Front Panel Operation
2-29
VOLTAGE BURDEN
The input resistance of the ammeter causes a small voltage
drop across the input terminals. This voltage is known as the
voltage burden. If the voltage burden is large in relation to
the voltage of the measured circuit, then significant measure-
ment errors will occur.
Refer to Figure 2-27 to see how voltage burden affects cur-
rent measurements. Assume V
S
is 5mV and R
S
is 5k
Ω
to
configure a 1µA current source (5mV/5k
Ω
= 1µA). An ideal
ammeter with zero voltage burden would measure the cur-
rent source as follows:
In practice however, every ammeter has a voltage burden. If
the voltage burden (V
B
) is 1mV, the current will be measured
as follows:
The 1mV voltage burden caused a 20% measurement error.
Percent error in a measured reading (I
M
) due to voltage bur-
den can be calculated as follows:
The voltage burden of the Model 6517A depends on the se-
lected range (see specifications). Voltage burden may be re-
duced by performing the offset adjustment procedure
explained in paragraph 2.19.3 (OFFSET-ADJ).
NOISE
Noise can seriously affect sensitive current measurements.
The following paragraphs discuss how source resistance and
input capacitance affect noise performance.
Source resistance
The source resistance of the DUT will affect the noise per-
formance of current measurements. As the source resistance
is reduced, the noise gain of the ammeter will increase, as we
will now discuss.
Figure 2-28 shows a simplified model of the feedback amme-
ter. R
S
and C
S
represents the source resistance and source ca-
pacitance, V
S
is the source voltage, and V
NOISE
is the noise
voltage. Finally, R
F
and C
F
are the feedback resistance and
capacitance respectively.
The source noise gain of the circuit can be given by the fol-
lowing equation:
Note that as R
S
decreases in value, the output noise increas-
es. For example, when R
F
= R
S
, the input noise is multiplied
by a factor of two. Since decreasing the source, resistance
can have a detrimental effect on noise performance, there are
usually minimum recommended source resistance values
based on measurement range. Table 2-10 summarizes mini-
mum recommended source resistance values for various
measurement ranges. Note that the recommended source re-
sistance varies by measurement range because the R
F
value
also depends on the measurement range.
I
M
E
S
R
S
------
5mV
5k
Ω
------------
1mA
=
=
=
I
M
V
S
V
B
–
R
S
------------------
5mV 1mV
–
5k
Ω
-----------------------------
0.8mA
=
=
=
I
M
%error
100%
V
S
V
B
⁄
(
)
-----------------------
=
Source
Rs
Vs
Meter
I
M
=
R
S
V
S
- V
B
I
V
B
(Voltage
Burden)
+
-
+
-
Figure 2-27
Voltage burden considerations
Table 2-10
Minimum recommended source resistance values
Range
Minimum recommended
source resistance
pA
nA
µA
mA
1 G
Ω
to 100 G
Ω
1 M
Ω
to 100 M
Ω
1 k
Ω
to 100 k
Ω
1
Ω
to 100
Ω
OutputV
NOISE
InputV
NOISE
1
R
F
R
S
⁄
+
(
)
=
