Voltage And Percent Error For Various Time Constants - Keithley 194 Instruction Manual

OPERATION
RS
L
wvv
I
1 ES
-:- c
r
-y-
Figure
3-24. Input Capacitance Effects
When ES is first applied, the voltage across the capacitance
(and thus, at the input of the instrument) does not rise
instantaneously to its final value. Instead, the capacitance
charges exponentially as follows:
VM = ES (l-e RC)
Note that RS is given in ohms, C is in farads, while t is
in seconds.
Because of the charging action of CI, the input voltage
follows the exponential curve shown in Figure 3-24. At the
end of one time constant (R&), the voltage will reach ap-
proximately 63% of its final value. At the end of two time
constants (2R&), the voltage will reach 86% of its final
value, and so on. Table 3-17 summarizes voltage and per-
cent error values for ten different time constants.
The response time will, of course, depend on the relative
values of Rs and CI. For example, if Rs has a value of IkfI,
and CI has a value of IOOOpF, a time constant of lesec
results. Thus, to allow the reading to settle to within O.l%,
approximately 7pec must be allowed.
While input capacitance does increase response time, it
can help to filter out some of the higher frequency noise
present in the signal by effectively limiting instrument
bandwidth. If we assume that all input capacitance is
lumped into a single element, the half-power (-3dB) point
of the circuit in Figure 3-25 will be:
1
f-3db = ~
27%C1
Thus, if RS has a value of lkl2, and Cl has a value of
lOOOpF, the half-power point will be 159kHz.
Table 3-17. Voltage and Percent Error For Various
Time Constants
Time*
VM
T 0.632 ES
27
0.86 Es
37 0.95
ES
41
0.982 ES
57 0.993 Es
61
0.9975 ES
71
0.999 Es
87
0.99966E,
YT 0.9999 ES
107 0.99995Es
%EIIor
36 %
14 %
5 %
1.8 %
0.674%
0.25 B
0.09 7'"
0.033%
0.0~12%
0.005%
*T = RsC
3.215 AC Frequency Response
Considerations
The strength of the Model 194 lies in its ability to analyze
complex waveforms. The following paragraphs discuss a
number of considerations to keep in mind when measur-
ing AC signals, including low frequency response limits,
as well as volt-hertz product considerations.
Low Frequency Response
When DC coupling is in effect, the instrument measures
down to DC levels (OHz). Thus, no consideration as to the
reponse of the instrument at low frequencies need be given
when using DC coupling.
3-45