AD73322: CD measurability

Q 

I want to make measurements at the tachometer output from dc-motors. This
signal is a dc-voltage with ac-parts (if the motor accelerate or stop). The
bandwith is therefore dc-max 2kHz. Can I use the ad-inputs of the ad73322 for
such a measurement (in the datasheet are examples of dc-in and dc-out
configurations) ?
I ask, because I had the infoemation, thjat sigma-delta converters are not able
to convert dc-signals.
Do you have an layout example for the ad73322 in the RU28 (tssop) package?
Do you also have some hints for decoupling if the supply voltage comes from an
DC-DC converter (operates at 300kHz) ?

 

A 

1)Firstly I must explain that the AD73322…family of parts are designed for the
telephony market, they are not characterised for DC operation, as you can see
from the datasheet, we do not give typical DC specs of linearity, offset and
gain errors. In addition , the analog front end is designed to work with a dc
bias at Vref, therefore  you may dc couple the i/p’s if the DC voltage of the
signal is at the same dc voltage of the reference, otherwise you need to ac
couple the signal.

Sigma delta converters that are designed for the industrial market are very
capable of converting dc signals, it is the ADC’s and DAC’s designed for the
audio market that are not specified for DC operation, such as the AD73322.
This  allows us to produce CODEC’s that are very cost effective due to the
elliminatin of the majority of DC testing that is carried out on our industrial
converters.


This address points to all the ap-notes available on sigma delta converter s
that we have posted on our web site.

We have a large range of converters that operate from DC to 2Khz, can you
detail your i/p signal range, the supply available, the resolution required,
the digital interface…plus any other relevant criteria, and I will get back to
you as soon as possible with a possible solution/solutions.

2)I take it you are refering to a land pattern for the AD73322 TSSOP package.

LANDS

For pattern of the solder lands for any of our packages we recommend the
standard layout.  You should be able to find it at the website of the IPC
(Institute for Interconnecting and Packaging Electronic Circuits). At
http://www.ipc.org/

Under "Design"  one will find a land pattern calculator that will
enable one to view the patterns in the current standard as well as calculate
new patterns from  component data. It contains a user searchable database of 
patterns for  components not yet approved by EIA or EIAJ.

Analog Devices does  not usually  issue recommendations on land patterns
because these are partially  dependent on the board design rules used, and on
the soldering processes used  by the board assemblers.


3) Low dropout regulators are often used following switching regulators for
better regulation and lower noise. Low dropout regulators such as the ADP3310
are desirable in these applications because they require only a small input to
output series voltageto maintain regulation.