AD7875LP datasheet(15/20 Pages) AD

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REV. B

AD7870/AD7875/AD7876

–15–

STAND-ALONE OPERATION

The AD7870/AD7875/AD7876 can be used in its Mode 2, par-

allel interface mode for stand-alone operation. In this case, con-

version is initiated with a pulse to the ADC

CS input. This

pulse must be longer than the conversion time of the ADC. The

BUSY output is used to drive the RD input. Data is latched

from the ADC DB0–DB11 outputs to an external latch on the

rising edge of

BUSY.

Figure 25. Stand-Alone Operation

APPLICATION HINTS

Good printed circuit board (PCB) layout is as important as the

overall circuit design itself in achieving high speed A/D perfor-

mance. 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 recommended as the switching

spikes will feed through to the comparator causing noisy code

transitions. Other causes of concern are ground loops and digi-

tal feedthrough from microprocessors. These are factors which

influence any ADC, and a proper PCB layout which 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 at the AGND pin or as close as

possible to the ADC. Connect all other grounds and the

AD7870/AD7875/AD7876 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

30 and 31 have both analog and digital ground planes which are

kept separated and only joined together at the AD7870/

AD7875/AD7876 AGND pin.

NOISE

Keep the input signal leads to VIN 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.

DATA ACQUISITION BOARD

Figure 28 shows the AD7870/AD7875/AD7876 in a data acqui-

sition circuit. The corresponding printed circuit board (PCB)

layout and silkscreen are shown in Figures 29 to 31. The board

layout has three interface ports: one serial and two parallel. 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 antialiasing filter. A component grid is pro-

vided near the analog input on the PCB, which may be used for

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

tate 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 ADC;

if this shorting plug is omitted, a wire link can be used to con-

nect 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 28) on the ADC 12/

8/CLK input configures

the ADC for the appropriate interface (see Pin Function

Description).

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 ADC CS input on

the data acquisition board. To avoid selecting on board RAM

sockets at the same time, LK6 on the ADSP-2100 board must

be removed. In addition, the ADSP-2100 expansion connector

has four interrupts labelled

EIRQ0 to EIRQ3. The ADC BUSY/

INT output connects to EIRQ0. There is a single wait state gen-

erator connected to EDMACK to allow the ADC to interface to

the faster versions of the ADSP-2100.

SKT4 is a 26-way (2-ROW) IDC connector. This connector

contains all the signal contacts as SKT6 with the exception of

EDMACK which is connected to SKT6 only. It also contains

decoded R/

W and STRB inputs which are necessary for

TMS32020 interfacing. The SKT4 pinout is shown in Fig-

ure 26.

Figure 26. SKT4, IDC Connector Pinout

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