Simple op-amp circuit

Hi,

So i’m trying to amplify the signal from a photometric licor sensor and record it with Arduino. There is only one problem: the signal peaks at low light intensity (please see attached picture of circuit)
I tried increasing the 100kohm resistor, what happens is that the upper voltage limit drops down to 2.9v but it remains as the maximum value.

Can you spot anything wrong with this circuit?

What is a "licor sensor"?

The AD822 op amp is not rated for 3V operation. It is rated for 5 to 30V operation.

That circuit looks bogus to me - where did you get it from?

What are the specifications of the sensor and what voltage output range do you want.

Then we can suggest the right opamp circuit and some component values.

It's not a non-inverting amplifier, inverting amplifier , voltage follower or any other known configuration. You have a 100k feedback resistor and NO input resistor. Doesn't that strike you as ODD ?. Even if you moved the 10 k resistor to the input, it's still not correct for a non-inverting amplifier. The 10 k resistor should go from the V- pin to GND instead of to the output.

Hi,

Thank you all for your input as i'm new to this field.

  1. The Licor sensor I'm using is similar to this: LI-COR LI-210R Photometric Sensor

  2. On the AD822 datasheet, it says:
    Single-supply capability from 5 V to 30 V
    Dual-supply capability from ±2.5 V to ±15 V
    Does that mean that the output is 2.5-15 only if i'm using the dual circuit? Could someone please clarify as what that means.
    http://www.analog.com/media/en/technical-documentation/data-sheets/AD822.pdf

  3. @raschemmel, I'll try doing what you suggested, thank you. Here's a short article (page 2) that shows a similar circuit.

Thank you,
Belal

  1. On the AD822 datasheet, it says:
    Single-supply capability from 5 V to 30 V
    Dual-supply capability from ±2.5 V to ±15 V
    Does that mean that the output is 2.5-15 only if i'm using the dual circuit? Could someone please clarify as what that means.
    http://www.analog.com/media/en/technical-documentation/data-sheets/AD822.pdf

Read it again. It doesn't say "2.5 -15"
It says "±2.5 V to ±15 V"

If you don't understand what that means, it means (-2.5V to +2.5V ) to (-15V to +15V) implying that the minimum +/- voltage is +/- 2.5V and the maximum +/- voltage is +/-15V.

The "Licor sensor" sounds like a nomal photo sensor, covered with a generous amount of snake oil.

All I could find in the pdf is a "0-10mV output".

The AD822 is a rail2rail out opamp, with inputs that include and can go below ground.
So the circuit has to amplify 0-10mV to 0-5volt.
The opamp can (must) run directly off Arduino's 5volt supply.
Leo..

The “Licor sensor” sounds like a nomal photo sensor, covered with a generous amount of snake oil.

@Wawa,
I seriously doubt the OP is old enough to know what you mean by SNAKE OIL

@OP,
You heard the man. Power the op amp off the arduino 5V.
You need a Gain of :

sensor max output voltage = 10mV (0.010 V)
Arduino max analog input voltage = 5V
5.00/0.010 = 500
A = 500.

Gain of Non-inverting amp = Rf/Rin +1
500-1=499
Let Rf = 500 k ohm
Rin = 500 k /A = 1 k ohm (technically 998 , but 1k is close enough)

Op Amp Non-Inverting Amplifier

Thank you both, that was very helpful. One last issue i'm having is that readings go up suddenly (not change in light intensity) then they go back to normal:
For example:
1.74
1.74
1.74
1.5
1.53
4.59
4.99
1.5
1.74
1.74
1.74

What could be the reason behind this spike?

Thanks,

Post your circuit.
You tell us our information is "helpful".
What is thar supposed to mean ?
Did you USE it ?
HOW ?

Also include WHAT you are measuring.
Light might appear constant to a human, but flicker to a fly.
Mains powered light is usually NOT constant.
Leo..

Low voltage signals can easily pick up noise - you have to filter this out and
beware of making blunders like sharing signal ground wiring with power.

Absolute Calibration: ± 5% traceable to the U.S. National Institute of Standards and Technology (NIST) Sensitivity: Typically 30 μA per 100 klux Linearity: Maximum deviation of 1% up to 100 klux Response Time: Less than 1 μs (2 m cable terminated into a 604 Ohm load) Temperature Dependence: ± 0.15% per °C maximum Cosine Correction: Cosine corrected up to 82° angle of incidence Azimuth: < ± 1% error over 360° at 45° elevation

Clearly not snake oil, this is a precision photometric sensor with spectral filter, cosine-correcting
optical path and precision mounted. The active device is a photodiode of course, they all are, so you
measure current under reverse bias, therefore a current->voltage opamp circuit is needed with
appropriate bias voltage (0V bias may be adequate, I can’t see a recommended circuit).

If you are not interested in repeatable accurate light metering you’ve wasted a lot of money though!

Post your (current) circuit

Belal1987:
Hi,

So i'm trying to amplify the signal from a photometric licor sensor and record it with Arduino. There is only one problem: the signal peaks at low light intensity (please see attached picture of circuit)
I tried increasing the 100kohm resistor, what happens is that the upper voltage limit drops down to 2.9v but it remains as the maximum value.

Can you spot anything wrong with this circuit?

Yes, I can spot something wrong with that circuit.

I think ...
Your original circuit is almost correct.
You need to connect Pin 3 to Pin 4 of the AD822 OP Amp.
This makes the circuit a Current-To-Voltage converter.

And use 4 AA batteries, instead of 2 AA batteries,
this will make a 6 Volt power supply.
The AD822 OP Amp needs 5 volts minimum.

SampleCircuitHere

Do you have the polarity of the LICOR correct?

raschemmel:
@Wawa,
I seriously doubt the OP is old enough to know what you mean by SNAKE OIL

@OP,
You heard the man. Power the op amp off the arduino 5V.
You need a Gain of :

sensor max output voltage = 10mV (0.010 V)
Arduino max analog input voltage = 5V
5.00/0.010 = 500
A = 500.

Gain of Non-inverting amp = Rf/Rin +1
500-1=499
Let Rf = 500 k ohm
Rin = 500 k /A = 1 k ohm (technically 998 , but 1k is close enough)

Op Amp Non-Inverting Amplifier

The LI-COR LI-210R is not a voltage source.
It is a current source, with rating of 30 microamps per 100 klux.
I would not use a non-inverting amplifier circuit.

mrsummitville:
And use 4 AA batteries, instead of 2 AA batteries,
this will make a 6 Volt power supply.
The AD822 OP Amp needs 5 volts minimum.

And you connect the opamp output directly to an Arduino pin?
With the risk of phantom powering.
What is the problem in using Arduino's 5volt supply.
Leo..

Wawa:
And you connect the opamp output directly to an Arduino pin?
With the risk of phantom powering.
What is the problem in using Arduino's 5volt supply.
Leo..

Because I do not see an Arduino or a 5 Volt Supply in the original schematic.

Belal1987:
So i'm trying to amplify the signal from a photometric licor sensor and record it with Arduino.

Post#0

I'm not familiar with that sensor but without knowing the maximum klux possible (Full Scale) I would have no idea what resistance to use to convert the current source to a voltage source that can be measured with an arduino.

Thank you all. I rewired the circuit by connecting v- of the amp. to Arduino's GND through the 10k resistor.

The voltage measured between v- and output is exactly how i wanted it to be (Doesn't max under low lights), however, readings in Arduini do not correlate to the amount of light. I think this is because Arduino's GND is different than (e.g. there is a resistor) the GND wired to the voltage meter. Any idea how to overcome this issue?

BTW. how can i attach a pic to a reply? is that possible?

B.