Q
What are the power and decoupling caps requirements for the AD7656?
A
As you can see from the figures below the static current drawn by the AD7656 is
the same as the operational current this is as a result of the comparator used
on the AD7656, the comparator draws more current during track mode. The VDD and
VSS supplies are only used for the track and hold switches on the analog input
and they don’t require much current, at 250 ksps they require 250 uA each.
These supplies will draw little current when in power down. The Avcc supply
takes most of the current as this is the supply used for the Ad7656 cores. This
is the supply that is used for the conversion process.
Here is the min decoupling that the part requires
AVCC pins 46/47 - 100uf poly tantalum.
AVCC pins 43/44 - 100uf poly tantalum.
AVCC pins 34/35 - 100uf poly tantalum.
VDD pin 31 - 10uf tantalum.
VSS pin 30 - 10uf tantalum.
Vdrive pin 9 - 10uf tantalum.
AVCC pin 60 - 10uf tantalum.
AVCC pin 50 - 10uf tantalum.
Refcap A/B/C pins 54/56/58 - 10uf tantalum.
Refin/Refout pin 51- 10uf tantalum.
However the 8 Avcc pins can alternatively be decoupled with a 10 uF and 0.1 uF
pair on each pin. Its important that the customer pays particular attention to
the Avcc layout and decoupling as this is the supply for the AD7656 core and
conversion.
