Keithley 2520, Руководство пользователя, Страница: 302 / 329

F-18 Measurement Considerations Model 2520 User’s Manual
Source capacitance
DUT source capacitance will also affect the noise performance of the Model 2520 amme-
ters. In general, as source capacitance increases, the noise gain also increases.
The elements of interest for this discussion are the capacitance (C
DUT
) of the DUT and the
internal feedback capacitance (C
F
) for the ammeter. Taking into account the capacitive
reactance of these two elements, our previous noise gain formula must be modified as fol-
lows:
Output V
NOISE
= Input V
NOISE
(1 + Z
F
/Z
DUT
)
where:
• Output V
NOISE
is the noise seen at the output of the ammeter.
• Input V
NOISE
is the noise seen at the input of the ammeter.
• Z
F
is the internal feedback impedance for the ammeter that is formed by C
F
and R
F
.
• Z
DUT
is the internal impedance of the DUT that is formed by C
DUT
and R
DUT
.
Furthermore,
and
Note that as C
S
increases in value, Z
DUT
decreases in value, thereby increasing the noise
gain. Again, at the point where Z
DUT
= Z
F
, the input noise is amplified by a factor of two.
High source capacitance can also cause overshoot ringing, and in the extreme, oscillation
of the feedback ammeter. Whenever possible, use low-capacitance photodiodes and short
cables.
Generated currents
Any extraneous generated currents in the test system will add to the measured detector
current, causing errors. Currents can be internally generated, as in the case of instrument
input offset current, or they can come from external sources such as insulators, cables, and
leaky photodiodes.
Z
F
R
F
2
π
fR
F
C
F
( )
2
1
+
-------------------------------------------- =
Z
DUT
R
S
2
π
fR
S
C
S
( )
2
1
+
-------------------------------------------- =