HP 3455A Operating And Service Manual page 101

Section VIII THEGRY OF OPERATION
Model 3455A
3455-84528
Figure 8-17. Ohms Reference Measurement.
Note 1: VOLTAGE LIMITED TOS V.
8-16 illustrates the circuit configuration for these measure-
ments. The DC Preamp input is grounded through a 100 kQ
resistor by FET switch A10Q2. The feedback path for the
X2 gain is through FET switches A10Q22 and A10Q21.
Feedback for X20 gain is through A10Q22, isolation amp-
lifier A10U3, switch A10Q27, the precision 10-to-1 divider
and switch Al0Q18. A separate measurement is made for
both gains and the results are stored by the main controller.
8-40. .1 kQ, 1 kQ, 10 kQ Reference Measurements. The
ohms reference voltage is developed across the reference
resistance. On the .1 kQ through 10 kQ ranges the refer-
ence resistance is 1 kQ (see Figure 8-17). The .7 mA
current source is connected to the 1 k2 reference through
telay A1OK7. The 999 kQQ reference resistor is shorted
by the combination of relay Al0K7 and A10K8. The refer-
ence voltage is applied to the DC Preamp input through
+10V +i0V
FET switch A10Q13. The reference is measured prior to
each measurement of the unknown resistance.
8-41. 100 kQ2, 1 MQ, 10 MQ Reference Measurements. On
the 100 kQ range, A10K7 is opened and the .7 mA current
source is applied to the combination of R1 and R5 (1 MQ).
The reference voltage developed across RI and RS is ap-
plied to the DC Preamp input through FET switch
A10Q14. On the 1 MQ and 10MQ ranges, relay A10K8 is
opened and the .7 uA current source is applied to the 1 MQ
reference resistance. The reference voltage is applied to
the DC Preamp input through A10Q14.
8-42. AUTO CALIBRATION-A/D CONVERTER.
8-43. Two Auto-Cal measurements are made to correct
errors which might be generated in the A/D Converter. One
POLARITY
Rg COMPARATOR BRR
Bas | B27
(OK 1.28M
RIS
input FROM _ !9.8K
DC PREAMP
3439-8-4473
OV DETECT
COMPARATOR:
OUTPUTS
TO
INGUARD
CONTROLLER
22V DETECT
COMPARATOR
Figure 8-18. Simplified A/D Converter Diagram.
8-10