Need OP-AMP for operation of input close to ground

yes you need an op amp that is classed as rail to rail output and either rail to rail input or single supply will do, there are many such as TS922 (dual) wich ive used a lot of although I think there must be better available.

Colin =^.^=

The LM324 or the dual version, LM358 should work well for this application, though you may need to add a loading resistor between ground and the output to allow the output to produce a signal all the way to zero volts. The output divider network to set the gain can perform this function.

What two resistor values did you try in your attempt to get a gain of 3.3?

The output can swing up to about 1.5 volts below its positive supply, so you cannot get a 5 vol output with a 5 volt supply, but should work fine if the supply is 7 to 15.

Linear Technology offer the LT1006 single- and LT1013 dual- and the LT1014 quad-op-amps for people who want something better than the LM358 dual- or the LM324 quad-op-amps.

They are still single supply op amps, but perform appreciably better than the LM324 and cost quite a lot more. Farnell stock them, along with the LT1077, LT1078, LT1079 low power eqivalents.

The LM324 should have performed a lot better than what you describe. I forgot to mention that the performance can be degraded if you let the unused inputs float. You should tie the unused + inputs to the negative rail, and tie the unused

- inputs to their respective outputs.

Billy wrote:

There are lots of op amps these days, optimized for many different applications. The 741 was a great part in its day, but you should be able to do far better for any specific application these days: cheaper AND a better match to what you are trying to do. Go to the web site of your favorite op amp supplier (and check one or two others to make sure you don't miss something obviously better) and search for a part that can run on a single 5V supply and has rail-to-rail input and output. As others have noted, the input doesn't need to be rail to rail, but does need to include the negative supply. Any op amp you apply properly should have adequate linearity. Now if you mean accuracy instead of linearity, that should be set by the feedback you apply: in a non-inverting configuration, the feedback resistor should be 2.3 times the resistor from (-) input to ground. A 1k and a 2.2k resistor should come close enough to be useful, and if you use 5% tolerance parts, it likely won't be above a gain of 3.3 (though could be as high as 3.43, worst case). Stability with environmental changes might be a bigger issue, and reason to use 1% parts (which are commonly inherently more stable). It doesn't need to be fast for your application. Low power may be a benefit. You should be able to find something that costs less than a dollar. Some suppliers will ship you a sample or two for free, without asking too many questions about your qualifications. Some suppliers to try: National Semiconductor, Linear Technology, Maxim, Texas Instruments, Analog Devices, Philips, ...

Cheers, Tom

Instead of using the micro's built-in ADC converter, which is not always good quality, you may want to consider using an external ADC. Analog devices offers some useful parts. I've been very happy with the AD7793, which has an internal amp, differential measurement, built-in reference, 50/60 Hz filtering and an easy to use SPI interface.

Hey, you dont need any op-amp. Just tie the reference input on the micro to 1.5 volts and feed the signal into the A/D directly.

Luhan

Car battery operation and total cluelessness- you must be a Ford or GM "engineer."

:) Nasty Fred! :)

Vasile