HP 740B Operating And Service Manual page 27

Section IV
output voltage. The value of the sixth digit is con-
trolled by a deck of the ten position STANDARD VER-
NIER switch. A second deck of the switch selects
a voltage proportional to the sixth digitfrom a voltage
divider. The outputfrom the second deck of the switch
isO to +0.1 Yin 0.01 V steps. The 0 to +0.1 V output
of the STANDARD VERNIER switch is applied as an
input to the Meter Loop.
4-10. The Meter Loop, like the Main Loop, is basically
a voltage feedback amplifier. In Standard Mode, the
feedback factor ({J) is always 0.1 resulting in a con-
stant closed loop gain (G) of 10.
4-11. The Meter Loop consists of a meter amplifier,
a meter, and a feedback network. The output of the
meter amplifier drives the meter. The meter drives
from 0 (mid scale) to +1. 0 in .1 steps as the STAN-
DARD VERNIER switch is rotated clockwise. The
decimal point in the meter indication is not considered
present in Standard mode, resulting in a 0 to 10 indi-
cation for the sixth digit.
4-12. DIFFERENTIAL VOLTMETER (Figure 7-1).
4-13. In the Differential Voltmeter mode of operation,
the unknown voltage is applied to the Main Loop.
Feedback voltage from the Main Loop output to the
input controls the closed loop gain. The magnitude
of the feedback voltage is proportional to the unknown
input voltage.
4-14. The Main Loop feedback voltage, or a portion
of it (depending on range), is applied in series op-
position to the Reference Loop output. The Reference
Loop generates a very stable reference voltage.
Magnitude of the reference voltage (0 to 1 V) is con-
trolled by the front panel VOLTAGE SET switches.
4 -15. The Meter Loop Monitors the difference between
the Main Loop feedback and the Reference Loop out-
put and indicates null (0) when the two voltages are
equal. The value of the input voltage is then indicated
on the digital readout tubes (first five digits) and the
meter (sixth digit).
4-16. The input voltage is applied through the Over-
load Protection circuit and Input Filter to the Main
Loop. The Overload Protection circuit protects the
instrument from voltages that exceed the selected
Model 740B
range. The input filter removes any ac signal and
noise superimposed on the dc input.
4 -1 7. The operation of the Main Loop in Differential
Voltmeter mode is similar to its operation in.Standard
Mode. The Main Loop Feedback Divider determines
the closed loop gain by controlling the amount of feed-
back returned to the input on each range. Table 4-2
shows the gain relationships.
4-18. Regardless of the range selected, the feedback
voltage to the Main Loop input is of t he same
polarity and nearly the same amplitude as the unknown
dc input. The Main Loopamplifies the difference be-
tween the two voltages. Since the feedback and input
voltages are almost equal, very little current is drawn
from the input voltage source by the Main Loop and
the input section presents a very high impedance to
the signal source.
4-19. On the 1 V range andbelow, the feedback voltage
from the Main Loop output is applied directly to the
Reference Loop. Closed loop gain of the Main Loop is
1, lO,10 2 and10 3 0nthe1V, 100mV, 10mVand1mV
ranges respectively, which result in a 0 to 1 V input
to the Reference Loop on these ranges. On the ranges
above 1 V, the feedback voltage from the Main Loop
output is divided by 10, 100 and 1000 on the 10 V,
100 V and 1000 V ranges respectively before being
applied to the Reference Loop. On all ranges, the
input to the Reference Loop is 0 to 1 V, proportional
to the unknown dc input voltage (Table 4-2).
4-20. The operation of the Reference Loop in Differen-
tial Voltmeter mode is very similar to its operation
in Standard mode. The reference supply furnishes
a stable and accurate 1 V reference voltage to the
binary coded decimal divider. Divider ratios are
controlled by the five VOLTAGE SET switches. The
STANDARD VERNIER control, however, is not used
in Differential Voltmeter mode. The resultant output
of the divider is 0 to +1 Vdc in 10 JlV steps. The
divider output is applied through the SENSITIVITY
switch and FUNCTION switch in series opposition to
the 0 to 1 Vfrom the Main Loop feedback. If the volt-
ages are not equal, current will flow through the Re-
ference Loop. This current is applied to the Meter
Loop.
Table 4-2. Main Loop Gain (AVM)
Closed Loop Gain, G
Range
{J
(! )
Main Loop Output with Input to Reference Loop with
{J
Full Scale Input Applied Full Scale Input Applied
1 mV .001
10 3
IV 1 V
10mV .01
10 2
IV 1 V
100 mV .1 10 IV 1 V
1 V 1 1 IV 1 V
10 V 1 1 10 V IV
100 V 1 1 100 V 1 V
1000
V
1 1 1000 V 1 V
4-2
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