Need OP-AMP for operation of input close to ground

[Billy]

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.

 

[colin]

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?

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 =^.^=

 

[John Popelish]

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?

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.

 

[[email protected]]

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?

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.

 

[John Popelish]

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.

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.

 

[Tom Bruhns]

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

 

[Arlet]

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.

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.

 

[Luhan]

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.

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

 

[Fred Bloggs]

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?

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

 

[vasile]

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

Nasty Fred!

1mV to 500mV means you need a low offset operational amplifier on a
large temperature
variation (as could be inside a car -50C to +70C).
So forget about 741.

Vasile