Source Capacitance; Engineering Units Conversion - Keithley 6512 Instruction Manual

Operation
The Model 6512 will typically show insignificant degrada-
tion in displayed performance with the noise gain of 2 result-
ing from allowing R = R
S
is about 9µV p-p over a bandwidth of 0.1-10Hz. Amplifier
E can be nulled by using suppress. The temperature coef-
OS
ficient of E is <30µV/°C, and these values can be used
OS
with the above equation to determine expected displayed
noise/drift given any source resistance. Note also that the
values given in Table 2-10 for minimum source resistance
also represent the value of R

2.13.8 Source capacitance

In amps, the Model 6512 is designed to accommodate up to
10,000pF input capacitance (C
will preclude problems in most test setups and allow ex-
tremely long input cable lengths without inducing instability
or oscillations.
Increasing source capacitance beyond this level may in-
crease noise and induce instrument instability. Again refer-
ring to Figure 2-27, the noise gain of the measurement circuit
can be found as follows:
OutputE
=
NOISE
where,
Z
= ---------------------------------------------- -
F
(
Z
= ---------------------------------------------- -
S
(
Note that as f 0, the above equation reduces to the low-fre-
quency equation discussed in paragraph 2.13.7.
The frequency range of interest is from 0.1 to 10Hz, which is
the noise bandwidth of the A/D converter. The value of C
5pF for pA ranges, 22pF for nA ranges, and 1,000pF for µA
ranges. Also, since the noise factor is given in volts, current
must be converted to volts. Table 2-11 summarizes equiva-
lent voltage sensitivity of Model 6512 amps ranges.
2-34
. Typical amplifier Input E
F
on that range.
F
in Figure 2-27). This limit
S
 
Z
F
×
 
InputE 1
+ ------ -
NOISE
 
Z
S
R
F
)
2
2πfR C 1
+
F F
R
S
) 2
2πfR C 1
+
S S
Table 2-11
Equivalent voltage sensitivity of Model 6512 amps ranges
NOISE
Range
2pA, 2nA, 2µA, 2mA
20pA, 20nA, 20µA, 20mA
200pA, 200nA, 200µA
In general, as C
application where C
age measurement of capacitors. In this case, Input E
must include the effects of the voltage source (E
bias the capacitor (any noise in the source voltage will in-
crease the input noise).
When measuring leakage currents on capacitors larger than
10,000pF, stability and noise performance can be maintained
by connecting a resistor in series with the capacitor under test.
The value of this resistor should be about 1MΩ. For large ca-
pacitor values (>1µF), the value of the series limiting resistor
can be made lower in order to improve settling times; how-
ever, values below 10kΩ are not generally recommended.
This resistor is not critical in terms of tolerance or stability.
Any carbon composition resistor will prove adequate.

2.14 Engineering units conversion

The Model 6512 is a highly sensitive instrument with wide-
ranging measurement capabilities. In the amps mode, for ex-
ample, the unit can detect currents as low as 0.1fA (10
At the other extreme, resistances up to 200GΩ can be mea-
sured. The instrument can display its reading either in engi-
neering units (such a mA) or in scientific notation (such as
-3
10 A). Table 2-12 lists engineering units and their equiva-
lent scientific notation values.
is
F
Equivalent voltage
sensitivity (µV/
count)
10µV
100µV
1mV
becomes larger, the noise gain increases. An
S
may be greater than 10,000pF is leak-
S
NOISE
) used to
S
-16
A).