AD8222BCPZ-R7 datasheet(17/25 Pages) AD

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AD8222

Data Sheet

Rev. B | Page 16 of 24

REFERENCE TERMINAL

The output voltage of an AD8222 channel is developed with

respect to the potential on the corresponding reference terminal.

Typically, the reference terminal is connected to ground, but it

can also be driven with a voltage to offset the output signal. For

example, connect a voltage to the reference terminal to level-

shift the output so that the AD8222 can drive a single-supply

ADC. Both REF1 and REF2 are protected with ESD diodes and

should not exceed either +VS or −VS by more than 0.3 V.

For best performance, source impedance to a reference terminal

should be kept below 1 Ω. As shown in Figure 45, the reference

terminal is at one end of a 10 kΩ resistor. Additional impedance

at the reference 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





REF



kΩ

Only the positive signal path is amplified; the negative path is

unaffected. This uneven amplification degrades the CMRR of

the amplifier.

INCORRECT

AD8222

CORRECT

AD8222

OP2177

–

CORRECT

AD8222

–

REF

Figure 46. Driving the Reference Pin

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 device; the landing pattern may be different. Refer to

the Outline Dimensions section to verify that the PCB symbol

has the correct dimensions.

The AD8222 comes in two package varieties, both with and

without a thermal pad.

Package Without Thermal Pad

The AD8222 ships with a package that does not include a thermal

pad; it is the preferred package for the AD8222. Unlike chip

scale packages where the pad limits routing capability, the AD8222

package allows routes and vias directly underneath the chip, so

that the full space savings of the small LFCSP can be realized.

Although the package has no metal in the center of the device,

the manufacturing process does leave a very small section of

exposed metal at each of the package corners, shown in Figure 56

in the Outline Dimensions section. This metal is connected to

−VS through the device. Because of a possibility of a short, vias

should not be placed underneath this exposed metal.

Package with Thermal Pad

This package is included primarily for legacy reasons. Because

the AD8222 dissipates so little power, there is little need for the

thermal pad.

The thermal pad is connected internally to −VS. The pad can

either be left unsoldered, soldered to an otherwise unconnected

PCB landing, or soldered to a landing connected to the negative

supply rail (−VS). If pin compatibility with the AD8224 is

desired, the pad should not be electrically connected to any net,

including −VS.

The solder process can leave flux and other contaminants on

the board. When these contaminants are between the AD8222

leads and thermal pad, they can create leakage paths that are

larger than the bias currents of the AD8222. A thorough

washing process removes these contaminants and restores the

excellent bias current performance of the AD8222.

LAYOUT

The AD8222 is a high precision device. To ensure optimum

performance at the PC board level, take care in the design of the

board layout. The AD8222 pinout is arranged in a logical

manner to aid in this task.

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. Take care to tie REF to the appropriate local ground.

Power Supplies

Use a stable dc voltage 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). Although the device

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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