AD7869
–12–
AD7869–TMS32020 Interface
Figure 18 shows an interface that is suitable for the TMS32020/
TMS320C25 processors. This interface is configured for syn-
chronous, continuous clock operation. Note the AD7869 will
not correctly interface to these processors if the AD7869 is con-
figured for a noncontinuous clock. Conversion starts and DAC
updating are controlled by an external timer.
TMS32020/
TMS320C25
FSX
DX
TCLK
DT
TFS
*ADDITIONAL PINS OMITTED FOR CLARITY
LDAC
FSR
CLKR
DR
RCLK
DR
CONVST
RFS
CONTROL
TIMER
AD7869*
4.7kΩ 2kΩ 4.7kΩ
5V
+
5V
–
CLKX
Figure 18. AD7869–TMS32020/TMS32025 Interface
APPLICATION HINTS
Good printed circuit board (PCB) layout is as important as the
circuit design itself in achieving high speed A/D performance.
The AD7869’s comparator is required to make bit decisions on
an LSB size of 366 µV. To achieve this, the designer has to be
conscious of noise both in the ADC itself and in the preceding
analog circuitry. Switching mode power supplies are not recom-
mended as the switching spikes will feed through to the com-
parator causing noisy code transitions. Other causes of concern
are ground loops and digital feedthrough from microprocessors.
These are factors that influence any ADC, and a proper PCB
layout that minimizes these effects is essential for best
performance.
LAYOUT HINTS
Ensure that the layout for the printed circuit board has the digi-
tal and analog signal lines separated as much as possible. Take
care not to run any digital track alongside an analog signal track.
Guard (screen) the analog input with AGND.
Establish a single point analog ground (star ground), separate
from the logic system ground, as close as possible to the
AD7869 AGND pins. Connect all other grounds and the
AD7869 DGND to this single analog ground point. Do not
connect any other digital grounds to this analog ground point.
Low impedance analog and digital power supply common re-
turns are essential to low noise operation of the ADC, so make
the foil width for these tracks as wide as possible. The use of
ground planes minimizes impedance paths and also guards the
analog circuitry from digital noise. The circuit layout of Figures
22 and 23 have both analog and digital ground planes that are
kept separated and only joined together at the AD7869 AGND
pins.
NOISE
Keep the input signal leads to V
IN
and signal return leads from
AGND as short as possible to minimize input noise coupling. In
applications where this is not possible, use a shielded cable be-
tween the source and the ADC. Reduce the ground circuit im-
pedance as much as possible since any potential difference in
grounds between the signal source and the ADC appears as an
error voltage in series with the input signal.
INPUT/OUTPUT BOARD
Figure 19 shows an analog I/O board based on the AD7869.
The corresponding printed circuit (PC) board layout and
silkscreen are shown in Figures 21 to 23.
The analog input to the AD7869 is buffered with an AD711 op
amp. There is a component grid provided near the analog input
on the PC board that may be used for an antialiasing filter for
the ADC or a reconstruction filter for the DAC or any other
conditioning circuitry. To facilitate this option, there are two
wire links (labeled LK1 and LK2) required on the analog input
and output tracks.
The board contains a SHA circuit that can be used on the out-
put of the AD7869 DAC to extend the very good performance
of the part over a wider frequency range. The increased perfor-
mance from the SHA can be seen from Figures 14 and 15 of
this data sheet. A wire link (labeled LK3) connects the board
output to either the SHA output or directly to the AD7869
DAC output .
There are three
LDAC link options on the board; LDAC can be
driven from an external source independent of
CONVST,
LDAC can be tied to CONVST or LDAC can be tied to GND.
Choosing the latter option disables the SHA operation and
places the SHA permanently in the track mode.
Microprocessor connections to the board are made by a 9-way
D-type connector. The pinout is shown in Figure 20. The
ADC’s digital outputs are buffered with 74HC4050s. These
buffers provide a higher current output capability for high
capacitance loads or cables. Normally, these buffers are not re-
quired as the AD7869 will be sitting on the same board as the
processor.
POWER SUPPLY CONNECTIONS
The PC board requires two analog power supplies and one 5 V
digital supply. Connections to the analog supply are made di-
rectly to the PC board as shown on the silkscreen in Figure 21.
The connections are labeled V+ and V–, and the range for both
of these supplies is 12 V to 15 V. Connections to the 5 V digital
supply are made through the D-type connector SKT6. The
±5 V analog supply required by the AD7869 is generated from
two voltage regulators on the V+ and V– supplies.
WIRE LINK OPTIONS
LK1, Analog Input Link
LK1 connects the analog input to a component grid or to a
buffer amplifier which drives the ADC input.
LK2, Analog Output Link
LK2 connects the analog output to the component grid or to ei-
ther the SHA or DAC output (see LK3).
LK3, SHA or DAC Select
The analog output may be taken directly from the DAC or from
a SHA at the output of the DAC.
LK4, DAC Reference Selection
The DAC reference may be connected to either the ADC refer-
ence output (RO ADC) or to the DAC reference (RO DAC).
