Series 3700A System Switch/Multimeter Reference Manual
Section 8: Theory of operation
3700AS-901-01 Rev. D/June 2018
8-9
Ratiometric method
For the 10
MΩ and 100 MΩ ranges, the ratiometric method is used to measure resistance. Test
current for this method is generated by a 6.4 V voltage source through a 10
MΩ reference resistance
(R
REF
), as shown in the figure below.
Basic circuit theory dictates that I
REF
is equal to the I
DUT
. Because the voltmeter of the Series 3700A
(V
MEAS
) has high input impedance (>10
GΩ), current through the voltmeter branch is insignificant and
can be discounted. Therefore, as shown in the following figures, I
REF
= I
DUT
.
Figure 110: Two-wire ratiometric method
Because I = V/R, Equation 1 is modified using the V/R equivalents in place of I
REF
and I
DUT
. Therefore:
I
SOUR
= (V
MEAS
/ R
REF
) + (V
MEAS
/ R
DUT
)
V
MEAS
is measured by the Series 3700A. With V
MEAS
, I
SOUR
, R
REF
known, the Series 3700A calculates
the resistance of the DUT and displays the result. R
REF
is learned during calibration and V
SOUR
is
routinely self-calibrated when autozero is enabled.
As shown, the 4-wire ohm function can also be used to measure ohms for the 10
MΩ and 100 MΩ
ranges. To minimize the effects of charge injection when autozero is enabled, the 10
MΩ to 100 MΩ
is actually a 3-wire ohm measurement. SENSE HI is not used (it can be left open). The measurement
method is similar to the ratiometric method for 2-wire ohms, but it performs an extra voltage
measurement (V
LEAD
) to compensate for voltage drop in the input test leads.
