Using an ir led - photodiode assembly to Chk the falling of saline drips , I want to do ambient compensation since I am nt enclosing the set up ... I am using instrumentation amplifier if for this, my frequency is very low from around mHz to around 2 hz.... What ic can give me a externally controlled gain frm say 10 - 300 and high cmrr to reject the ambient light ...

 

If this is a question about eliminating ambient light from an IR sensor why not use a IR filtered sensor in the first place?

 

IR sensor should have a filter to eliminate ambient light

 

Try the Analog Devices AD8132. You would need a second photodetector to sense the ambinet, and then carefully adjust the level of the ambient sensor to cancel the ambient falling on the drip detector. Personally, I think that if it works, it will be tricky to maintain cancellation over an extended period of time or under varying ambient conditions. If you are doing this as a commercial project and are serious about finding a good solution, you might consider using a high frequency modulator using a regular square wave signal or a pseudo noise generator to drive and LED to illuminate the drips and a high pass filter and detector or synchronous demodulator to pull the "drip" signal out of the signal + noise. With such an approach you can achieve dramatic S/N ratios.

Of course, pwdixon's suggestion of using an IR filter should be tried first because depending upon your "receiver" circuit and the kind and amount of ambient noise, that might be all you need.

 

...I want to do ambient compensation since I am nt enclosing the set up ...

Every little bit of mechanical solution here would go a LONG way. It may be impractical to eliminate all ambient light but it wouldn't be hard to cut it by, say 5X or so. That would be a 5X gain in signal:noise ratio and that would make the electronics SO much easier.

 

With such an approach you can achieve dramatic S/N ratios.

Ooh, I was assuming this was already in place. Maybe a bad assumption. It's the state-of-the-art solution and should be a part of this project.

 

Tk you for your help guys.
I am not using a filtered source , as one of the major constrainsts is to reduce the cost of the component - I assume a filtered source is gonna be of a much higher cost when compared to a normal IR Led .

 

get an IR filter from a broken remote control

 

The real-world IV drop detectors use IR filters and honeycomb collimators on both the emitter and detector.

Ken

 

The source does not need filtering, just the detector. If a wider detector window is needed, You could use 5 surface mount photo transistors covering 5 mm with outputs connected to a 8 input OR gate. Worked with a separation of 50 cm to count drops.

 

Tk you for your help guys.
I am not using a filtered source , as one of the major constrainsts is to reduce the cost of the component - I assume a filtered source is gonna be of a much higher cost when compared to a normal IR Led .

Just buy an IR sensor they almost all have the plastic IR filter these days, in fact I think it's pretty much impossible to get one without it.

 

By IR sensor, does it mean: IR Receiver, IR Photo Module. or IR Receiver Module ? Which all require burst type modulation which might not work with drop detection. High frequency continuous modulation or no modulation might be best. I found no difference at close distances, say 1 ft. The 5 photo-transistors are Everlight PT91-21C, 860 nm. clear lens with an IR pass filter and IR emitter from a remote control.