AD8221
GAIN SELECTION
Placing a resistor across the R
G
terminals set the gain of
AD8221, which can be calculated by referring to Table 5 or
by using the gain equation.
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.
R
G
=
49.4 kΩ
G
−
1
Calculated Gain
1.990
4.984
9.998
19.93
50.40
100.0
199.4
495.0
991.0
Table 5. Gains Achieved Using 1% Resistors
1% Standard Table Value of R
G
(Ω)
49.9 k
12.4 k
5.49 k
2.61 k
1.00 k
499
249
100
49.9
The AD8221 defaults to G = 1 when no gain resistor is used.
Gain accuracy is determined by the absolute tolerance of R
G
.
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 R
G
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.
03149-044
Figure 44. Top Layer of the AD8221-EVAL
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.
Figure 45. Bottom Layer of the AD8221-EVAL
Rev. B | Page 17 of 24
03149-045
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