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How to use a single supply amplifier?

Hello NG I'd like to amplify the signal of a photodiode to read it with a ad- converter.

Signal photodiode: 0-50mV Voltage range adc: 0-5V

I tried to use two types of amplyfiers, AD623 and INA126u. But in the tests some very strange clipping happened. (See below.) I think there's something basic about single supply amplifiers which I understand wrong.

Could you please tell me where the mistake is with my wiring or make a better proposal. If possible I want to use one of these amplifiers since they are lying right on my desk...

Thank you

Test1 with AD623 Pin1 -RG is connected to Pin8 +RG by a 1kOhm resistor (Gain = 100) Pin2 -IN is connected to GND Pin4 -VS is connected to GND Pin5 REF is connected to GND Pin7 +VS in connected to +5V or +10V (The result doesn't depend on this. - Capacitors to GND are applied...)

Test results: Pin3 +IN Pin6 OUT

0mV 0V 10mV 1V 20mV 1.2V 30mV 1.3V 40mV 1.5V 50mV 1.7V

Test2 with INA127u Pin1 RG is NC (Gain = 5 for testing purpous) Pin2 V-IN is connected to GND Pin4 V- is connected to GND Pin5 REF is connected to GND Pin7 V+ in connected to +5V Pin8 RG is NC

Test results: Pin3 V+IN Pin6 VO

0V 3.6V 1V 3.6V 2V 3.6V 3V 3.6V 3.6V 3.6V 3.7V 4.1V 3.8V 4.6V 3.9V 4.6V
Reply to
g.weilenmann
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For the INA126, you're trying to operate it outside its input common mode range, and until that's fixed, all bets are off.

For the AD623, look at Figure 21 of the data sheet. At even 1V out, you're outside the range suggested by that figure.

I don't see why you don't just use a common rail-to-rail input/output op amp to do the job. You can configure for a non-inverting gain easily. I'm a bit surprised you are using the photodiode in a voltage mode; it's common to run them into a low impedance current-to-voltage converter for good linearity. It should be easy to find op amp ap noted about that application.

Cheers, Tom

Reply to
Tom Bruhns

Hi Tom

The input common mode range was exactly my problem, for both amplifiers.

Thank you so much for your answer!

have a good day, Gabriel

Reply to
g.weilenmann

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