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Part Number: THS4631
Tool/software: WEBENCH® Design Tools
I have designed a wideband (1MHz) TIA using the THS4631. The feedback elements are 400kOhm and 0.5pF respectively. The issue I have is that my noise calculations by hand, mainly based on Graeme's book (Photodiode Amplifiers – Op amp solutions) as well as the step by step noise calculation detailed in chapters 10 and 11 of Art Kay’s excellent book (Operational Amplifier Noise), is way smaller than WEBENCH's in particular the total current noise. Photodiode current varies from 1.28 to 2.51uA and WEBENCH total current output noise OpAmpI = 2.345mVrms while my hand calculations gives 14.16uVrms. How does WEBENCH calculates the noise?
Thanks for your help and regards,
Sorry for the delay in reply. Would it be possible for you to attach the THS4631 schematic for which you are trying to calculate the total output noise? Also, what is the photo-diode model/part number you are using or it's photo-diode capacitance?
As you may have noted from the book references, the total output noise is primarily dependent upon the noise gain of the op-amp. At higher frequencies, the noise gain is given by NG = 1+(Cg/Cf) which determines bulk of the integrated total output noise. The Cg in the noise gain term consists of the photo diode capacitance as well as the input capacitance (both differential and single-ended cap) of the THS4631; whereas Cf is the feedback capacitance. I believe the WEBENCH calculates the total output noise based on the above noise gain calculation. It would be helpful if you can attach the WEBENCH schematic for further understanding of your problem.
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In reply to Rohit Bhat:
Thanks for your reply. I have attached both design reports (WEBENCH and mine). I hope I made mistake in my calculations that will explain the discrepancies in noise, specially with laser light on photodiodes.
0412.TI Forum - Help with TIA.pdf2112.WebenchReportsServlet.pdf
In reply to Herve Dongmo:
Sorry for missing this post.
I don't think you have made a mistake in your calculations for the total output referred noise. For some reason, the WEBENCH noise numbers seem to be too high for the circuit you are designing. I have asked the WEBENCH team to look into this and they are yet to get back.
I have simulated your circuit in TINA-TI and able to correlate with your calculation. The only difference between my simulation and your calculation is the gain bandwidth product (GBP) where I am simulating ~150MHz of GBP based on Aol curve, whereas in the calculation you have considered 325MHz (fc). Simulating a GBP of 150MHz gets me closer to the total output referred noise of 977uVrms. In the device datasheet, the GBP is mentioned to be around 210MHz which is the fc number I would recommend to use in your calculation.
Now, regarding the issue of measuring 10 times the output referred noise than calculated, are you making there is no other source of parasitic capacitance at the inverting input of THS4631 other than the ones considered in your calculation?
Figure 1: Aol and Noise Gain (1/Vfb) curves
Figure 2: Total Output Referred Noise
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