AD7871/AD7872

–13–

REV. D

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 a 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 at the AD7871/AD7872 AGND

pin or as close as possible to the AD7871/AD7872. Connect all

other grounds and the AD7871/AD7872 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

26 and 27 have both analog and digital ground planes that are

kept separated and joined together only at the AD7871/AD7872

AGND pin.

NOISE

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.

DATA ACQUISITION BOARD

Figure 24 shows the AD7871/AD7872 in a data acquisition cir-

cuit. The corresponding printed circuit board (PCB) layout has

three interface ports: one serial and two parallel. Note that the

AD7871/AD7872 serial lines are buffered by a 74HC244. This

allows long lines with large capacitive loads to be driven. One of

the parallel ports is directly compatible with the ADSP-2100

evaluation board expansion connector.

The only additional component required for a full data acquisi-

tion system is an anti-aliasing filter. There is a component grid

provided near the analog input on the PCB, which may be used

for such a filter or any other input conditioning circuitry. To fa-

cilitate this option, there is a shorting plug (labelled LK1 on the

PCB) on the analog input track. If this shorting plug is used, the

analog input connects to the buffer amplifier driving the AD7871/

AD7872; if this shorting plug is omitted, a wire link can be used to

connect the analog input to the PCB component grid.

INTERFACE CONNECTIONS

There are two parallel connectors labeled SKT4 and SKT6,

and one serial connector labeled SKT5. A shorting plug option

(LK3 in Figure 24) configures the ADC for the appropriate

interface.

SKT6 is a 96-contact (3-row) Eurocard connector that is directly

compatible with the ADSP-2100 Evaluation Board Prototype

Expansion Connector. The expansion connector on the

ADSP-2100 has eight decoded chip enable outputs labeled

ECE1 to ECE8. ECE6 is used to drive the AD7871

CS input

on the board. To avoid selecting the onboard RAM sockets at

the same time, LK6 on the ADSP-2100 board must be removed.

In addition, the ADSP-2100 expansion connector has four in-

terrupts labelled EIRQ0 to EIRQ3. The AD7871

BUSY/INT

output connects to EIRQ0. There is a single wait state generator

connected to EDMACK to allow the AD7871 to interface to the

faster versions of the ADSP-2100.

SKT4 is a 26-way (2-row) IDC connector. This contains the

same signal contacts as SKT6 except for EDMACK, which is

connected to SKT6 only. It also contains decoded R/

W and

STRB inputs necessary for TMS32020 interfacing.

SKT5 is a 5-way D-type connector meant for serial interfacing

only. An inverted DB11/SCLK output is also provided on this

connector for systems that accept data on a rising clock edge.

SKT1, SKT2 and SKT3 are three BNC connectors providing

connections for the analog input, the

CONVST input and an

external clock.

Figure 22. SKT4 Pinout

Figure 23. SKT5 Pinout

POWER SUPPLY CONNECTIONS

The PCB requires two analog power supplies and one 5 V logic

supply. The analog supplies are labelled V+ and V–, and the

range for both supplies is 12 V to 15 V. Connection to the 5 V

digital supply is made through any of the connectors SKT4 to

SKT6. The

±5 V supply required by the AD7871 and AD7872

is generated from voltage regulators on the V+ and V– power

supplies input (IC6 and IC7 in Figure 24).

SHORTING PLUG OPTIONS

There are seven shorting plug options which must be set before

using the board. These are outlined below:

LK1

Connects the analog input to a buffer amplifier. The

analog input may also be connected to a component

grid for signal conditioning.

LK2

Selects either the AD7871/AD7872 internal clock or

an external clock source.

LK3

Configures the AD7871 14/

8/CLK input for the

appropriate serial or parallel interface.

LK4

Connects the AD7871

RD input directly to the two par-

allel connectors or to a decoded

STRB and R/W input.

LK5

Connects the pull-up resistor R3 to

SSTRB.

LK6

Connects the pull-up resistor R4 to SCLK.

LK7

Connects the pull-up resistor R5 to SDATA.

Note that LK5 to LK7 should be removed for parallel interfacing.