Keithley 6512, Instruction Manual

Page: 54 / 156

Operation
2-34
The Model 6512 will typically show insignificant degrada-
tion in displayed performance with the noise gain of 2 result-
ing from allowing R
S
= R
F
. Typical amplifier Input E
NOISE
is about 9µV p-p over a bandwidth of 0.1-10Hz. Amplifier
E
OS
can be nulled by using suppress. The temperature coef-
ficient of E
OS
is <30µV/°C, and these values can be used
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
F
on that range.
2.13.8 Source capacitance
In amps, the Model 6512 is designed to accommodate up to
10,000pF input capacitance (C
S
in Figure 2-27). This limit
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:
where,
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
F
is
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.
OutputE
NOISE
InputE
NOISE
1
Z
F
Z
S
------- +
 
 
 
×
=
Z
F
R
F
2
π
fR
F
C
F
( )
2
1
+
----------------------------------------------- =
Z
S
R
S
2
π
fR
S
C
S
( )
2
1
+
----------------------------------------------- =
In general, as C
S
becomes larger, the noise gain increases. An
application where C
S
may be greater than 10,000pF is leak-
age measurement of capacitors. In this case, Input E
NOISE
must include the effects of the voltage source (E
S
) used to
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 1M
Ω
. 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 10k
Ω
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
-16
A).
At the other extreme, resistances up to 200G
Ω
can be mea-
sured. The instrument can display its reading either in engi-
neering units (such a mA) or in scientific notation (such as
10
-3
A). Table 2-12 lists engineering units and their equiva-
lent scientific notation values.
Table 2-11
Equivalent voltage sensitivity of Model 6512 amps ranges
Range
Equivalent voltage
sensitivity (µV/
count)
2pA, 2nA, 2µA, 2mA
20pA, 20nA, 20µA, 20mA
200pA, 200nA, 200µA 10µV
100µV
1mV