Auto-Calibration-Ohms; General - HP 3455A Operating And Service Manual

Model 3455A
8-34. AUTO—CALIBRATION - OHMS.
8-35. General.
8-36. During the ohms function the ohms converter sup-
plies a current through both the unknown resistance and
the reference resistance (see Figure 8-15). Since the same
' current flows through both resistors, their respective voit-
age drops are proportional. As with the DC Auto-Cal
sequence, the offset errors are measured and subtracted
from the unknown and reference resistance measurements.
The voltage developed across the unknown resistor is mea-
sured by closing S1 while the reference voltage, developed
across the reference resistor, is measured by closing S2. The
value of the unknown resistance is computed by the main
controller. An equation describing this computation is:
—{(VRx + Eg! Gy — Eo G1]
(VreF + Eo) G2 — Ey G2
Kr
VIRTUAL GROUND
THEORY OF OPERATION
Section VII
Figure 8-15, Basic Ohms Measurement Diagram.
where Rx is the unknown resistance value, VRx is the volt-
age drop across the unknown resistance, VR ref. is the volt-
age drop across the reference resistance, Eg is the input
offset error, Gy and G2 are the circuit gains of the particu-
lar measurements, and K, is the range factor. This equation
simplifies to:
_ VRx G1
x = Kr
VREF G2
8-37. Circuit Description.
8-38. .1kQ, 1 kQ, 1 MQ Offset Error Measurements, The
offset error constants derived for the .1 V de and 1 V de
ranges are also used for the .1 kQ, 1 kQ, and 1 MQ offset
error constants, since the circuit configurations are the
same. Refer to Paragraph 8-25 for a description of these
offset error measurements.
8-39. 10 kQ, 100 kQ, 10 MQ Offset Error Measurements.
Two additional offset measurements are made to compen-
sate for errors which might be present when making mea-
surements on the 10 kQ, 100 kQ, or 10 MQ ranges. Figure
3455-8-4531
Figure 8-16. 10 k2, 100 k&2, 10 MQ Offset Error Measurement.
5 8-9