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AD7988-1_1208 Datasheet(PDF) 21 Page - Analog Devices

Part No. AD7988-1_1208
Description  16-Bit Lower Power PulSAR ADCs in MSOP/LFCSP (QFN)
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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AD7988-1_1208 Datasheet(HTML) 21 Page - Analog Devices

 
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Data Sheet
AD7988-1/AD7988-5
Rev. C | Page 21 of 24
APPLICATIONS INFORMATION
INTERFACING TO BLACKFIN® DSP
The AD7988-x can easily connect to a DSP SPI or SPORT. The
SPI configuration is straightforward, using the standard SPI
interface as shown in Figure 40.
AD7988-1/
AD7988-5
SCK
SDO
CNV
SPI_CLK
SPI_MISO
SPI_MOSI
BLACKFIN
DSP
Figure 40. Typical Connection to Blackfin SPI Interface
Similarly, the SPORT interface can be used to interface to this
ADC. The SPORT interface has some benefits in that it can use
direct memory access (DMA) and provides a lower jitter CNV
signal generated from a hardware counter.
Some glue logic may be required between SPORT and the
AD7988-x interface. The evaluation board for the AD7988-x
interfaces directly to the SPORT of the Blackfin-based (ADSP-
BF-527) SDP board. The configuration used for the SPORT
interface requires the addition of some glue logic as shown in
Figure 41. The SCK input to the ADC was gated off when CNV
was high to keep the SCK line static while converting the data,
thereby ensuring the best integrity of the result. This approach
uses an AND gate and a NOT gate for the SCK path. The other
logic gates used on the RSCLK and RFS paths are for delay
matching purposes and may not be necessary where path
lengths are short.
This is one approach to using the SPORT interface for this ADC;
there may be other solutions equal to this approach.
SCK
SDO
CNV
TSCLK
DR
TFS
RFS
RSCLK
VDRIVE
AD7988-1/
AD7988-5
BLACKFIN
DSP
Figure 41. Evaluation Board Connection to Blackfin Sport Interface
LAYOUT
Design the printed circuit board (PCB) that houses the AD7988-x
so that the analog and digital sections are separated and confined
to certain areas of the board. The pinout of the AD7988-x, with all
the analog signals on the left side and all the digital signals on
the right side, eases this task.
Avoid running digital lines under the device because these couple
noise onto the die, unless a ground plane under the AD7988-x is
used as a shield. Fast switching signals, such as CNV or clocks,
should never run near analog signal paths. Avoid crossover of
digital and analog signals.
Using at least one ground plane is recommended. It can be
common or split between the digital and analog section. In the
latter case, join the planes underneath the AD7988-x devices.
The AD7988-x voltage reference input, REF, has a dynamic input
impedance. Decouple REF with minimal parasitic inductances
by placing the reference decoupling ceramic capacitor close to,
but ideally right up against, the REF and GND pins and connect-
ing them with wide, low impedance traces.
Finally, decouple the power supplies of the AD7988-x, VDD and
VIO, with ceramic capacitors, typically 100 nF, placed close to
the AD7988-x and connected using short and wide traces to
provide low impedance paths and to reduce the effect of glitches
on the power supply lines.
An example of a layout following these rules is shown in Figure 42
and Figure 43.
EVALUATING THE PERFORMANCE OF THE
AD7988-x
The evaluation board package for the AD7988-x (EVAL-AD7988-
5SDZ) includes a fully assembled and tested evaluation board,
documentation, and software for controlling the board from a
PC via the EVAL-SDP-CB1Z.
AD7988-1/
AD7988-5
Figure 42. Example Layout of the AD7988-x (Top Layer)
Figure 43. Example Layout of the AD7988-x (Bottom Layer)


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