Keithley 630 Instruction Manual page 14

MODEL 630 POTENTIOMETRIC ELECTROMETER OPERATION
2-7. NULL SENSITIVITY AND SOURCE RESISTANCE.
a.
Source resistance affects null sensitivity. Because of its high input resistance,
the Model 630 is less affected.
The following paragraphs show the value of using a poten-
tiometric electrometer for accurate measurements from a high resistance source.
b.
The equation below gives the relationship of the meter reading to source resistance:
Equation 2
where V, is the source voltage in volts;
R, is the source resistance in ohms;
V is the nuli detector meter reading in volts;
Ed is the setting of the Reference Voltage Dials in volts;
R,, is the input resistance of the null detector in ohms
With no source resistance, the null detector meter will read exactly the difference be-
tween the reference voltage and the source voltage (V = V, - Ed). For increasing source
resistance, the meter will indicate only a portion of the difference between the two
voltages.
This discrepancy becomes more important the more off null a reading is.
c.
Figure 7 contains graphs showing the effect source resistance has on off-null read-
ings for the Model ,630. The curves are the percent the meter reads the voltage differ-
ence. At loo%, V = V, - Ed.
Note that as the Model 630 approaches null, the meter
indication as an absolute value becomes insignif+cant regardless of source resistance.
For instance, if the Model 630 is 0.1 millivolt off null on the l-millivolt range and the
source resistance is such that the meter indicates only 10% of actual value, the metier
will be within -09 millivolt. At .Ol millivolt off null, the meter will be within 9
microvolts.
NOTE
When the Model 630 is as near to null as possible, the Reference Voltage Dial
setting is always correct within the instrument's specified limit of error.
2-8. AC EFFECTS ON MEASUREMENTS. The Model 630 has greater than 60 db line frequency re-
jection on its l-millivolt range.
Greater line frequency pickup may reduce off-null sen-
sitivity and cause needle quiver or a zero shift. To reduce this, use an R-C filter at
the input. The component values depend on the source resistance.
might be a 109 resistor and a lOO-picofarad capacitor.
1010
A good compromise
With a
ohm source, this adds
only about two seconds to the rise time.
NOTE
Use a larger R or C if rise time is not important.
Always use a polystyrene
capacitor or one with as good an insulation resistance to avoid degenerating the
input resistance.
2-9. CURRENT MEASUREMENTS.
a. When used with a high-value precision resistor,
the Model 630 Potentiometric Elec-
trometer can be used to accurately measure currents as low as lo-13 amperes.
The choice
0266R
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