piclist 2003\02\21\005334a >
Thread: Differential Amplifiers
face BY : Ned Konz email (remove spam text)

On Thursday 20 February 2003 08:37 pm, Ian McLean wrote:
> You are right about using a differential, followed by non-inverting
> stage for the job with this op-amp ...
> The solution to my problem as it happens is to do with gain as much
> as the maximum voltage swing I could expect from an LM358.
> Somebody has mentioned that if my negative is at 0, then I could
> expect clipping above 3.5V.  This is indeed what I saw, clipping,
> and is what gave me the clue, and after further simulation, have
> concluded that the gain is pretty much limited to a 0-3.5V swing
> for an input range of 0-400mV (can't be bothered working that out).
>  I could settle for that, or as I have now tried, use both op-amps
> in the package to get a swing from 0-4V which is what I was after.

I don't think you'll get that kind of swing on a 5V supply out of
these, no matter what gain you're running at. Your positive output
swing is still going to be limited.

> Thank you for all the advise.
> PS: Must get my hands on some better op-amps ?  Any suggestions as
> to a suitable one for this job ?

There are many dual op amps out there with better specs. A lot of CMOS
op amps have rail-to-rail inputs *and* outputs.

The choice of op amp depends on what kind of precision, drift, output
drive, supply voltage (5V?), frequency response, noise, etc. you need
for your application.

If you could share these, we could help better. Or you could look at
the op amp selection guides at National, TI, Linear Tech, and Analog
Devices. They all have parametric selection guides; some also give
price estimates (these will be much lower than you'll pay in small
quantities, of course, but they can be used to compare amps).

Ned Konz

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In reply to: <000201c2d962$fd9d92e0$0c00a8c0@deepthought>
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