Datasheet AD8221 (Analog Devices) - 20

AD8221
REFERENCE TERMINAL +VS
As shown in Figure 43, 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
AD8221
is useful when the output signal needs to be offset to a precise
REF
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 al owable reference voltage range is a function
–VS
of the gain, input, and supply voltage. The REF pin should not
TRANSFORMER
exceed either +V or –V by more than 0.5 V. S S
+VS
For best performance, source impedance to the REF terminal should be kept low, because parasitic resistance can adversely affect CMRR and gain accuracy.
AD8221 POWER SUPPLY REGULATION AND BYPASSING REF
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
–VS
the amplifier.
THERMOCOUPLE
A 0.1 µF capacitor should be placed close to each supply pin.
+VS
As shown in Figure 47, a 10 µF tantalum capacitor can be used
C
further away from the part. In most cases, it can be shared by other precision integrated circuits.
1 R fHIGH-PASS = 2ʌRC AD8221 +VS C REF R 0.1µF 10µF
7
–V
4
S
-0 9 4
+IN
1
CAPACITOR COUPLED
3 0
VOUT
Figure 48. Creating an IBIAS Path
AD8221 LOAD INPUT PROTECTION –IN REF
All terminals of the AD8221 are protected against ESD, 1 kV Human Body Model. In addition, the input structure allows for
0.1µF 10µF
dc overload conditions below the negative supply, −V . The 6 S 4 -0 9 internal 400 Ω resistors limit current in the event of a negative 4
–V
1
S
3 0 fault condition. However, in the case of a dc overload voltage Figure 47. Supply Decoupling, REF, and Output Referred to Local Ground above the positive supply, +V , a large current flows directly S
INPUT BIAS CURRENT RETURN PATH
through the ESD diode to the positive rail. Therefore, an external The input bias current of the AD8221 must have a return path resistor should be used in series with the input to limit current to common. When the source, such as a thermocouple, cannot for voltages above +Vs. In either scenario, the AD8221 can provide a return current path, one should be created, as shown safely handle a continuous 6 mA current, I = V /R for IN EXT in Figure 48. positive overvoltage and I = V /(400 Ω + R ) for negative IN EXT 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. Rev. C | Page 19 of 24