AD7883

REV. 0

–9–

APPLICATION HINTS

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

circuit design itself in achieving high speed A/D performance.

The AD7883’s comparator is required to make bit decisions on

an LSB size of 0.8 mV. To achieve this, the designer must 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 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 digital tracks alongside analog signal tracks.

Guard (screen) the analog input with AGND.

Establish a single point analog ground (star ground) separate

from the logic system ground at the AD7883 AGND pin or as

close as possible to the AD7883. Connect all other grounds and

the AD7883 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.

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.

V+

+

–

C1

10

µF

C2

0.1

µF

IC1

ANALOG

INPUT

V+

V–

AB

V–

AB

LK2

LK3

TO ADC

LK1

SKT1

C3

10

µF

C4

0.1

µF

V+

Figure 13. Analog Input Buffering

ANALOG INPUT BUFFERING

To achieve specified performance, it is recommended that the

source. This necessitates the use of an input buffer amplifier.

The choice of op amp will be a function of the particular appli-

cation and the desired analog input range.

A single supply op amp is recommended for such an implemen-

tation. This will allow for operation of the AD7883 in the 0 to

and V– of the op amp. Recommended single-supply op amps

are the OP-195 and AD820.

In bipolar operation, positive and negative supplies must be

connected to V+ and V– of the op amp.

The AD711 is a general purpose op amp which could be used

to drive the analog input of the AD7883, in this input range.