the 100- and 1000-volt ranges to the equivalent of the 1- and
10-volt ranges. In the differential mode, the voltage difference (unknown
voltage, or unknown voltage divided by 100, minus reference voltage) is
reduced as necessary by positions on the TVM attenuator selected by null
switch sections S3H, S3G, and S3E to provide the basic null detector inputs
of 1 millivolt or 100 microvolts.
e. In the AC TVM mode, null switch section S3H and ac-dc polarity
switch section S4E (fig 5-15) provide connection to only one position on the
TVM attenuator regardless of where the range switch is set. Also, in the ac
differential mode, the voltage difference (converter output voltage minus
reference voltage) is reduced by the same positions on the TVM attenuator as
for 1 volt dc differential measurements. This is because the output of the
ac-dc converter is 1 volt dc for full input on each range.
f. For the TVM, low sensitivity, and medium low sensitivity modes, the
input resistance of the TVM attenuator is 10 megohms (R4 through R7). For
the medium high and high sensitivity modes, the input resistance of the TVM
attenuator is 1 megohm (R4 and R7).
However, this is not the input
resistance of the 887A for the dc TVM and dc differential mode.
For the
1- and 10-volt range, the input resistance is determined by dividing the
unknown voltage by the current drawn from the unknown.
The current drawn
from the unknown is equal to the difference between the unknown terminal
voltage and the internally known voltage divided by the resistance of the
TVM attenuator. The equation for input resistance can therefore be written
as:
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