HP 740B Operating And Service Manual page 21

Section III
VOLTAGE SOURCE CIRCUIT
>-----0
CHASSIS GROUNO
>-----0
, ___
§~~RP~~~~Q
___ ,
I
CIRCUIT GROUND I
+
~
f----o
+
-
[
7408
liVM
+-INPUT LEAD -
j
-=
t
L _- ,
-INPUT LEAD
[-
... _-- -
r}z'
-¢='
I
rv
~
1_-1-50010
~
Figure 3-8. Measurement Error Caused by AC Ground Currents
Model 740B
a. Connect + and - INPUT terminals to circuit
under test. Do not turn on circuit or apply
drive voltage.
b. Set 740B RANGE to I mY; depress X10 3 SEN-
SITIVITY.
c. Adjust ZERO control for a zero meter indi-
cation.
- - - - - NOTE - - - - -
If the resistance connected between
the + and - INPUT terminals is
greater than 10kn, this adjustment
may be difficult due to stray volt-
age pickup caused by electrostatic
charges, low humidity, etc.
VOL TAGE SOURCE
CIRCUIT
3-49. GUARDING.
3-50. The Model 740B is a floating instrument; how-
ever, a finite impedance exists from the - INPUT
terminal (circuit ground) to earth ground (
-=1-).
This
im pedance is represented by Z I and Z2 in Figure 3 -8.
3-51. Induced ac ground currents can generate a po-
tential between the voltage source ground and the
740B ground. This current will cause an ac voltage
to appear at the - INPUT terminal which may cause
a measurement error, especially when making low
level measurements. Also, the - INPUT terminal
may be driven at a potential above ground in the
voltage source circuit (Figure 3-9). The impedance
from - INPUT to chassis ground (Z I and Z2) parallels
part of the voltage source circuit (R4) and causes a
larger current to flow through RI, R2 and R3, in-
creasing the apparent voltage across R3. (Arrows
3-10
r
.,
~ov
.,
CHASSIS GROUND
""0
~v ,---§~~Q~~~~~---,
I I
I
CIRCUIT GROUND
1
~
.,
I I
"'0
I I
+
+20' +IHPUT LEAO
I I
~
+
I
I I
-
I I
.3
I
7408
I .,.0
1 liVM
I
II
+IOV
1 I
-INPUT LEAD 1 1
.~ I=>
-==>
-
1
I I
I 1
..
I
-=
---1
l:' _ --
j
""0
1
----S;
1
1
I
~
- ---
L Jl
r}z'
-¢:
I
-- -
...
Figure 3-9. Measurement Error Caused by DC Ground Currents
01794-1