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Op Amp help requested

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DoctorLes

New Member
Hi,

I have been working on a project for quite awhile now but keep running into detours and dead ends, so thanks in advance.

I've searched the Net and several books high and low but am still pretty much confused about what I need. Also, none of the "Practical" circuits I have come across give sufficient component data for me to build my own, or else they are not appropriate for my application.

Goal: I want to boost my infrared thermopile's voltage output from about 0.2 mv to high enough to drive an analog input for my Arduino microprocessor but I'm not sure how much that will need to be. The thermopile has an internal ambient temperature compensating resistor; the changing value needs to be sent to the Op Amp too.

The output of my Arduino then drives a serial port/USB converter cable/circuit. Then I convert that data to a dynamic graph to be read on my computer (I have Mac, Linux and Microsoft platforms available, but Mac seems easiest to use right now).

My Arduino is using a single 5V USB power supply, so I need to use this single voltage source for my Op Amp circuit. I don't want to fuss with an external twin battery supply but I will if I have to, I guess. So I figure the Op Amp circuit has to split the USB supply voltage, giving the max output voltage of just under 2.5 VDC, at full amplification.

So, for all this I think I also need a low noise Op Amp with a low minimum operating voltage range. I think I need to use an instrumentation amp design with some kind of buffering stage. Right now, I have an LM324N Quad Op Amp chip but can get others, of course.

Summing up, I would like advice on which type of Op Amp circuit to use, what type of chip characteristics should I look for (perhaps with a suggestion of one or two chips to try) and where I might be able to track down component values. I realize components determine the amplification and such, using formulas, but it's mostly the other components that leave me in doubt, such as bypass capacitors, etc.

Thanks Again,

Les
 

Sydney

New Member
Just a small thought, can you use one of the many designs using a 324 as an intrument amp.?. There are a multitude of reliable designs on the net, I have used several on strain guage weighing and stress monitoring systems.
 

DoctorLes

New Member
Hi Sydney,
First, thanks for the quick reply! (and any thoughts, no matter how "small" are welcome :)
I thought the 324 was an old, relatively leaky op am and required +/- voltage supply? At any rate, I haven't found a circuit yet that is sufficiently detailed and close enough to what I need for me to adapt it. If you've seen any likely suspects I'd appreciate them too.

Regards,

Les
 

MikeMl

Well-Known Member
Most Helpful Member
Hi Les,

The LM324 can operate on a single supply of 5V; the only problem is that its output can only pull to 5-1.5V when high. It can pull all the way to ground when low. The inputs can operate all the way to ground, meaning that you can reference your sensor to ground.

If your uprocessor's A/D input range is 0-5V, then the LM324 will fall short. You would be much better off with a modern CMOS rail-to-rail input and output opamp which operates well at 5V, and can pull its output within mV of ground or +5V.

DoctorMike
 

mneary

New Member
I wouldn't throw away an op amp just because it can't reach 5V. With a load resistor, you can get a 0-3V output which only wastes one bit of resolution.

Your bigger problem is offset voltage. The LM324 offset voltage is max 5mV. Your input of 200 microvolts is tiny compared to that. Fortunately when you get low offset, it often includes rail-to-rail operation. I assume you're only building a few, so $5 isn't a big deal. I looked at Linear and found the LTC2053, and a handy app note. [edit] Its offset is an impressive 10 microvolts which could still be 5% of your signal, :( [/edit]

[edit] After a gain of 1000 on one LTC2053, you'll have 200mV so you'll need a gain of maybe 10 or 20 or whatever to get 2V, 4V, or whatever you want for the Arduino. [/edit]

Rather than myself typing what they wrote, why not review what they published and come back with your questions...

With such a tiny input, the problems don't end with the op amp, the galvanic interconnections could be an issue.
 
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MikeMl

Well-Known Member
Most Helpful Member
What is the sampling rate in the A/D?
What is the frequency response of the sensor?
 

DoctorLes

New Member
I just wanted to say a quick but earnest THANKS! to all who have posted responses. I won't be able to work on the project again until weekends so I'll let folks know what progress I'm making. The thermopile like I have is used in cheap little handheld, non-contact, IR thermometers, so I know this is feasible. I'm trying to replicate one of those circuits but ultimately feed the output into a computer and graph the results. That's the essence of my project, and I'm committed to get this thing working.

I'll be back; meanwhile, thanks again.

Les
KE7SLX
 
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