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ambient light rejection with AD8304

Question asked by rbv on Aug 21, 2012
Latest reply on Sep 7, 2012 by rbv

I'm making a sensor to measure light absorbance in a sample where the geometry varies over time.  I currently have the AD8304-EVAL board ordered, I just want someone to look over my circuit before I hook everything up.

I'm planning to use the circuit from Figure 7 of the datasheet:

 

enter image description here

Vref and Vout will feed the corresponding inputs on an SPI analog-to-digital converter.

The light source will be pulsed (sequenced through several wavelengths, actually), and the ADC measurements will be synchronized with the source pulses. Pulses will be on the order of 100us in duration and the sequence will repeat at 2kHz.

I'd like to remove the error introduced by ambient light, which can be detected during the intervals while the source is turned off. I think I can remove the ambient light by adding a parallel inductor which "learns" the ambient power level. Can I use a solid-state switch to make the inductor only track the ambient level, as shown?

enter image description here

My idea is that the transistor acts as a switch -- while the source is turned off, the transistor disconnects the TIA and the RLC circuit charges until the current is all passing through the inductor. When the source turns on, the inductor holds its current, subtracting it from the photodiode signal, and the remaining (non-ambient) current passes through the transistor to the TIA.

Will this work? What parameters of the transistor and inductor do I need to pay special attention to?

On further consideration, I think that the node is supposed to be at VSUM, not ground, when the inductor current is constant. So I think I should connect the inductor to (buffered) VSUM, as follows:

enter image description here

Any advice would be much appreciated.

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