Keithley 6514, Инструкция по эксплуатации

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11-16 Digital I/O, Analog Outputs, and External Feedback
Frequency compensation/stabilization is accomplished by adding a feedback capacitor, C
FB
.
The value of this capacitor depends on the particular transistor being used and the maximum cur-
rent level expected. Compensation at maximum current is required because the dynamic imped-
ance will be minimum at this point. It should be noted that the response speed at lower currents
will be compromised due to the increasing dynamic impedance, which is given by the following
formula:
Using the above transistors, a minimum RC time constant of 100µsec at maximum input cur-
rent would be used. At I
IN
(max) of 100µA, this value would correspond to 0.4µF. Note that at
100nA, this value would increase the RC response time constant to 100msec. A minimum capac-
itance of 100pF is recommended.
Although the input signal to this particular circuit is assumed to be a current, conversion to
voltage input could be performed by placing a shunt resistor across the input. However, the nom-
inal voltage burden of 1mV must be considered as an error signal that must be taken into
account.
Further processing of the current response can be achieved by using the suppress feature. For
example, REL could be enabled with a reference input current applied. For all subsequent cur-
rents, the natural logarithm of the ratio of the measured current to the suppressed current would
then be displayed:
V
DISP
= V
REL
kT/q (ln (I
READ
/I
O
) - ln (I
REL
/I
O
))
= kT/q (ln (I
READ
/I
REL
))
= 0.26/I (ln (I
READ
/I
REL
)) @ 25˚C
NOTE
The circuit topology of Figure 11-9 works for positive input currents only. For bipolar
input signals, an external offset bias must be applied, or use a PNP transistor for Q1.
Non-decade current gains
Model 6514 electrometer input uses internal decade resistance feedback networks for the
current ranges. In some applications, non-decade current gains may be desirable. As shown in
Figure 11-10, an external feedback resistor, R
FB
, can be used to serve this purpose. Limitations
on the magnitude of the feedback current require that the value of R
FB
be greater than 10
2
Ω .
Input
HI
Current
Input
COM
Preamp
Out
Op Amp
+
-
Zero
Check
<1
Ω
(Chassis)
LO
S
To Ranging
Amplifier
10M
Ω
R
FB
Figure 11-10
Non-decade
current gains