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Zero offset via PIC

Hello!

I would like to address a problem I'm mulling over at the moment. I have a micro-controller based circuit with a front end consisting of an Instrumentation amplifier. I thought of applying zero volts to the inputs (via pins from PIC) as a way of finding the sum of offset voltages - via A/D converter. My worry is that this could affect the high CMRR (120dB) needed at the input - can this be quantified?

Another alternative is to use Comparators of the PIC, but I'm already using one of them. This circuit is battery operated (9V split supply) and would therefore have to perform (lowest point) at 3V - ruling out a CMOS switch.

These are just a few thoughts whizzing around me at the moment. Any help on which way to go would be helpful.

Regards,

Ozzy.

Reply to
ozzy
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Use a relay instead to short the inputs. Much easier to keep that high CMRR, as a relay is a nearly perfect switch. They come pretty small these days as well.

Dave

Reply to
Dave

Dave,

Thanks for the suggestion. My circuit will operate off a 9V battery (+/- 4.5V rail to rail), so i'm really looking at something that could switch with at least 3V - I'm sure a relay will need more than this. That's why I was thinking back to the Comparator, as it will provide a low output impedance - as if the connection was directly to ground.

The CMRR won't be important here (contradicting my previous statement!) as no signal is passing through - strictly 0V DC needed. By the way my idea was not to have this continually nulled - mimicking a chopper amp. But to have this done only on Power ON. A time out after 10-15 minutes will ensure that the drift of Vos with time and temp will not be significant.

regards,

Osman

Reply to
ozzy

There are lots of cmos analog mux chips that will work at very low supply voltages.

John

Reply to
John Larkin

For something like a simple shorting of the input a 2N7000 or BSS123 should be fine. If it absolutely has to get to lower Z at 3V then IRF offers some nice FETs. In case capacitance is an issue there are RF dual-gate MOSFETs.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

Joerg,

I was'nt aware that MOSFETS could switch so well at 2.7V - RDS(on) of

30mOhms! Is this partly due to the increase in surface area provided by the HEX structure (similar in a way to U-MOS by ZETEX, although i'm not sure if they ever took off - huge problems in gate oxide integrity delayed project by 2 years).

regards,

Ozzy

Reply to
ozzy

Don't short the signal, just disconnect it and short the inputs of the signal-conditioning front-end. That doesn't require a very low on resistance. So you need a spdt or maybe a dpdt analog switch.

John

Reply to
John Larkin

It's market driven. Stuff has to work with newer 3.3V or lower voltage node logic. Then there are all the gadgets that run off 4V LiIon and thus need a low Rdson switcher FET. Not that mankind really needs all these gadgets but...

AFAIR the lowest Vgs FET I have seen has a guaranteed Rsdon limit at

1.8V Vgs. Wasn't even terribly expensive.

I usually do short it but with a resistor up front :-)

If Ozzy decides on DPDT switches and uses the typical CD405x or 74HC405x then he should take extra care of all the noise paths. It can't hurt to RC filter the VCC, plus some filtering on the control lines maybe. Don't know about leakage of these devices.

Of course, the best of all switches is the SD5400 but these have become rare and expensive. Must have fallen from grace.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

These sorts of things are interesting:

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Lots of people make gadgets like these... TI, Fairchild, Onsemi. Some are sold as logic bus switches, but they still make nice analog switches, if you're not too afraid of charge injection.

John

Reply to
John Larkin

These are indeed great. It always feels good for an analog guy to hitch a bargain ride on a digital chip. Almost like the advent of the unbuffered CMOS inverters a few decades ago. Initially I had shunned bus switches because of their high prices but that has come down nicely.

--
Regards, Joerg

http://www.analogconsultants.com
Reply to
Joerg

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