AD8221ARZ-R7 datasheet(17/24 Pages) AD

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AD8221

Rev. B | Page 17 of 24

GAIN SELECTION

Placing a resistor across the RG terminals set the gain of

AD8221, which can be calculated by referring to Table 5 or

by using the gain equation.

kΩ

−

Table 5. Gains Achieved Using 1% Resistors

1% Standard Table Value of RG (Ω)

Calculated Gain

49.9 k

1.990

12.4 k

4.984

5.49 k

9.998

2.61 k

19.93

1.00 k

50.40

499

100.0

249

199.4

100

495.0

49.9

991.0

The AD8221 defaults to G = 1 when no gain resistor is used.

Gain accuracy is determined by the absolute tolerance of RG.

The TC of the external gain resistor increases the gain drift of

the instrumentation amplifier. Gain error and gain drift are kept

to a minimum when the gain resistor is not used.

LAYOUT

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 AD8221, as shown

in Figure 46, or connected to a voltage that is referenced to the

local ground of the AD8221.

Common-Mode Rejection

One benefit of the high CMRR over frequency of the AD8221 is

that it has greater immunity to disturbances, such as line noise

and its associated harmonics, than do typical instrumentation

amplifiers. Typically, these amplifiers have CMRR fall-off at

200 Hz; common-mode filters are often used to compensate for

this shortcoming. The AD8221 is able to reject CMRR over a

greater frequency range, reducing the need for filtering.

A well implemented layout helps to maintain the high CMRR

over frequency of the AD8221. Input source impedance and

capacitance should be closely matched. In addition, source

resistance and capacitance should be placed as close to the

inputs as permissible.

Grounding

The output voltage of the AD8221 is developed with respect to

the potential on the reference terminal. Care should be taken to

tie REF to the appropriate local ground.

In mixed-signal environments, low level analog signals need to

be isolated from the noisy digital environment. Many ADCs

have separate analog and digital ground pins. Although it is

convenient to tie both grounds to a single ground plane, the

current traveling through the ground wires and PC board may

cause hundreds of millivolts of error. Therefore, separate analog

and digital ground returns should be used to minimize the

current flow from sensitive points to the system ground. An

example layout is shown in Figure 44 and Figure 45.

Figure 44. Top Layer of the AD8221-EVAL

Figure 45. Bottom Layer of the AD8221-EVAL

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