AD8220BRMZ-R7 datasheet(20/28 Pages) AD

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AD8220

Rev. 0 | Page 20 of 28

GAIN SELECTION

Placing a resistor across the RG terminals sets the AD8220 gain,

which can be calculated by referring to Table 5 or by using the

gain equation

−

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 AD8220 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. In

applications that need to take advantage of the AD8220’s low

input bias current, avoid placing metal under the input path to

minimize leakage current. To maintain high CMRR over

frequency, layout the input traces symmetrically and layout the

RG resistor’s traces symmetrically. Ensure that the traces maintain

resistive and capacitive balance; this holds for additional PCB

metal layers under the input and RG pins. Traces from the gain

setting resistor to the RG pins should be kept as short as possible

to minimize parasitic inductance. An example layout is shown in

Figure 57 and Figure 58. To ensure the most accurate output, the

trace from the REF pin should either be connected to the

AD8220 local ground (see Figure 59) or connected to a voltage

that is referenced to the AD8220 local ground.

Figure 57. Example Layout. Top Layer of the AD8220 Evaluation Board

Figure 58. Example Layout. Bottom Layer of the AD8220 Evaluation Board

Common-Mode Rejection

The AD8220 has high CMRR over frequency which gives it

greater immunity to disturbances such as line noise and its

associated harmonics in contrast to typical in-amps whose

CMRR falls off around 200 Hz. These in-amps often need

common-mode filters at the inputs to compensate for this

shortcoming. The AD8220 is able to reject CMRR over a greater

frequency range, reducing the need for input common-mode

filtering.

A well implemented layout helps to maintain the AD8220’s high

CMRR over frequency. Input source impedance and capacitance

should be closely matched. In addition, source resistance and

capacitance should be placed as close to the inputs as possible.

Grounding

The AD8220’s output voltage is developed with respect to the

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

REF to the appropriate local ground (see Figure 59).

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 can

cause a large error. Therefore, separate analog and digital

ground returns should be used to minimize the current flow

from sensitive points to the system ground.

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