Datasheet

AD8220
Rev. B | Page 21 of 28
REFERENCE TERMINAL
The reference terminal, REF, is at one end of a 20 kΩ resistor
(see Figure 55). 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 voltages other than
common. For example, a voltage source can be tied to the REF
pin to level-shift the output so that the AD8220 can interface
with an ADC. The allowable reference voltage range is a function of
the gain, common-mode input, and supply voltages. The REF
pin should not exceed either +V
S
or −V
S
by more than 0.5 V.
For best performance, especially in cases where the output is
not measured with respect to the REF terminal, source impedance
to the REF terminal should be kept low, because parasitic
resistance can adversely affect CMRR and gain accuracy.
POWER SUPPLY REGULATION AND BYPASSING
The AD8220 has high PSRR. However, for optimal
performance, a stable dc voltage should be used to power
the instrumentation amplifier. Noise on the supply pins can
adversely affect performance. As in all linear circuits, bypass
capacitors must be used to decouple the amplifier.
A 0.1 μF capacitor should be placed close to each supply pin.
A 10 μF tantalum capacitor can be used further away from the
part (see Figure 59). In most cases, it can be shared by other
precision integrated circuits.
AD8220
+V
S
+IN
–IN
LOAD
REF
0.1µF
10µF
0.1µF 10µF
–V
S
V
OUT
0
3579-001
Figure 59. Supply Decoupling, REF and Output Referred to Ground
INPUT BIAS CURRENT RETURN PATH
The AD8220 input bias current is extremely small at less than
10 pA. Nonetheless, the input bias current must have a return
path to common. When the source, such as a transformer,
cannot provide a return current path, one should be created
(see Figure 60).
+V
S
REF
TRANSFORMER
–V
S
AD8220
AC-COUPLED
+V
S
REF
C
R
R
C
–V
S
AD8220
1
f
HIGH-PASS
=
2RC
03579-002
Figure 60. Creating an I
BIAS
Path
INPUT PROTECTION
All terminals of the AD8220 are protected against ESD.
(ESD protection is guaranteed to 4 kV, human body model.) In
addition, the input structure allows for dc overload conditions
a diode drop above the positive supply and a diode drop below
the negative supply. Voltages beyond a diode drop of the supplies
cause the ESD diodes to conduct and enable current to flow
through the diode. Therefore, an external resistor should be
used in series with each of the inputs to limit current for
voltages above +Vs. In either scenario, the AD8220 safely
handles a continuous 6 mA current at room temperature.
For applications where the AD8220 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.