Q
What are some common pitfalls to avoid while designing with AD8422?
A
AD8422 is a high performance precision instrumentation amplifier. In order to
achieve the optimum performance, it must be applied properly.
Floating sensors: Instrumentation amplifier inputs are not isolated, and
therefore cannot measure floating voltages. For floating sensors, such as
thermocouples or transformers, a dc path to ground must be provided for the
bias current. This guarantees that the AD8422 inputs don’t float to the edge of
the input range and saturate.
For single-supply applications, the inputs must be biased to a voltage within
the input range, such as mid-supply. See AN0937
(http://www.analog.com/static/imported-files/application_notes/AN-937.pdf) for
some more single-supply in-amp considerations.
REF input impedance: AD8422’s output is developed relative to the voltage at
the REF pin, which is most often tied directly to ground. If the output must be
level-shifted, the REF pin can be driven to accomplish this. However, in order
to avoid excessive CMRR and gain errors, the REF pin must be driven with a
low-impedance source.
RFI Rectification: The AD8422 is a precision amplifier. Therefore, when
high-frequency signals such as RF couple into the inputs, they can be rectified
into a dc voltage at the output. Since these signals vary with time, this
rectification can appear as a train of pulses, which cannot be removed by
additional filtering or calibration. The data sheet recommends an RFI filter to
reduce RFI rectification. In applications with higher levels of RF
interference, chip ferrite beads or common-mode chokes may be used at the
inputs to further attenuate RF signals. See A Designers Guide to
Instrumentation Amplifiers for more information.
More considerations for applying the AD8422 can be found in the “THEORY OF
OPERATION” section of the data sheet.