Re: Need OP-AMP for operation of input close to ground
- From: "Tom Bruhns" <k7itm@xxxxxxx>
- Date: 20 Oct 2006 12:56:49 -0700
Billy wrote:
I need to amplify a small signal coming from a temp sensor. The temp
sensor has an output voltage range from 0V to 1.5V. I want to
interface the temp sensor to A/D converters on a micro-computer, so I
would like to use the full 0V to 5V range of the micro. So, I want to
amplify the temp sensor signal by a Gain of 3.3 in order to get a 0V to
5V range. I want to use an op-amp in a non-inverting configuration for
this gain. My problem is that when I used a 741 op-amp I didn't get
the correct output when my signal was near ground (ex.1mV to 500mV). I
was told this was because the 741 doesn't operate well near it's
voltage rails, which in my case are 0V and 12.6V (car battery). So I
tried an LM324 single power supply op-amp, which gave me better
results, but the linearity on the op-amp wasn't the greatest. Instead
of a Gain of 3.3, I got a gain close to 3.7 or so. I was wondering,
does anybody know of a better op-amp with good linearity that can
operate on inputs very close to ground and give me correct gain values?
Thank you very much.
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
.
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