Rfi; Source Capacitance - Keithley 595 Instruction Manual

OPERATION
Methods of minimizin g electrostatic interference include:
1.
2.
Shielding. Possibilities include: a shielded room, a
shielded booth, shielding the sensitive circWtid using
shielded cables. The shield should always be connected
to a solid connector that is connected to signal low
(GUARD). If signal low is floated above ground, observe
safety precautions when touching the shield. Meshed
screen or loosely braided cable could be inadequate for
high impedances, or in strong fiekis. The Keithley Model
6104 Test Shield can provide shielding under many cir-
cumstances. Note, however, that shielding can increase
capacitance in the measuring cimuit. The effects of
capacitance are discussed in paragraph 3.13.5.
Reduction of electrostatic fields. Moving power line or
other sources away from the experiment reduces the
amount of electrostatic interference seen in the
measurement.
3.13.3 Thermal EMFs
Thermal EMFs are small electrical potentials generated by
differences in temperature at the junction of two dlssin6l.a~
metals. Low thermal connections should be used whenever
thermal EMFs are known to be a problem. Crimped cop-
per connections can be used to minimize these effects.
3.13.4 RFI
Radio Frequency Interference (RFI) is a general term fre-
quently used to desaiie electromagnetic interference over
a wide range of frequencies across the spectrum. RFI can
be especially troublesome at low signal levsls, but it may
also affect higher level measurements in extreme cases.
RFI can be caused by steady-state sources s&as TV or
radio broadcast signals, or it can result from impulse
sources as in the case of arcing in high voltage en-
vironments. In either case, the effect on instrument per-
formance can be considerable, if enough of the unwanted
signal is present. The effects of RFI can often be seen as
an mm.sally large offset, or in the case of impulse source-s,
sudden err&c variations in the displayed reading. In ex-
treme situations it may cause resetting or latch-up of
microprocessor-based systems.
RFI calvbe minimized by taking one or more of several
precautions when operating the Model 595 in such en-
vironments. The most obvious method is to keep the in-
strument and experiment as far away from the RFI source
as possible. Shielding the instrument, experiment, and test
leads wiIl often reduce RFI to an acceptable level. In ex-
treme cases, a specially constructed screen room may be
necessary to sufficiently attenuate the troublesome signal.
3.13.5 Source Capacitance
The Model 595 specifications assume that the instrument
is used with the supplied Model 4801 Low Noise BNC
cables. In practice, it is advisable in both capacitance and
current measurements to keep cable lengths as short as
possible without~ applying undue stress to cables or con-
nectors. It is also suggested that fixtures be designed to
minimize the stray capacitance between the Model 595
INPUT HI and GUARD terminals.
O&asionally, an application wiII require that longer cables
or test fixtures With significant capacitance from INPUT HI
to GUARD be used. The device under t&t may also have
a large capacitance associated with it which is connected
from INPUT Hl to either GUARD or the VOLTAGE
SOURCE OUTPUT This capacitance is referred to as source
capacitance. Source capacitance degrades the instrument
performance from the level specified using the supplied
cables.
In the current function, the Model 595 is designed to~ac-
comodate up to 20,OOOpF of source capacitance without
oscillating or becoming unstable. Increasing source
capacitance beyond this level may cause instrument in-
stability Even within the limit of 20,00OpF, source
capacitance increases measurement noise. The amount of
noise on the current measurement depends on the value
of the source impedance (resistance and capacitance com-
bined), the impedance in the feedback loop of the Model
595, and the magnitude of the voltage noise source.
The feedback impedance oft the Model 595 in the current
function is a resistance (I&) in parallel with a capacitance
(C,). This combination results in a feedback impedance at
the frequency, f, of:
21 _ R, I d (2 x g x f x RF x C,) + 1
A generalized soorce impedance can be considered a
parallel resistance and capadtance (RP and C,) in series with
B v&%ance &). The impedance of this combinatioti at the
frequenq f will be:
22 = Rs + R, I 4 (2 x K x f x R, x C,) + 1
The Model 595 can beg thought of as having a noise source
in series with its input-of EN = 10~V peak to peek in a 0.1
tom 1OHz bandwidth. In addition to this voltage noise soorce,
the noise of any other source in the drcuit, including the
Model 595 voltage source must be appropriately added to
E,. The total noise of ail sources is referred to bask E,
3-23