Datasheet

AD8251
Rev. B | Page 18 of 24
POWER SUPPLY REGULATION AND BYPASSING
The AD8251 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 per-
formance. As in all linear circuits, bypass capacitors must be
used to decouple the amplifier.
Place a 0.1 µF capacitor close to each supply pin. A 10 µF tantalum
capacitor can be used farther away from the part (see Figure 55)
and, in most cases, it can be shared by other precision integrated
circuits.
AD8251
+
V
S
+IN
–IN
LOAD
REF
0.1µF
10µF
0.1µF 10µF
–V
S
DGND
OUT
DGND
A0
A1
WR
06287-054
Figure 55. Supply Decoupling, REF, and Output Referred to Ground
INPUT BIAS CURRENT RETURN PATH
The AD8251 input bias current must have a return path to its
local analog ground. When the source, such as a thermocouple,
cannot provide a return current path, one should be created
(see Figure 56).
THERMOCOUPLE
+V
S
REF
–V
S
AD8251
CAPACITIVELY COUPLED
+V
S
REF
C
C
–V
S
AD8251
TRANSFORMER
+V
S
REF
–V
S
AD8251
INCORRECT
CAPACITIVELY COUPLED
+V
S
REF
C
R
R
C
–V
S
AD8251
1
f
HIGH-PASS
=
2RC
THERMOCOUPLE
+V
S
REF
–V
S
10M
AD8251
TRANSFORMER
+V
S
REF
–V
S
AD8251
CORRECT
06287-055
Figure 56. Creating an I
BIAS
Return Path
INPUT PROTECTION
All terminals of the AD8251 are protected against ESD. Note
that 2.2 k series resistors precede the ESD diodes as shown in
Figure 51. The resistors limit current into the diodes and allow
for dc overload conditions 13 V above the positive supply and
13 V below the negative supply. An external resistor should be
used in series with each input to limit current for voltages
greater than 13 V beyond either supply rail. In either scenario,
the AD8251 safely handles a continuous 6 mA current at room
temperature. For applications where the AD8251 encounters
extreme overload voltages, external series resistors and low
leakage diode clamps, such as BAV199Ls, FJH1100s, or SP720s,
should be used.