1. Home
  2. Electronic Components
  3. >30V output AND 0V offset from a single op-amp?

>30V output AND 0V offset from a single op-amp?

CA3140 data sheets show that the output can approach the positive rail and (allegedly) reach the negative rail. This might be false. On a single rail supply the offset cannot be trimmed to get a true zero output, you MUST have some negative.

I want to measure up to 30 watts at 1W/V from a thermopile sensor, on a multimeter, so my idea is a split rail supply whose total is 34 volts, with

30 volts on the positive side and just a few hundred mV for negative, to

get through-zero adjustment for offset error.

Will this work without frying the IC? Can anyone suggest common, low price, good performing IC's for this? It doesn't have to be a single rail op-amp, it just has to accept a VERY asymmetrical split rail. I'm hoping the LF411 or LF412 (with external offset trim) can do this.

I can make the dual supply stage, that's not the tough bit, but I'd like some advice about the extreme asymmetry I want for the gain stage. (Both will hopefully be workable from one dual op-amp IC).

-------------------------------------------------------------------------- PS. Sorry about the repost, but I really want some help here, and my last post got entirely lost behind the mark-up debate. I've reduced this post a LOT to try to gain some attention to it. :)

Reply to
Lostgallifreyan
Loading thread data ...

On Wed, 26 Apr 2006 01:00:11 GMT, Lostgallifreyan put finger to keyboard and composed:

Why not just use a resistive potential divider to reduce the input to a manageable level? Does the source need to see a high impedance, ie will it be unduly loaded by a potential divider?

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

Franc Zabkar wrote in news: snipped-for-privacy@4ax.com:

Sorry, I nearly missed this one. :)

It's not the input that's the problem though, that's lower than +3V. What I was after was up to +30V output for measuring on a single range of a voltmeter. I was concerned that dual rail op-amps might only be designed for symmetrical supply rails, and might fail if ground was biased almost to negative while output was driven close to positive rail.

I reposted this in simpler form when this post appeared to get lost, with subject "How asymmetrical is an op-amp supply allowed to be?"

I've been told there by Tim Williams that any op-amp will allow a strongly biased ground, just 2 volts or so above negative, and still allow an output to get close to the positive rail even with max supply voltage; he says that both transistors in the output stage (and presumably the others) must be able to handle the full supply voltage.

If this is all I need to consider when using an LF412 with the small negative voltage to help zero the offset to my ground (made by the first stage in the op-amp and two resistors as potential divider), then the question is answered. :)

Reply to
Lostgallifreyan

On Thu, 04 May 2006 10:47:43 GMT, Lostgallifreyan put finger to keyboard and composed:

Sorry, brain fart. Sadly they're getting more frequent as I get older.

- Franc Zabkar

--
Please remove one 'i' from my address when replying by email.
Reply to
Franc Zabkar

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.