AD8222 Datasheet, PDF (16/24 Pages) Analog Devices – Precision, Dual-Channel Instrumentation Amplifier

AD8222
REFERENCE TERMINAL
The output voltage of the AD8222 is developed with respect to
the potential on the reference 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 AD8222 can drive a single-supply
ADC. The REF pin is protected with ESD diodes and should
not exceed either +VS or −VS by more than 0.3 V.
For best performance, source impedance to the REF terminal
should be kept below 1 Ω. As shown in Figure 44, the reference
terminal, REF, is at one end of a 10 kΩ resistor. Additional
impedance at the REF terminal adds to this 10 kΩ resistor and
results in amplification of the signal connected to the positive
input. The amplification from the additional RREF can be
computed by
( ) 2 10 kΩ + R_REF
20 kΩ + R_REF
Only the positive signal path is amplified; the negative path is
unaffected. This uneven amplification degrades the amplifier’s
CMRR.
INCORRECT
CORRECT
CORRECT
AD8222
V_REF
V_REF
AD8222
V_REF
AD8222
+
OP2177
–
+
AD8222
–
Figure 45. Driving the Reference Pin
LAYOUT
The AD8222 is a high precision device. To ensure optimum
performance at the PC board level, care must be taken in the
design of the board layout. The AD8222 pinout is arranged in a
logical manner to aid in this task.
Package Considerations
The AD8222 comes in a 4 mm × 4 mm LFCSP. Beware of
blindly copying the footprint from another 4 mm × 4 mm
LFCSP part; it may not have the same thermal pad size and
leads. Refer to the Outline Dimensions section to verify that
the PCB symbol has the correct dimensions. Space between the
leads and thermal pad should be kept as wide as possible for the
best bias current performance.
Thermal Pad
The AD8222’s 4 mm × 4 mm LFCSP comes with a thermal pad.
This pad is connected internally to −VS. The pad can either be
left unconnected or connected to the negative supply rail.
To preserve maximum pin compatibility with future dual
instrumentation amplifiers, leave the pad unconnected. This
can be done by not soldering the paddle at all or by soldering
the part to a landing that is a not connected to any other net.
For high vibration applications, a landing is recommended.
Because the AD8222 dissipates little power, heat dissipation is
rarely an issue. If improved heat dissipation is desired (for example,
when driving heavy loads), connect the thermal pad to the
negative supply rail. For the best heat dissipation performance,
the negative supply rail should be a plane in the board. See
the section for thermal coefficients with and without the pad
soldered.
Common-Mode Rejection over Frequency
The AD8222 has a higher CMRR over frequency than typical
in-amps, which gives it greater immunity to disturbances, such
as line noise and its associated harmonics. A well-implemented
layout is required to maintain this high performance. Input
source impedances should be matched closely. Source resistance
should be placed close to the inputs so that it interacts with as
little parasitic capacitance as possible.
Parasitics at the RGx pins can also affect CMRR over frequency.
The PCB should be laid out so that the parasitic capacitances at
each pin match. Traces from the gain setting resistor to the RGx
pins should be kept short to minimize parasitic inductance.
Reference
Errors introduced at the reference terminal feed directly to the
output. Care should be taken to tie REF to the appropriate local
ground.
Power Supplies
A stable dc voltage should be used to power the instrumentation
amplifier. Noise on the supply pins can adversely affect
performance.
The AD8222 has two positive supply pins (Pin 5 and Pin 16)
and two negative supply pins (Pin 8 and Pin 13). While the part
functions with only one pin from each supply pair connected,
both pins should be connected for specified performance and
optimum reliability.
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