The same applies for EOS. The Model 617 will typically show
insignificant degradation in displayed performance with the
noise gain of 2 resulting from allowing Rg = Rm. Typical
amplifier input E,,ise is about 9pV p-p in a bandwidth of
0.1.1OHz. Amplifier EOS can be nulled by using suppress.
The temperature coefficient of Eos is< 304~V/“C. These
numbers can be used with Equation (1) to determine ex-
pected displayed noise/drift given any source resistance.
Note also that the values given in Table 2-9 for minimum
source resistance also represent the value of RF on that
range.
2.14.8 Source Capacitance
2.15 ENGINEERING UNITS CONVERSION
In amps, the Model 617 is designed to accommodate up to
10,OOOpF input capacitance CC,). This limit will preclude
problems in most test setups and allow extremely long input
cable lengths without inducing instability or oscillations.
Increasing capacitance beyond this level may increase noise
and induce instrument instability. The noise gain of the
measurement circuit can be found from:
Equation 2.
Output E, = Input E, x ( 1 +$ )
where ZF =
RF
and Zs =
\I (2a fRpC# + 1
RS
vGziifRSCs)2 +u
Clearly as f - 0 equation (2) reduces to equation (1)
Refer to Table 2-11 for equivalent voltage sensitivity of 617
amps ranges.
The frequency range of interest is 0.1 to lOI& which is the
noise bandwidth of the A/D converter. The value of CF is 5pF
for pA ranges, 22pF for nA ranges and 1CGOpF for pA ranges.
In general, as Cs becomes larger, the noise gain becomes
larger. An application where Cs may be greater than
10,COOpF is leakage measurement of capacitors. In this case
Input E, must include the effects of the voltage source (Es)
used to bias the capacitor (any noise in the source voltage will
increase the input noise).
When measuring leakage currents on capacitors larger than
lO,OOOpF, stability and noise performance can be maintained
by adding a resistor in series with the capacitor under test.
The value of this resistor should be around 1MQ. For large
capacitor values (> lpF), the value of the series limiting
resistor can be made lower in order to improve settling times;
however, values below 10kR are not generally recommended.
This resistor is not critical in terms of tolerance or stability.
Any carbon composition resistor will prove adequate.
The Model 617 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 O.lfA (lo--lbA).
At the other extreme, resistances in the 1OMl (10160) range
can be measured. The instrument can display its reading
either in engineering units (such a mA) or in scientific nota-
tion (such as lo-3A). Table 2-10 lists engineering units and
their equivalent scientific notation values.
Table 2-10. Engineering Units Conversion
Symbol 1 Prefix Exponent
f
/ femto- 1 lo-15
P
”
h
ml
k
M
G
T
P
pico-
“S”O-
i
milli-
kilo-
mega-
giga-
tera-
peta-
10-v
10-S
10-G
10-3
103
106
109
1012
1015
Table 2-11. Equivalent Voltage Sensitivity
of 617 Amps Ranges
c
Range
Sensitivity hV/count)
---.-
2pA, 2nA. 2pA 2mA
ZOpA, 20nA. ~OJLA 20mA
200pA. 2OOnA, 200pA
1
-
2-3512-36
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