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R327
,
Output
staze configuration
for the am!x and coulombs
func-
.
-
tions is shown in Figure 6-7. 4306
and Q307 are the active
devices
for
this
configuration.
Since
the
power
supply
voltages
are much lower (&24V),
only a single pair of tran-
sistors is required.
As with the high voltage
configuration,
signal input is applied
through
the two diodes to the bases of
the transistors.
This input
causes
a current,
V/RE to flow
through
R309 and R323. This current
develops
an output
voltage
-V(RL/RE).
R327 and R324 limit power dissipation
at high current
outp&s
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
terminal.
R305 and R308 provide
protection
for the output
stage in this same situation.
Note that the output
stage pro-
vides an inversion
in amps and coulombs,
while in volts and
ohms it does not.
w
Q306
CR306 21
T
1:
24"
R305
r]PUT
-v/k
3OM
i*IN
TAGE
RE,
wJ9
RE,=Ry=RE
RL
SAIN
>F--Qvv
:'
)
TAGE
OUTPUT
3MMON
k2
R323
xZCR316
R308
1
"vu
-
L
24v
+
Q307
CR307
zy
vvb
R324
6.3.4 Ohms Voltage Source
In the constant-current
ohms mode,
a bootstrapped
voltage
source is connected
in series with a range resistor
to force a
constant
current
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
com-
ponents.
W.301 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
1OV output
through
R315. R316, R313 and R317 divide
the output
to
Figure 8-7. Output Stage Configuration (Amps and
provide
feedback
for U304. R313 and VR301 are selected at
Coulombs)
the factory
to provide
an accurate
1OV output.
The actual
c ov
J
i
i
J
FROM
<
PREAMP
OUTPUT
Figure 6-8. Ohms Voltage Source Simplified Schematic
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