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ADL5304 design with dual photodiodes

Are there any application notes describing how to use ADL5304 with dual photodiodes? I'm interested to know if ADL5304 is suitable to be used for absorbance measurements in a liquid chromathography UV monitor. We use a reference (R) and a sample (S) channel and the absorbance is simply calculated as log(R/S). Since the current at the sample channel is very small the photodiode needs to be zero-biased.

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  • Hello there,

    Good question, sorry we don't have such an App Note. 

    ADL5304 does support log ratio measurement capability. The two inputs are INUM and IDEN. Connect these to each of the PD anodes. This means IREF pin is now left open (floating) because it's reference output current is no longer needed. Note that the AC responses of the two channels are not identical, as seen by comparing datasheet Figures 21 and 24, but hopefully the highest speed performance is not needed for the application. Datasheet figures 5 and 9 illustrate the ratiometric response of the two inputs. It may be necessary to alter the output scale factor or output offset to accommodate the desired ratio measurement range. INPS and the SCLx pins are provided for this purpose.  -Bruce H. 

  • Another thought about the IREF pin:

    Since it's a current source output, best not to leave it open. Provide a path to ground, for example R= 1.5V / 100nA = 15M ohms to ground. This keep the voltage within the compliance range of the current source, which helps protect functionality of the bias and Vref block.   -Bruce H. 

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  • Another thought about the IREF pin:

    Since it's a current source output, best not to leave it open. Provide a path to ground, for example R= 1.5V / 100nA = 15M ohms to ground. This keep the voltage within the compliance range of the current source, which helps protect functionality of the bias and Vref block.   -Bruce H. 

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