ADS823 datasheet(13/20 Pages) TI1

13 / 20 page

ADS823, ADS826

SBAS070B

www.ti.com

GROUNDING AND DECOUPLING

Proper grounding and bypassing, short lead length, and the

use of ground planes are particularly important for high-

frequency designs. Multilayer PC boards are recommended

for best performance since they offer distinct advantages like

minimizing ground impedance, separation of signal layers by

ground layers, etc. The ADS823 and ADS826 should be

treated as analog components. Whenever possible, the sup-

ply pins should be powered by the analog supply. This will

ensure the most consistent results, since digital supply lines

often carry high levels of noise which otherwise would be

coupled into the converter and degrade the achievable per-

formance. All ground connections on the ADS823 and ADS826

are internally joined together, obviating the design of split

ground planes. The ground pins (1, 16, 26) should directly

connect to an analog ground plane which covers the PC

board area around the converter. While designing the layout,

it is important to keep the analog signal traces separated

from any digital lines to prevent noise coupling onto the

analog signal path. Due to the high sampling rate, the

ADS823 and ADS826 generate high frequency current tran-

sients and noise (clock feedthrough) that are fed back into

the supply and reference lines. This requires that all supply

and reference pins are sufficiently bypassed. Figure 10

FIGURE 10. Recommended Bypassing for the Supply Pins.

GND

ADS823

ADS826

0.1

µF

0.1

µF

GND

µF

+5V

VDRV

0.1

µF

+3/+5V

shows the recommended decoupling scheme for the ADS823

and ADS826. In most cases 0.1

µF ceramic chip capacitors at

each pin are adequate to keep the impedance low over a

wide frequency range. Their effectiveness largely depends

on the proximity to the individual supply pin. Therefore, they

should be located as close to the supply pins as possible. In

addition, a larger bipolar capacitor (1

µF to 22µF) should be

placed on the PC board in proximity of the converter circuit.

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