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Topic: Simple op-amp circuit (Read 8592 times) previous topic - next topic

Belal1987

Nov 05, 2015, 06:18 pm Last Edit: Nov 05, 2015, 06:29 pm by 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?

jremington

What is a "licor sensor"?

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

MarkT

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.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

raschemmel

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.
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Belal1987

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: http://www.licor.com/env/products/light/photometric.html

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


raschemmel

Quote
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 
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.

Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Wawa

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..

raschemmel

#7
Nov 06, 2015, 03:04 am Last Edit: Nov 06, 2015, 04:51 am by raschemmel
Quote
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
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Belal1987

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,

raschemmel

#9
Nov 06, 2015, 02:36 pm Last Edit: Nov 06, 2015, 09:51 pm by raschemmel
Post your circuit.
You tell us our information is "helpful".
What is thar supposed to mean ?
Did you USE it ?
HOW ?
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

Wawa

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..

MarkT

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.

Quote
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!
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

raschemmel

Quote
Post your (current) circuit
Arduino UNOs, Pro-Minis, ATMega328, ATtiny85, LCDs, MCP4162, keypads,<br />DS18B20s,74c922,nRF24L01, RS232, SD card, RC fixed wing, quadcopter

mrsummitville

#13
Nov 07, 2015, 02:22 am Last Edit: Nov 07, 2015, 02:58 am by mrsummitville
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?

mrsummitville

@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.

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