Hello,

I'm trying to amplify the signal of a pyranometer (Solar Power Meter), here is the device:
http://www.ambientweather.com/amsp216.html

It doesn't have a datasheet, but I was able to open it and place a 150ohm resistance between the sensor and get a voltage proportional to the readings, The bad part is I'm only getting measurements between 1 to 10mV so I need to amplify the signal to be able to use the arduino ADC.

I tried using Op Amps to amplify the signal, but I'm not able to get any readings using the ADC. It's the first time I use Op Amps so I'm not sure if I'm doing something wrong, here is the diagram:

Theoretically it works, but I'm not able to read any voltage at all with my voltmeter. I'm using a LM324 with 7.5V as source. Any suggestion is welcomed. I'm not clear if Op Amps amplify DC steady signals or only changing signals like AC? According to what if have read the Op Amps try to drive the signal to zero, so I'm guessing that's is what's happening. I will be grateful if someone cares to explain this!

Regards,
Pedro

 

Please clarify - your schematic shows a dual supply but you referred to it as a 7.5V supply, implying to me that it's a single supply. The picture shows a differential reading at your multimeter, but your schematic shows everything referenced to a common ground.

You need only a single op-amp, as far as I can tell. And yes, op-amps are happy with DC.

Find out what your maximum reading is, and that will allow you to set the gain. For instance if you need to amplify 10mV up to 5V for the Arduino, you would want a gain of 500. Now that I write that out, it is a bit much for a single stage, so I guess that's why you're using both amps.

Your first non-inverting input is tied to ground. When the inverting (-) input voltage exceeds the non-inverting (+) input, the op-amp output goes to the negative. So basically your first stage is locked at zero if you are using a single supply.

 

You may try to using another method, the gain of amplifier is 50 at the first stage and the gain is 10 at the second stages, and two amplifiers all using non-inverting amplifiers.

When you labeling the babel, you better labeling them from left side to right side.

 

Please clarify - your schematic shows a dual supply but you referred to it as a 7.5V supply, implying to me that it's a single supply. The picture shows a differential reading at your multimeter, but your schematic shows everything referenced to a common ground.

I'm using a LM324, it has a single supply, as far as I understand it take cares of the negative supply. I did some research and it probably splits the supply to +3V and -3V, so I should supply like 12V.

What should I reference my readings to?

 

it take cares of the negative supply.

No, it doesn't.

I did some research and it probably splits the supply to +3V and -3V

It won't do that by itself. Perhaps you're thinking of a 'virtual earth', where half the supply voltage (7.5/2) is derived using a voltage-divider and is then treated as the reference point, giving +3.75V and -3.75V with respect to that reference?

 

You have committed a cardinal error in applying the scheme operational amplifier lm324. The amplifier amplifies the sum of the useful signal and the error (offset) of the operational amplifier. You must enter an additional offset adjustment. There are far better amplifiers with very low offset voltage (microvolts).

 

Are you sure you can't replace the 150 Ohm resistor with a bigger one? That would solve your problem immediately and without any extra circuitry. If that's not possible, then LM324 is definitely not the right opamp, because its offset voltage is two times the lowest value you want to measure. I would suggest the relatively cheap OP07.

 

Hi,

I'd like to second (and now third) that nomination that the offset voltage is a big problem with the LM324 and related general purpose op amps when used with high gain DC coupling, one stage or ten stages.

The problem is that even with only 2mv input offset, a gain of 500 means that the output would be 1v even without any input signal. That means that there is always some reading even when the sensor does not pick up anything.

How you deal with this is tricky. You might get away with subtracting (or added if needed) the offset in code but that would reduce your dynamic range. You'd have to try that with your application.

Another way is to provide an offset adjustment, or chop up the input, or just get a better op amp with a known very low input offset. They are more expensive however.

 

I'm using a LM324, it has a single supply, as far as I understand it take cares of the negative supply. I did some research and it probably splits the supply to +3V and -3V, so I should supply like 12V.

What should I reference my readings to?

you are struggling to understand opamps you need to find a good intro to the subject, there are some good resources on this site.

As far as your project is concerned you need an instrumentation amp, so I suggest you spend some time reading the data sheet of a part that can run on a single 5V rail, AD623:

http://uk.farnell.com/analog-devices/ad623arz/ic-instr-amp-800khz-110db-8soic/dp/9426213
http://www.farnell.com/datasheets/82242.pdf
In a circuit like this:


You need to decide if your voltage is always positive or both positive and negative. That will determine how you connect up Vminus and Vref.

Peter

 

How you deal with this is tricky. You might get away with subtracting (or added if needed) the offset in code but that would reduce your dynamic range. You'd have to try that with your application.

Actually, subtracting the offset in code is not a bad idea, if you really can't get your hands in a better opamp right now. Just be careful to subtract the correct amount of voltage (LM324 has a typical offset of 2mV, but it can get as high as 3mV). Apart from that, its voltage drift is good enough to ensure adequate results. Keep in mind, though, that you lose some precision with that method.

 

Actually, subtracting the offset in code is not a bad idea, if you really can't get your hands in a better opamp right now. Just be careful to subtract the correct amount of voltage (LM324 has a typical offset of 2mV, but it can get as high as 3mV). Apart from that, its voltage drift is good enough to ensure adequate results. Keep in mind, though, that you lose some precision with that method.

Hi,

Yeah i agree, especially if you dont need super accuracy. Sometimes loosing a little dynamic range doesnt matter that much really.
I've worked with the LM358 enough to know that the offset is typically lower like 0.5mv to maybe 1.2mv. But for one of my applications (a current shunt amplifier) even a gain of 100 was producing far too much error, so some compensation technique had to be used.
BTW just for a little information, the LM358 has an internal design the same as the LM324 i believe.

 

I'm using a LM324, it has a single supply, as far as I understand it take cares of the negative supply. I did some research and it probably splits the supply to +3V and -3V, so I should supply like 12V.

You need to do a lot more research. No standard opamps split a single power supply and create a centerpoint ground internally. Also, while some opamps are called "single-supply", this is because they have extended features, such as the input common mode range or output voltage swing range, and does not mean that they behave differently from "normal" dual supply opamps in the way they deal with DC signals. Finally, your resistor values around IC2 are not correct for a normal differential configuration.

BUT - In your first schematic you show the input signal as referred to GND. Because of this, there is no explicit reason to use an instrumentation amp or a fully differential amplifier configuration. And you probably can get away with only one power supply rail and GND. In this case you do need an opamp with an input common mode range that includes the negative rail. The LM324 was one of the very first to do this, but there are newer devices with better bandwidth, noise, and error performance. You can use one, or two non-inverting amp stages in series to start, and maybe add an offset trim.

ak