Evaluation Board Circuitry; Analog Inputs; Reference; Serial Interface - Analog Devices EVAL-AD7903SDZ User Manual

EVAL-AD7903SDZ User Guide

EVALUATION BOARD CIRCUITRY

ANALOG INPUTS

The analog inputs to the evaluation board are SMA Connectors J6,
J7, J10, and J15. These inputs are buffered with dedicated amplifier
circuitry (U12, U14, U16, and U17), as shown in Figure 26 and
Figure 27. The circuit not only allows different configurations,
input range scaling, and filtering but also allows adding a dc
component and using different op amps and supplies. The analog
input amplifiers are set as unity-gain buffers by factory default.
The amplifier positive rail is driven from 7.5 V (from U13—
ADP7104). The negative amplifier rail is driven from –V
(generated by U3—ADP2301).
The default configuration sets all op amps (U12, U14, U16, and
U17) at midscale, generated from a buffered reference voltage
divider (VCM).
The evaluation board is configured by factory default for providing
a differential analog input path to ADC1 and ADC2 (see Figure 16).
For dynamic performance, an FFT test can be done by applying
a very low distortion ac source.
For low frequency testing, the audio precision source can be
used directly because its outputs are isolated. Set the outputs for
a balanced and floating ground. Different sources can be used;
however, most are single-ended sources that use a fixed output
resistance.
By default, the evaluation board uses the amplifiers as unity-gain
buffers. The noninverting input has a common-mode adjustment
that involves a 49.9 kΩ shunt resistor. This resistor creates a
voltage divider with the source, which must be accounted for
when using the board.

REFERENCE

S
An external 5 V reference (U6—ADR435) is used to supply the
ADCs directly.

SERIAL INTERFACE

The evaluation board uses the SPORT interface from the
ADSP-BF527
Multiple AND gates (U9, U10, and U11) are used to clock and
gate the SPORT transfer to the ADC device.
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DSP.
UG-609