Datasheet

ADS823, ADS826
SBAS070B
13
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.
+V
S
27
26
GND
ADS823
ADS826
+
0.1µF 0.1µF
+V
S
15
16
GND
10µF
+5V
VDRV
28
0.1µF
+3/+5V
shows the recommended decoupling scheme for the ADS823
and ADS826. In most cases 0.1µF 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 (1µF to 22µF) should be
placed on the PC board in proximity of the converter circuit.