GROUNDING AND DECOUPLING
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 its high sampling rate, the
ADS2806 generates high-frequency current transients and
noise (clock feedthrough) that are fed back into the supply
and reference lines. This requires that all supply and refer-
ence pins are sufficiently bypassed. Figure 8 shows the
recommended decoupling scheme for the ADS2806. 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. If system
supplies are not a low enough impedance, adding a small
tantalum capacitor will yield the best results.
Proper grounding, bypassing, short trace lengths, and the
use of power and ground planes are particularly important
for high-frequency designs. Multilayer PC boards are rec-
ommended for best performance since they offer distinct
advantages, such as minimizing ground impedance, sepa-
ration of signal layers by ground layers, etc. The ADS2806
should be treated as an analog component. Whenever
possible, the supply 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 that
otherwise would be coupled into the converter and degrade
the achievable performance. The ground pins should di-
rectly connect to an analog ground plane that covers the PC
board area under the converter. While designing the layout
ADS2806
+VS
57
+VS
3 (46)
+VS
5 (43)
GND
55, 58
GND
1, 2, 64
(47, 48, 49)
GND
4 (44)
GND
7 (41)
VDRV
8 (40)
GND
23, 25
0.1µF
0.1µF
0.1µF
0.1µF
+5V
+3V/+5V
Numbers in Parenthesis Indicate Pins for Channel A
FIGURE 8. Recommended Bypassing for the Supply Pins.
ADS2806
SBAS178B
16
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