Analog Front End (Afe) Considerations; Input Signal Filtering; Power Vs. Bandwidth Vs. Noise; Gain - Analog Devices EVAL-AD4080-FMCZ User Manual

User Guide EVAL-AD4080

ANALOG FRONT END (AFE) CONSIDERATIONS

The AFE of EVAL-AD4080-FMCZ can be modified to suit the
needs of specific applications. Applying changes to the AFE usually
involves trade-offs, and designing the right AFE for an application
requires careful consideration. The following sections discuss some
of the items that must be taken into consideration when designing
or modifying the AFE for this hardware evaluation platform.

INPUT SIGNAL FILTERING

Limiting the bandwidth at the input of the signal chain to the region
where the signal of interest lies helps reduce excess noise. This
reduction of noise can be achieved by the provided mechanisms in
the form of the RC filter at the board input, the addition of the do not
install (DNI) capacitors in the amplifier feedback network bandwidth
adjustment in Stage 1, or the increasing of the capacitance value
for the existing capacitors in the FDA feedback loop. Note that the
value of filter resistors may have an impact on the overall SNR.
The internal digital filters available inside the AD4080 are a useful
feature when considering the filtering in the signal chain as a whole.
The user can programmatically set a low pass filter, choosing a
simple sinc1 filter or a higher order sinc5 with a sharp roll-off to
compliment the AFE filter or to help relax its design requirements.

POWER vs. BANDWIDTH vs. NOISE

Choosing the lowest noise, lowest distortion, highest precision, wid-
er bandwidth amplifiers may require more power to be consumed in
the AFE to reach the required target performance. Therefore, power
constrained applications must carefully consider the choice of each
amplifier.

GAIN

Care must be taken in the signal chain to consider where it is
optimum to place gain to maximize the SNR. As an example, it may
be better to add the required signal gain in a preceding low noise
amplifier stage rather than in the FDA that is tasked with driving the
AD4080 because the FDA noise gain may have a bigger impact on
the signal chain SNR than the gain set by the preceding lower noise
stage.

ADC DRIVER STAGE

See the Easy Drive Analog Inputs section in the AD4080 data sheet
for details about this topic.
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