AD7242/AD7244

REV. A

–11–

APPLYING THE AD7242/AD7244

Good printed circuit board layout is as important as the overall

circuit design itself in achieving high speed converter perfor-

mance. The AD7242 works on an LSB size of 1.465 mV, while

the AD7244 works on an LSB size of 366

µV. Therefore, the

designer must be conscious of minimizing noise in both the

converter itself and in the surrounding circuitry. Switching

mode power supplies are not recommended as the switching

spikes can feed through to the on-chip amplifier. Other causes

of concern are ground loops and digital feedthrough from

microprocessors. These are factors that influence any high

performance converter, 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 separated

digital and analog lines as much as possible. Take care not to

run any digital track alongside an analog signal track. Establish a

single point analog ground (star ground) separate from the logic

system ground. Place this star ground as close as possible to the

AD7242/AD7244. Connect all analog grounds to this star

ground and also connect the AD7242/AD7244 DGND pins to

this ground. Do not connect any other digital grounds to this

analog ground point.

Low impedance analog and digital power supply common

returns are essential to low noise operation of high performance

converters. Therefore, the foil width for these tracks should be

kept as wide as possible. The use of ground planes minimizes

impedance paths and also guards the analog circuitry from

digital noise.

NOISE

signal return leads to AGND as short as possible to minimize

noise coupling. In applications where this is not possible, use a

shielded cable between the DAC outputs and their destination.

Reduce the ground circuit impedance as much as possible since

any potential difference in grounds between the DAC and its

destination device appears as an error voltage in series with the

DAC output.