Circuit Operation During Resistance Measurements - Keithley 195 Instruction Manual

M"l.TlPLEXER
FETS
/
\
-2v
REFERENCE
Figure 6-6. Circuit Operation During DC Voltage Measurements
The FET switching sequence is shown in Figure 6-9. Note that
reading calculations are done at several phases during the in-
put sequence.
Once the digitized values for each input signal are in memory.
the microcomouter calculates the disolaved or IEEE value from
I the following iormula:
It can be seen that the,zero error is subtracted from both the
signal and reference values, and the ratio is then taken. Multi-
plication by two is necessanl because of the -2V reference
source value.
6.7.2 Circuit Operation During Resistance
Measurements
For resistance measurments, four inputs are used so the Model
195 can obtain a measurement through the use of ratiometric
techniques.
These four inputs include:
1. Ohms Reference HI
2. Ohms Reference LO
3. Ohms Sense HI
4. Ohms Sense LO
Figure 6.9. FET Switching Sequence During DC Voltage
Measurements
6-9