Keithley 7706 User Manual page 21

Loading effects
Loading of the voltage source becomes a consideration for low resistance loads. As the source resistance increases, the error
caused by loading increases.
V is the programmed analog output of the Model 7706
s
R is the total lead resistance of the wiring and connections
Lead
R is the resistance of the user's circuit
Load
V is the measured voltage
M
The voltage actually measured by the meter is attenuated by the voltage divider action of R
follows:
This relationship can be modified to directly compute for percent error:
Using the above equation, to keep loading error within 0.1%, the resistance of the Model 7706 system must be at least 1/999
the value of load resistance.
Figure 17
Loading effects
R R
Lead
+ +
V V
S S
Source Source
Voltage Voltage
– –
DAC output errors
The DAC output is most accurate when the Model 7706 is operated in stable temperature conditions that are as close as possible
to the environmental conditions used for calibration. Offset voltage drift over temperature is 1mV/°C. Also, the offset voltage
value may change when changing from slot 1 to slot 2.
Figure 17 shows the method used to determine the percent error due to loading where:
V
M
Percent Error
V V
M M
Measured Measured
R R
Load Load
Voltage Voltage
V R
S Load
= ---------------------------------- -
R + R
Load Lead
R
Load
×
= ---------------------------------- - 100
R + R
Load Lead
Connection
and R , and it can be calculated as
S I
th
17