Offset Compensated Ohms; Input Switching For Ohms Measurements; Autozero - 4 Wire Ohm; Level Shift For Low Ohms Offset Circuit - HP 3456A Operating And Service Manual

Model 3456A

8-129. Offset Compensated Ohms.

sated Ohms makes two measurements on the circuit
connected to the input terminals. The first measurement
is a convential ohms measurement. The second is the
same except the current source is turned off. The second
reading is subtracted from the first prior to the
resistance calculation. This compensates for small offset
voltages present on the external circuit. When the cur­
rent source is turned off, Q207 (diode connected FET)
provides a current-path between the output and the
negative input of U205 so that the positive and negative
input terminals can remain at the same potential.

8-130. Input Switching for Ohms Measurements.

simplified schematic shows the components in the Input
Switching that complete the paths
Amplifier for both the 2-wire and 4-wire ohms
measurements (4-wire shown with dashed line). Relay
KJ03 and FET switch Q I 1 6 are clo�ed during 2-wire
ohms measurements. Two-wire ohms is exactly the same
as a DCV measurement except that current is supplied
through the VOLTS HI and LO terminals (K101
closed).
8- 1 3 1 . Relay KJOJ and FET switch Q I 16 are open dur­
ing 4-wire ohms measurements. At this time the VOLTS
HI and the VOLTS lO terminals still supply the ohms
current but floating terminals "4 WRn SENSE HI" and
"4 WRO SENSE LO" are used to measure the dc
voltage drop. The voltage drop across the unknown
resistance and the lead resistance (R lead ) is measured
via the "4 WRn SENSE HI" terminal (through FET
switches Q I 1 5 and QJOJ). the voltage across R lead may
cause an incorrect reading-4 wire n Autozero corrects
for this measurement error.
8-132. Autozero-4 WIRE OHM.
on during a 4-wire ohms measurement, two measure­
ment are made. The first measurement is made with
QJ05 closed (Input Amplifier shorted to "4 WRn
SENSE LO" terminal) and QIOJ open. This is the
4 WRn Autozero measurement. A second measurement
is made with QI05 open and QJ03 closed (measurement
of 4 WRO SENSE HI terminal). The first measuremem
is then subtracted from the second by the Inguard Logic
prior to sending the measurement data to the Outguard.
FET QI04 is open and FET Q I 1 5 is closed at all times
for both the normal and Autozero 4-wire ohms
measurements. Lead resistance (R lead ), in addition to
the dc offset of the Input Amplifier, is subtracted out
during 4-wire ohms Autozero measurements.
8-133. Low Ohms Offset.
Amplifier Schematic, for the following circuit descrip­
tion. The Low Ohms Offset circuit alters the Input
Amplifier operation to allow accurate 4-wire ohms
measurements when using low ranges (10 K ohms range
and below). The circuit level shifts the dc reference level
at the output of the Input Amplifier.
Offset Compen­
The
10
the Input
When Autozero is
Refer to schematic 3 , Input
8-134. Figure 8-29 and 8-30 illustrate how this circuit
operates. The negative voltage swing at the output of
the Input Amplifier is normaUy between 0 V and
(A). The low ohms offset circuit level shifts the dc
reference at the amplifier output from 0 V to
(C).
(8) which increases the amplifier voltage swing range
from 12 V (A) to 13.J V
bottom of. part of the U200 resistor divider from
ground to - 12 V a .
Y
, 1 2
+ 1 . 3 Y
c
A
- 12
Figure 8·29. Level Shift for low Ohms Offset Circuit.
8-135. Figure 8-30 illustrates the voltage drops that oc­
cur across R x ( . 1 1 5 V) and R lead
{}
range measurement. The 4-wire ohms measurement is
taken between the 4 WR
the VOLTS LO terminal (ground). In this case, the
voltage read is the voltage across R x ( . 1 1 5 V) and
R lead I (.01 V). For the lOO ohms range, the input
amplifier gain is lOO. This would result in an amplifier
output voltage of 12.5 V if there is no Low Ohms Offset
circuit (see Figure 8-3IA). This would be an AID Con­
verter overload condition. Figure 8-3 1 8 illustrates how
this level shift increases the dynamic range to allow for
lead resistance. Note that the measurement is within the
AID Converter limits. The R lead I error (.01 V
after XIOO gain) plus the Input Amplifier error is sub­
tracted out during the 4-wire ohm Autozero.
8-136. The input to the positive terminal (pin 3) of com­
parator U30la is high (HOHM) for all of the ohms
ranges. The open collector transistor in the output stage
of U30la is off for this condition. The inputs of the
U306 inverters are pulled to ground by R323 at this
time. The resulting output of the inverters ( - 12 V) is
applied to pin 12 of U200. When the ohms function is
not selected, HOHM is low and the output of U30la is
- 18 V. The resistor network (R322 and R323) at the in­
put of U306 divides this - 18 V down to - 1 2 V which is
inverted to zero volts by UJ07.
Service
- 12 V
+
1 . 3 V
This is done by moving the
A: No,".al AmpliHe, Vol'age $w.ng
B: l.".1 ShOt,
C: A •• u�.n' Amplif;e, Vol'age Sw;ng
I
(.01 V) for a l OO ohm
HI terminal (top of R x ) and
1 . 0 V
-
8-25