Theory of Operation
6-5
sistors. This input causes a current, V/R
E
, to flow through
R309 and R323. This current develops an output voltage:
-V(R
L
/R
E
). R327 and R324 limit power dissipation at high
current outputs for Q306 and Q307 respectively. CR306 and
CR307 provide protection for the ±24V supplies in case a
high external voltage is applied to the PREAMP OUT termi-
nal. R305 and R308 provide protection for the output stage
in this same situation. Note that the output stage provides an
inversion in amps and coulombs, while in volts and ohms it
does not.
Figure 6-7
Output stage configuration in amps and coulombs
6.3.4 Ohms voltage source
In the ohms function, a bootstrapped voltage source is con-
nected in series with a range resistor to force a constant cur-
rent through the resistance being measured. A simplified
schematic diagram of this source is shown in Figure 6- 8.
The source itself is made up of U304 and associated compo-
nents. VR301 provides the voltage reference, while VR304 is
a clamping diode to protect the circuit in case of line over
voltage. R347 limits power dissipation in Q302. The current
through the reference is produced by the 10V output through
R315. R316, R313, and R317 divide the output to provide
feedback for U304. R313 and VR301 are selected at the fac-
R325
Q307
R324
R308
OUTPUT
24V
CR306
R327
CR316
GAIN
STAGE
COMMON
INPUT
FROM
GAIN
STAGE
R323
CR307
24V
RL
R305
RE2
RE1
R309
RE1=RE2=RE
CR315
Q306
tory to provide an accurate 10V output. The actual source
output is 10V, but this value is divided to 1V and 0.1V by a
voltage divider made up of elements of R304. Q302 buffers
the output of U304 since R304 will draw 10mA. Only a sin-
gle voltage source value is used for a given range, with the
value being selected by the appropriate relay contacts. For
the 2M
Ω
range, however, all relay contacts are open, and a
1µA current is sourced through the precision 900k
Ω
resistor
R303 and feedback element R322.
Figure 6-8
Ohms voltage source simplified schematic
6.3.5 Zero check
The zero check mode provides a means for determining and
cancelling offsets internal to the instrument. The exact con-
figuration of the zero check circuitry will depend on the se-
lected function.
A simplified schematic of the zero check circuitry for volts and
ohms functions is shown in Figure 6-9. When zero check is en-
abled, K307 is energized, providing a path to signal common
through the normally closed contacts of K301. A 10M
Ω
resis-
tor, R334, is placed across the electrometer input when zero
check is enabled, while the preamp input is shorted to input low.
In amps and coulombs, contacts on K301 connect the invert-
ing input and the output of the op amp together. This config-
uration gives the circuit unity gain, allowing any input offset
voltage to appear at the output. Note that when zero check is
enabled, the input impedance is the combined impedance of
the feedback element in parallel with R334, which has a val-
ue of 10M
Ω
(see Figure 6-10).
R347
FROM
PREAMP
OUTPUT
+24V
U304
R317
R313
R316
R304A
R304B
R304C
0V
R303
900k
Ω
0.1V
VR304
VR301
Q302
R315
10V
1V
Summary of Contents for 6512
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