AD8221
REFERENCE TERMINAL
As shown in Figure 42, the reference terminal, REF, is at one
end of a 10 kΩ resistor. The output of the instrumentation
amplifier 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 +V
S
or –V
S
by more than 0.5 V.
For best performance, source impedance to the REF terminal
should be kept low, because parasitic resistance can adversely
affect CMRR and gain accuracy.
+V
S
AD8221
REF
–V
S
TRANSFORMER
+V
S
POWER SUPPLY REGULATION AND BYPASSING
A stable dc voltage should be used to power the instrumentation
amplifier. Noise on the supply pins can 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 46, a 10 μF tantalum capacitor can be used
further away from the part. In most cases, it can be shared by
other precision integrated circuits.
+V
S
AD8221
REF
–V
S
THERMOCOUPLE
+V
S
C
1
f
HIGH-PASS
= 2πRC
C
R
–V
S
03149-047
R
AD8221
REF
0.1µF
+IN
10µF
CAPACITOR COUPLED
AD8221
–IN
REF
V
OUT
LOAD
Figure 47. Creating an I
BIAS
Path
INPUT PROTECTION
All terminals of the AD8221 are protected against ESD, 1 kV
Human Body Model. In addition, the input structure allows for
dc overload conditions below the negative supply, −V
S
. 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, +V
S
, a large current flows 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 = V
IN
/R
EXT
for
positive overvoltage and I = V
IN
/(400 Ω + R
EXT
) 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.
–V
S
Figure 46. Supply Decoupling, REF, and Output Referred to Local Ground
INPUT BIAS CURRENT RETURN PATH
The input bias current of the AD8221 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 47.
03149-046
0.1µF
10µF
Rev. B | Page 18 of 24
AD [ ANALOG DEVICES ]
