AC photodiode amplifier design question - synchronous demodulation

Hello,

I asking advice on the following issue.

I need to design a photodiode amplifier with the following characteristics:

- photovoltaic mode

- AC signal with with few kHz

- very low noise

- high gain required

- photodiode properies: capacitace ~ 2000 pF , Rs=3000 MOhm

- amplifier output to be fed to ADA2200 Synchronous Demodulator -> AD779x ADC

- eventually DC servo to compensate offset due to ambient light

As this thread points out the opamp input bias current for AC applications only results in a DC offset.

https://ez.analog.com/message/122455#122455

- Is my interpretation correct that as long as the induced offset is not too large I can choose a low voltage noise opamp with an appropriate bandwith ?

- Is there a better way to deal with the DC component of the signal ( due to ambient light ) than using a DC servo circuit as it might introduce a lot of noise ?

- What opamps would be suitable for this kind of application ?

Any advice is highly appreciated!

Thanks!

Peter

  • Hi, Peter.

    There are several factors that contribute to the error in a system. First, as you say, is the input offset voltage of the amplifier. The higher it is, the higher it will amplify at the output, the higher the error. One of the solutions to reduce this offset is to ac couple the inputs or pick an amplifier with very low input offset voltage in the range of uV or less. Second is the amplifier's input bias current. There are amplifier's that has verly low bias in the range of nA or even pA especially the CMOS and JFET amplifiers. Third is the PCB (printd circuit board). Having a poor layout by not putting a guard trace at the trace carrying the photodiode current would cause leakage current. Fourth are the amplifier's input voltage noise and the feedback resistor's noise. This was discussed by Matt in the link you provided. Fifth is the amplifier's bandwidth. The higher the bandwidth, the higher the noise gain. Ther are many more to enumerate but these five are the most critical specs to consider in selecting an amplifier and in designing a photodiode amplifier.

    To remove the DC light level, you can create a chopping circuit or you can use a sine wave modulation or implement it through a firmware.

    ADA4817-1 is a good candidate becuase of its low bias current of typically 2pA, input voltage noise of 4nV/rtHz and a unity gain bandwidth of 400MHz which is very applicable for photodiode applications. Another candidates are the AD8615 with maximum input bias current of 1pA and low offset voltage of 80uV. The oldest and guaranteed ADI part that you can use is the AD549 with max input bias current of 60fA.

    A good read on photodiode design is this link - http://www.analog.com/media/en/technical-documentation/technical-articles/Optimizing-Precision-Photodiode-Sensor-Circuit…

    Let me know if you have further questions.

    Regards,

    Anna

  • Hi Anna,

    thank you very much for your reply. I appreciate!

    I will try out the AD8615 first since my bandwith requirements are rather moderate and high bandwith opamps can sometimes be tricky to tame ...

    Thanks again

    Peter