AD8295BCPZ-R7 datasheet(20/28 Pages) AD

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AD8295

Rev. 0 | Page 20 of 2

Reference Terminal

The output voltage of the AD8295 instrumentation amplifier

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 dc level.

The reference pin input can be driven slightly beyond the rails.

The REF pin is protected with ESD diodes, and the REF voltage

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

For best performance, the source impedance to the REF terminal

should be kept below 1 Ω. Additional impedance at the REF

terminal can significantly degrade the CMRR of the amplifier.

When the reference source has significant output impedance

(for example, a resistive voltage divider), buffer the signal before

driving the REF pin. Internal Op Amp A1 or A2 can be used for

this purpose, as shown in Figure 54.

+VS

REF

AD8295

REF

+VS

OP AMP

BUFFER

AD8295

INCORRECT

CORRECT

Figure 54. Driving the Reference Pin

Noise at the reference feeds directly to the output. Therefore, in

Figure 54, Capacitor C is added to filter out any high frequency

noise on the positive power supply line. For very clean supplies,

the capacitor may not be needed. The filter frequency is a trade-

off between noise rejection and start-up time, and is given by

the following equation:

LOWPASS

LAYOUT

The AD8295 is a high precision device. To ensure optimum

performance at the PCB level, care must be taken in the board

layout. The AD8295 pins are arranged in a logical manner to

aid in this task. Unlike most LFCSP packages, the AD8295

package was designed without the thermal pad to allow routes

and vias directly beneath the chip.

Careful board layout maximizes system performance. Traces

from the gain setting resistor to the RG pins should be kept as

short as possible to minimize parasitic inductance. To ensure

the most accurate output, the trace from the REF pin should

either be connected to the local ground of the AD8295 or to a

voltage that is referenced to the local ground of the AD8295.

Common-Mode Rejection over Frequency

The AD8295 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. The AD8295 pinout

and hidden paddle package were designed so that the board

designer can take full advantage of this performance with a

well-implemented layout.

Poor layout can cause some of the common-mode signal to be

converted to a differential signal before it reaches the in-amp.

Such conversions occur when one input path has a frequency

response that is different from the other. To keep CMRR across

frequency high, the input source impedance and capacitance of

each path should be closely matched. Additional source resistance

in the input path (for example, for input protection) should be

placed close to the in-amp inputs to minimize their interaction

with parasitic capacitance from the PCB traces.

Parasitic capacitance at the gain setting pins can also affect

CMRR over frequency. The traces to the RG resistor should be

kept as short as possible. If the board design has a component at

the gain setting pins (for example, a switch or jumper), the part

should be chosen so that the parasitic capacitance is as small as

possible.

Unused Op Amps

When not in use, the internal op amps should be connected

in a unity-gain configuration, with the noninverting input

connected to a bias point in the input range of the op amp.

These connections ensure that the AD8295 op amp uses

minimum power and does not disturb the internal power

supplies of the AD8295. These connections are shown as

dotted lines in several of the applications figures.

Reference

The output voltage of the instrumentation amplifier section of

the AD8295 is developed with respect to the potential on the

reference terminal (REF); care should be taken to tie the REF

pin to the appropriate local ground.

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