Engineering Units Conversion - Keithley 595 Instruction Manual

The noise I on the measured current can then be
calculated as follows:
IN = (Er / RF) x (1 + Zl / 22)
Values of the source impedance (22) must be calculated~for
the specific measurement circuit. External noise sources
must be taken into account when determining E,. Values
of RF and Zl are determined by the range of the Model 595
and are presented below at the approximate noise band-
width (fJ. Use f, for f in the calculation of 22, since it has
been used for the value of Zl shown.
PA
2nA, 20nA
2OOnA, 2pA
wJ.4,
mw.
1OGQ 57pF
2SOMS 0.28Hz
lOOMa 1OOpF 84MR 1OI-k
lMR 1OlOpF lM!-I 1OHz
10 kR 1OpF 10 kS lOH2
Note that when 22 is small, as in the case of a high source
capacitance with low series resistance, the quantity 21122
is large so that the current noise (IJ is high. In such a situa-
tion, the user may add his own series resistance (Rs) to in-
crease ZZ, thereby lowering the noise. A side effect of ad-
ding &is that the response time of the source impedance
will increase, because it is proportional to (Rs x C,). De-
pending on the noise level desired and the measurement
time constraints, an acceptable value of l& can be chosen.
If source capacitances over 20,OOOpF cannot be avoided,
series resistance (RJ can be added to prevent oscillation.
Values below 1OkR are not suggested, and agood rule is
to choose a value of Rs close to Zl. Keep in mind that the
device response time increases proportionally to (rt x C,).
The resistor type is not critical in terms of ~tolerance or
temperature coefficient. Carbon composition or similar
resistor construction should prove adequate.
In the capacitance function, source capacitance will degrade
noise performance as it does for the current function. In
this case, however, the Model 595 performance specifica-
tions take into account the noise~~due to the value of the
capacitance under test and the supplied input cables. Thus
only the shunt capacitance between INPUT HI and GUARD
in addition to the supplied cables needs to be considered
to determine measurement noise.
If the capacitance from INPUT HI to GUARD in addition
to the supplied Model 4801 cables is Cs, then the capaci-
tance noise- (CJ for the measurement is:
CN~e (1 + C, I (C, ~+ l5OpF)) x C, (specified)
with the Model 4801 cables
The value of Ci depends on the measurement circuit, but
the values~of C, presented below represent the capacitance
range selected.
Range
20nF
20%
CF
lllF
WpF
1OpF
The capacitance of the Model 4801 cables is approximately
WOpF. If these cables are not used, then l30pF should be
subtracted from the value of C, ~&hen calculating
capacitance noise.
Values of C, above 20,OOOpF are not recommended in the
capacitance function because they may compromise stab%-
tyof the measurement. If high shunt capacitance is
unavoidable, its effect can be reduced by adding resistance
(RJ in series with a & value of approximately &IQ. The
tolerance and composition of this resistor are not critical. a
3.13.6 Engineering Units Conversion
The user may find it helpful in interpreting operation;
specifications, and discussion of the Model 595 to under-
stand engineering unit notation. Table 3-3 lists engineer-
ing units and their equivalent scientific notation values.
Table 3-3. Engineering Units Conversion
Symbol
f
P
n
s
m
k.
M M
G G
T T
P P
P&.X
Exponent
femto-
pica-
nano-
IllhO-
milli-
kilo-
mega-
E
peta-
10-s
lo-'=
10-p
10-e
10-1
;:
109
lo'2
10'5
3-24