AD8295 데이터 시트보기 (PDF) - Analog Devices
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AD8295 Datasheet PDF : 28 Pages
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Power Supplies
A stable dc voltage should be used to power the instrumentation
amplifier. Noise on the supply pins can adversely affect perfor-
mance. See the PSRR performance curves in Figure 14 and
Figure 15 for more information.
A 0.1 μF capacitor should be placed as close as possible to each
supply pin. An additional capacitor, a 10 μF tantalum for the
lower frequencies, can be used farther away from the IC. In
most cases, the 10 μF bypass capacitor can be shared by other
integrated circuits on the same PCB.
+VS
0.1µF
+IN
RG
AD8295
IN-AMP
–IN
REF
10µF
VOUT
LOAD
0.1µF
10µF
–VS
Figure 61. Supply Decoupling, REF, and Output Referred to Local Ground
INPUT PROTECTION
All terminals of the AD8295 are protected against ESD by
diodes at the inputs. If voltages beyond the supplies are anti-
cipated, resistors should be placed in series with the inputs to
limit the current. Resistors should be chosen so that current
does not exceed 6 mA into the internal ESD diodes in the over-
load condition. These resistors can be the same as those used
for RFI protection. (See the RF Interference section for more
information.)
For applications where the AD8295 encounters extreme
overload voltages, as in cardiac defibrillators, external series
resistors and low leakage diode clamps, such as BAV199Ls,
FJH1100s, or SP720s can be used.
INPUT BIAS CURRENT RETURN PATH
The input bias currents of the AD8295 must have a return path
to common. When the source, such as a thermocouple, cannot
provide a return current path, one should be created, as shown
in Figure 62. Otherwise, the input currents charge up the input
capacitance until the in-amp is turned off or saturated.
INCORRECT
+VS
AD8295
CORRECT
+VS
AD8295
IN-AMP
REF
–VS
TRANSFORMER
+VS
AD8295
IN-AMP
REF
–VS
TRANSFORMER
+VS
AD8295
IN-AMP
REF
–VS
THERMOCOUPLE
AD8295
IN-AMP
REF
10MΩ
–VS
THERMOCOUPLE
+VS
C
+VS
C
AD8295
C
IN-AMP
fHIGH-PASS
=
1
2πRC
R
C
AD8295
IN-AMP
REF
REF
R
–VS
–VS
CAPACITIVELY COUPLED
CAPACITIVELY COUPLED
Figure 62. Creating an Input Bias Current Return Path
RF INTERFERENCE
RF interference is often a problem when amplifiers are used in
applications where there are strong RF signals. The precision
circuits in the AD8295 can rectify the RF signals so that they
appear as a dc offset voltage error. To avoid this rectification,
place a low-pass filter before the input. Figure 63 shows such a
network in front of the instrumentation amplifier. The filter
limits both the differential and common-mode bandwidth, as
shown in the following equations:
f FILTER (Diff )
=
1
2πR(2CD
+
CC
)
f
FILTER
(CM)
=
1
2πRCC
where CD ≥ 10CC.
Rev. A | Page 21 of 28
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