Analog Input Interface - Analog Devices ADSP-21992 EZ-KIT LITE Manual

The source of reset for the ADSP-21992 can be chosen by configuring jumper JP2. When JP2 is
in the 2-3 position, the POR output pin of the ADSP-21992 is tied to the reset pin of the ADSP-
21992. When JP2 is in the 1-2 position, the source of reset for the ADSP-21992 is the reset button
(S1), the external power-on reset IC (ADM708), or a reset command from VisualDSP++ through
the USB interface. The 1-2 position is the default and recommended configuration when using the
VisualDSP++ development tools.

4.2.3 Analog Input Interface

The ADSP-21992 EZ-KIT Lite board permits up to eight analog inputs to be fed from the analog
inputs connector, P4, to the eight ADC channels of the ADSP-21992. All eight analog inputs at
the analog inputs connector may range from -1V to +1V.
There is a separate interface circuit for each of the eight ADC channels of the ADSP-21992.
These analog interface circuits convert the nominal ±1 V signals at the analog inputs connector
P4 to signals centered on the ADSP-21992 reference voltage level (either the internally derived
1.0V level or the externally provided 1.024V level). The analog interface circuits, consisting of
high-performance operational amplifiers and precision resistors, effectively offset the analog
inputs by the reference voltage level.
Two AD8044 quad operational amplifiers (A5 & A6) are used for the analog interface and are
configured as summing unity-gain stages. A5 is used to interface the VIN0, VIN1, VIN2, and
VIN3 analog inputs. A6 is used to buffer the VIN4, VIN5, VIN6, and VIN7 inputs.
Precision 10 KΩ resistors for input and feedback ensure accurate gain matching of all channels.
In addition, 47 pF feedback capacitors provide simple low-pass filtering with a very high cut-off
frequency (339 kHz) on all analog inputs.
The analog inputs are applied to the ADSP-21992 in a single-ended fashion, so that the inverting
inputs to the sample-and-hold amplifiers of the ADSP-21992 (ASHAN and BSHAN) are
connected to a buffered version of the reference voltage. A representation of the analog interface
circuit for one of the ADC channels is shown in
end of this document, each analog input stage also contains a small RC filter at the operational
amplifier output.
Figure 4-3: One Analog Interface Circuit of ADSP-21992 EZ-KIT Lite Board
ADSP-21992 EZ-KIT Lite Evaluation System Manual
R
R
V1
R
VREF
Figure 4-3. As shown in the schematics at the
C
R
+AVDD
-
VIN1
AD8044
+
-AVDD
4-31