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Photodiode amplifier interface with serial ADC

Dhruvdeep Narwat
Prodigy 10 points
Other Parts Discussed in Thread: LMP7721, ADS7822

hi all,

I am using LMP7721 as photodiode amplifier and ADS7822 as serial ADC. The light pulse input are coming at the rate of 1KHz. I studied in some of the papers/notes that a RC is required while interfacing to ADC. This external C will load the TransImpedance Amplifier (TIA). I have read papers regarding out of the loop compensation,in the loop compensation and other compensation techniques for capacitor loading. Also somewhere it was written that the feedback capacitor designed for TIA is sufficient to compensate for capacitor loading. I am unable to understand this as until now what i have understood is that feedback capacitors are employed to reduce gain peaking of TIA closed-loop gain. How come the feedback capacitors are able to compensate for capacitor loading.  To summarize; my Questions are:

1. What is the S/H capacitor value of ADC to determine the external C required(if required)?

2. Does feedback capacitor is able to compensate for the capacitive loading?

3. What is the output resistance of LMP7721?

Thanks

 

  • 0
    Prodigy 180 points

    Hi,

    You should read these

    http://www.analog.com/library/analogdialogue/archives/46-12/sar_front_end.html

    http://www.google.gr/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&cad=rja&ved=0CDAQFjAA&url=http%3A%2F%2Fe2e.ti.com%2Fcfs-file.ashx%2F__key%2Ftelligent-evolution-components-attachments%2F00-232-01-00-00-12-39-75%2FPA_2D00_001--Optimize_5F00_SAR_5F00_converter_5F00_design-REV-b.pdf&ei=Nal3UufaBsLG4gSryYDICw&usg=AFQjCNFg1o_jvNknm7sP8Q0KT3xZWC262g&sig2=8KRdfSfaTT-pQnz0kvaihQ&bvm=bv.55819444,d.bGE

    in order to have a clue how to determine the RC values required to "load" your ADC inputs.

    For the output impedance, I am also curious, and I expect someone from TI to give an answer.

    The capacitance of your ADC is in the datasheet of that specific ADC.

    Info on the feedback capacitor you can find here.

    http://www.analog.com/static/imported-files/tutorials/MT-058.pdf