AD8221AR(2003) Просмотр технического описания (PDF) - Analog Devices
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AD8221AR Datasheet PDF : 20 Pages
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REFERENCE TERMINAL
As shown in Figure 42, the reference terminal, REF, is at one end
of a 10 kΩ resistor. The instrumentation amplifier’s output is
referenced to the voltage on the REF terminal; this is useful
when the output signal needs to be offset to a precise midsupply
level. For example, a voltage source can be tied to the REF pin to
level-shift the output so that the AD8221 can interface with an
ADC. The allowable reference voltage range is a function of the
gain, input and supply voltage. The REF pin should not exceed
either +VS or –VS by more than 0.5 V.
For best performance, source impedance to the REF terminal
should be kept low, since parasitic resistance can adversely affect
CMRR and gain accuracy.
POWER SUPPLY REGULATION AND BYPASSING
A stable dc voltage should be used to power the instrumenta-
tion amplifier. Noise on the supply pins may adversely affect
performance. Bypass capacitors should be used to decouple the
amplifier.
A 0.1 µF capacitor should be placed close to each supply pin. As
shown in Figure 47, a 10 µF tantalum capacitor may be used
further away from the part. In most cases, it may be shared by
other precision integrated circuits.
Figure 46
+VS
0.1µF
+IN
AD8221
–IN
REF
10µF
VOUT
LOAD
0.1µF
10µF
–VS
Figure 47. Supply Decoupling,. REF and Output Referred to Local Ground
INPUT BIAS CURRENT RETURN PATH
The AD8221’s input bias current 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 48.
AD8221
+VS
AD8221
REF
–VS
TRANSFORMER
+VS
AD8221
REF
–VS
THERMOCOUPLE
+VS
C
fHIGH-PASS =
1
2πRC
R
C
AD8221
REF
R
–VS
CAPACITOR COUPLED
Figure 48. Creating an IBIAS Path
INPUT PROTECTION
All terminals of the AD8221 are protected against ESD1. In
addition, the input structure allows for dc overload conditions
below the negative supply, –Vs. The internal 400 Ω resistors
limit current in the event of a negative fault condition. However,
in the case of a dc overload voltage above the positive supply,
+Vs, a large current would flow directly through the ESD diode
to the positive rail. Therefore, an external resistor should be
used in series with the input to limit current for voltages above
+Vs. In either scenario, the AD8221 can safely handle a
continuous 6 mA current, I = VIN/REXT for positive overvoltage
and I = VIN/(400 Ω + REXT) for negative overvoltage.
For applications where the AD8221 encounters extreme
overload voltages, as in cardiac defibrillators, external series
resistors and low leakage diode clamps such as BAV199Ls,
FJH1100s, or SP720s should be used.
1 1 kV—Human Body Model.
Rev. A | Page 15 of 20
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