HP 740B Operating And Service Manual page 12

Model 740B
SECTION III
OPERATING INSTRUCTIONS
Section ill
3-1. INTRODUCTION.
3 -2. This section contains instructions and informa-
tion on operating the Model 740B. Included is a dis-
cussion of the accuracy specifications, operating
precautions, identification of controls, indicators and
connectors and step-by -step operating instructions
for each instrument function.
3-3. ACCURACY SPECIFICATIONS.
3 -4. When operated as a dc standard, the Model 740B
accuracy specification is: ±(O. 002% of setting+O. 0004%
of range). The differential voltmeter accuracy speci-
fication is ± (0.005% of reading + 0.0004% of range +
1
JlV).
The instrument should operate within these
limits providing it hasbeencalibratedwithin the past
30 days and is operated at the ambient temperature
present at the time of calibration. If operated at a
different ambient temperature, the accuracy speci-
fications should be derated according to the following:
a. Standard Mode Tern perature Coefficient: Less
than (2 ppm of setting or 1 ppm of range,
whichever is greater) per °C; 10
0
C to 40
0
C.
Example: Normal allowable accuracy devia-
tion for a 1 V output on the 1 V
range is ±24
JlV
(0. 002% of setting
+ 0.0004% of range). If the am-
bient temperature differs by 5
0
C
from the ambient temperature at the
time of calibration, the allowable
accuracy deviation becomes ± 34
Jl
V
for the 1 V output (original deviation
of 24
JlV
plus the TC correction
factor of 10
JlV).
b. Differential Voltmeter Mode Temperature
Coefficient: Less than ± (2 ppm of reading +
1
Jl
V) per °C; 10
0
C to 40
0
C.
Exam pIe: Normal allowable accuracy devia -
tion for a 1 V reading on the 1 V
range is ± 55
JlV
(0. 005%of reading
+ 0.0004% of range + 1
JlV).
If the
ambient tern perature differs by 5
0
C
from the ambient temperature at
the time of calibration, the allow-
able accuracy deviation becomes
± 70
JlV
for the 1 V reading (original
devia tion of 55 Jl V plus the TC cor-
rection factor of 15
Jl
V).
3 -5. Figure 3 -1 shows how the output voltage accuracy
in Standard Mode varies with percentage of range.
01794-1
Notice that the output accuracy at 100% of range is
±0.0024% (24 ppm). However, as the magnitude of the
output decreases with respect to full scale, the ac-
curacy decreases. At 10% of range, the accuracy is
±0.006% (60 ppm) of setting. At 1% of range, the ac-
curacy decreases to ±0.042% (420 ppm) of setting.
Whenever optimum accuracy is desired, the Model
740B shouldbe operated as near full scale as possible.
3-6. Figure 3-2 shows how the voltage reading ac-
curacy in Differential Voltmeter Mode varies with
percentage of range. Again, greatest accuracy is
achieved at 100% of range. Separate curves show the
accuracy characteristics on the millivolt ranges. The
accuracy decreases on each descending millivolt range
due to the effects of signal noise on low level dc inputs.
Inputs of 1 m V and below are measured with nearly the
same accuracy on the 1 mV and 10 mV ranges. A 1
mV input, for example, would be measured with an
accuracy of ±0.105%of reading on the 1 mV range and
±0.11% of reading on the 10 mV range. However,
measurement resolution (the capability of detecting
small changes in the input) wouldbe ten times greater
on the 1 mV range.
3-7.
OPERATING PRECAUTIONS.
3 -8. The following precautions should be observed
whenever using the Model 740B.
a. BEFORE APPLYING POWER TO THE MODEL
740B, VERIFY THAT THE REAR PANEL
LINE VOLTAGE SWITCH INDICATES THE
LINE VOLTAGE TO BE USED.
b. DO NOT FLOAT -INPUT OR -OUTPUT TER-
MINAL MORE THAN 500 VDC ABOVE OR
BELOW CHASSIS (POWERLINE) GROUND.
c. DISCHARGE INPUT BY MOMENTARILY
SETTING INPUT, Z SWITCH (ON INPUT BOX)
TO 2 MEG POSITION FOLLOWING ALL HIGH
VOLTAGE MEASUREMENTS.
-----NOTE - - - - - -
Be sure to return switch to
00
posi-
tion after discharging input to avoid
introducing a loading error in the
following measurements.
3-9.
CONTROLS, INDICATORS AND
CONNECTORS.
3 -10. Each operating control, indicator and connector
located on the Model 740B is identified in Figure 3-3.
The description of each cpmponent is keyed to the il-
lustrations which are included within the figure.
3-1