Simulation of Photodiode connection to ADA4350

Question

Hello Fellows, 
 
 I am attaching a LTSpice simulation circuit. Inverting input of ADA4350 is connected to a recommended equivalent circuit of a photodiode. The stimulus is a 2MHz sinusoidal signal of 2.5uAp value. 
 To calculate the R feedback (R1) and C feedback (C1), used the literature - snapshot is attached. 
 
 /cfs-file/__key/communityserver-discussions-components-files/384/2086.Sinusoidal.zip 
 My questions are: 
 1. When C2 (900pF) is almost nonexistent, input/output waveforms are not distorted. The gain is still not what is expected according to the formulas from the attached sheet. What I might be misinterpreting in setting up the circuit? 
 2. Datasheet for ADA4350 states that it can be operated from single supply source of 3.3VDC. When I replace VEE with a ground, The output seems to have been disabled. There is hints of sinusoidal signal at output, but no gain. Again, how should I correct this issue. 
 Note: The photodiode I am looking for will have nominal terminal capacitance of 900pF and will have up to 2000pF. 
 
 Thanks for helping.

Hooman · Accepted Answer

Hi Mohi, 
 I'm not on expert on the ADC's and the driver you've noted. However, here is my 2-cents: 
 As far as driving the input of a SAR ADC like AD7685: 
 Usually an RC as shown in Figure 27 of AD7685 datasheet (33ohm series and 2.7nF shunt cap) alleviates most issues with charge kickback from the ADC frontend. You may want to make some measurements with the RC combination to see if you see any issues. 
 You may want to consult this excellent article (link below) from Alan Walsh which goes into the design of the front-end RC network for charge kickback mitigation: 
 "Front-End Amplifier and RC Filter Design for a Precision SAR Analog-to-Digital Converter." 
 I took Alan's idea and simplified it a bit here in this article: 
 The Perilous Path from the Transducer to the ADC: What&rsquo;s an Engineer to Do? 
 There is also some help from ADI on the ADC driver Tool in configuring the input network here which does some of the tedious computations automatically for you: 
 Precision ADC Driver Tool 
 Hopefully this answers your question. 
 
 Regards, 
 Hooman)

Hooman · Answer

Hi Mohi, 
 To answer your specific questions about the ADA4350 interface with your photodiode: 
 1. Minimum discernible input current from your photodiode: I'd use the information in Table 17: 
 
 This would be output noise. With a high impedance photodiode, the low frequency noise gain is 1V/V (as Figure 60 bode plot also shows). 
 The input noise current contribution will not be dominant, as stated in the paragraph just above Table 17: 
 "The effect due to the current noise is negligible in comparison". 
 You can then compare your output "signal" [ = i_Photodiode * RF] against the output "noise" [ = RMS sum of the RF noise and the VNOISE from Table 17]. You'll be able to detect the 300nA photodiode current as long this ratio is at least a few dB's. 
 2. Individual Noise Terms Effect / Contribution : As the datasheet states and as Table 17 shows, your dominant noise sources are RF thermal and the input voltage noise. Apparently, input current noise is of little significance here. 
 
 Regards, 
 Hooman

Hooman · Answer

Hi Mohi, 
 I don't see anything obvious, but to be honest I did not spend an inordinate amount of time reviewing your ADA4350 TIA design feeding into the ADC :-). 
 I also wanted to point out a TIA device Preliminary datasheet for LTC6563 with ADC driver built-in if you've not seen it before, for your reference, if you wish to go that route: 
 https://www.analog.com/media/en/technical-documentation/data-sheets/ltc6563.pdf 
 
 I'm hoping other people on the forum offer suggestions (if any) to your ADA4350 schematic. 
 
 Regards, 
 Hooman

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Simulation of Photodiode connection to ADA4350

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