Delta-Sigma Converter

[leonel]

Hi.
I'm trying to read directly from a PIR sensor (3.3mVpp) using a PIC.
I read in Microchip AN700 it's possible using Delta-Sigma Converter to increase resolution (to 20 bits !!!!) then sense small signals, like this one.
How i can do that? I'm a little confused...

[BigWumpus]

Äh,
flux-compensator ?

[leonel]

Sorry but i don't understand... ???

[RSSHARPE]

I have used a pic and a cs5524 (24 bit a/d ) this uses shift in / out commands. I had 19 bits efffectivly. Remember your input circuits need to be layed out carefully to take advantage of available dynamic range.

regards

[Dave]

leonel, have a look at the MCP355x series of Delta-Sigma Converters. I have done some work with the MCP3553 and am planning on incorporating into a new design. They are fully differential and the MCP3553 has a 16Ms. update rate. The series is made by Microchip.

Dave Purola,
N8NTA

[leonel]

Using MCP3553 probably will work, but my circuit is to be supplied with 4 batteries of 1.5V, +-700mAh. MCP3553 has 170uA of current supply it's a little too much.
I would like to have a consume of about +-20uA.

[Dave]

leonel, In shutdown mode it only draws 1 uA...... In sleep mode it only draws 10uA......

Dave Purola,
N8NTA

[leonel]

You're right Dave. I took a better look after, and i read what you say.
Probably i will follow you're advise, but first i will see if there are another cheaper solution... (but i don't think so...)

Thank you.

[Charles Linquis]

Have you thought of amplifying the sensor's output with an op-amp before you feed it into the A/D?

A gain of 1000 isn't hard to do with one or two stages. Your 3.3mV p-p would then be 3.3V p-p.

[leonel]

yes, i already thought... It was my first solution.
But micro-power opamps are expensive, and consumes a little more than MCP3553.