LM363 Datasheet, PDF (10/22 Pages) National Semiconductor (TI) – LM363 Precision Instrumentation Amplifier

Application Hints (Continued)
c Single Supply Unipolar Output
d Single Supply Bipolar Output
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FIGURE 1 Reference Connections (Continued)
OUTPUTS
The LM363’s output can typically swing within 1V of the
supplies at light loads While specified to drive a 2 kX load
to g10V current limit is typically 15 mA at room tempera-
ture The output can stably drive capacitive loads up to
400 pF For higher load capacitance the amplifier may be
overcompensated (see COMPENSATION section follow-
ing) The output may be continuously shorted to ground
without damaging the device
OFFSET VOLTAGE
The LM363’s offset voltage is internally trimmed to a very
low value Note that data sheet values are given at
T_je25 C V_CMe0V and V^a eV^be15V For other condi-
tions warm-up drift temperature drift common-mode rejec-
tion and power supply rejection must be taken into account
Warm-up drift due to chip and package thermal gradients is
an effect separate from temperature drift Typical warm-up
drift is tabulated in the Electrical Characteristics settling
time is approximately 5 minutes in still air At load currents
up to 5 mA thermal feedback effects are negligible
(DV_OS^s2mV at Ge1000)
Care must be taken in measuring the extremely low offset
voltages of the high gain amplifiers Input leads must be
held isothermal to eliminate thermocouple effects Oscilla-
tions due to either heavy capacitive loading or stray capaci-
tance from input to output can cause erroneous readings
In either case overcompensation will help High frequency
noise fed into the inputs may be rectified internally and pro-
duce an offset shift A simple low-pass RC filter will usually
cure this problem (Figure 2 ) Use film type resistors for their
low thermal EMF In highly noisy environments LC filters
can be substituted for increased RF attenuation
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FIGURE 2 Low Pass Filter Prevents RF Rectification
Instrumentation amplifiers have both an input offset voltage
(V_IOS) and an output offset voltage (V_OOS) The total input-
referred offset voltage (V_OSRTI) is related to the instrumen-
tation amplifier gain (G) as follows V_OSRTIeV_IOSaV_OOS
G The offset voltage given in the LM363 specifications is
the total input-referred offset As long as only one gain is
used offset voltage can be nulled at either input or output
as shown in Figures 3a and 3b When the 16-pin device is
used at multiple gain settings both V_IOS and V_OOS should
be nulled to get minimum offset at all gains as shown in
Figure 3c The correct procedure is to trim V_OOS for zero
output at Ge10 then trim V_IOS at Ge1000
FIGURE 3 Offset Voltage Trimming
10
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