Application Hints
(Continued)
Because the LM363’s offset voltage is so low to begin with,
offset nulling has a negligible effect on offset temperature
drift.Forexample,zeroinga100
m
Voffset,assumingexternal
resistor TC of 200 ppm/
§
C and worst-case internal resistor
TC, results in an additional drift component of 0.08
m
V/
§
C.
For this reason, drift specifications are guaranteed, with or
without external offset nulling.
GAIN ADJUSTMENT
Gain may be increased by adding an external voltage divid-
er between output force and sense and reference; the pre-
ferred connection is shown in Figure 4. Since both the
sense and reference pins look like 50 k
X
(
g
20 k
X
) to V
b
,
impedances presented to both pins must be equal to avoid
offset error. For example, a 100
X
imbalance can create a
worst-case output offset of 50 mV, creating an input-re-
ferred error of 5 mV at G
e
10 or 50
m
V at G
e
1000.
Increasing gain this way increases output offset error. An
LM363H-100 may have an output offset of 5 mV, resulting in
input referred offset component of 50
m
V. Raising the gain
to 200 yields a 10 mV error at the output and changes input
referred error by an additional 50
m
V.
External resistors connected to the reference and sense
pins can only increase the gain. If ultra-low output imped-
ance is not critical, the technique inFigure 5 can be used to
trim the gain to nominal value. Alternatively, the V
OS
adjust-
ment terminals on the 16-pin package may be used to trim
the gain (Figure 10b).
TL/H/5609–12
R1 and R2 should be as low as possible to avoid errors due to 50 k
X
input impedance of reference and sense pins. Total resistance
(R2
a
2R1) should be above 4 k
X
, however, to prevent excessive load
on the LM363 output. The exact formula for calculating gain (G) is:
G
e
G
O
#
1
a
2R1
R2
a
R1
50k
J
G
O
e
preset gain
The last term may be ignored in applications where gain accuracy is not
critical. The table below gives suggested values for R1 and R2 along
with the calculated error due to ‘‘closest value’’ standard 1% resistors.
Total gain error tolerance includes contributions from LM363 G
O
error
and resistor tolerance (
g
1%) and works out to approximately 2.5% in
every case.
Pinout shown is for 16-pin package. This same technique can also be
used with 8-pin versions.
Gain Increase
1.5
2
2.5
3
4
5
6
7
8
9
10
R1
1.21k
1.21k
2k
2k
1.78k
2k
2.49k
2.94k
3.48k
3.92k
4.42k
R2
5k
2.49k
2.74k
2.05k
1.21k
1k
1k
1k
1k
1k
1k
Error (typ)
a
0.6%
b
0.2%
0
b
0.3%
b
0.6%
a
0.8%
a
0.5%
b
0.9%
a
0.4%
b
0.9%
b
0.7%
FIGURE 4. Increasing Gain
Pinout shown is for 8-pin versions.
This same technique can also be used
with 16-pin version.
TL/H/5609–13
FIGURE 5. Adjusting Gain, Alternate Technique
11