I'm trying to design a general purpose photodiode transimpedance amplifier for my laser optics lab, but haven't yet entirely mastered the art of the datasheet. The OPA380 and OPA657 seem promising, but the difference in how the specs are given on the datasheets gives me some trouble in making an apples to apples comparison. For what it's worth, here's some info on the applications I have in mind:
--Low End: DC level measurements of changes in laser power (this is a relative, not absolute measurement).
--High End: Small, fast (microsecond) changes in fluorescence levels (again, relative measurement).
--Interchangeable Photodioide: In conjunction with a switchable selection of feedback capacitors, I'd like to be able to change between photodiodes for different purposes.
The 380 boasts "a transimpedance bandwidth of > 1MHz in most configurations" and a GBW of 90MHz, while the specs of the 657 show a Small-Signal Bandwidth of 90MHz at Gain = +20, and a GBW of 1600 for Gain > +40. How does all this translate in TIA configuration with a 10 Mega-ohm resistor? Is the 657 a hands-down better choice if an output DC offset of a few microVolts one way or another isn't an issue?
Thanks in advance for sharing some wisdom. Have a great day.
Here is a good application note that may help you with your transimpedance amplifier design: Transimpedance
Considerations for High-Speed Operational Amplifiers - sboa122
The app note discusses gain, bandwidth, noise, and stability as they pertain to designing transimpedance (photodiode) amplifiers.
The big difference here between using the OPA657 or the OPA380 is that you will be able to achieve roughly 4.2 times the transimpedance bandwidth with the OPA657 than with the OPA380 due to the much higher GBW (1600MHz vs 90MHz). Bandwidth can be estimated using eq. 12 on p. 5 of the application note.
For the higher speed application where you need to detect fluorescence level changes in the time scale of microseconds, the OPA657 is a better choice. Since relative measurements are being taken, the OPA657 will be suitable for the DC measurements as well.
How do you plan to implement the switchable selection of feedback capacitors and photodiodes? Do you mean that different boards will have different capacitor/photodiode combinations, or do you intend to use a switch mechanism to allow switching components on-the-fly? Switches will add capacitance and introduce possible leakage paths that may cause issues in the design.
Thanks Kristoffer. I'll have my nose in that app. note for a while to come I think.
I haven't worked out all the details of the capacitors yet. I've seen switchable feedback resistors in other designs, and I figured why not do the same with some low-value capacitors. I could have different sizes/speeds of photodiodes around the lab, all set up to plug directly into the TIA circuit. Once a photodiode has been picked for a given application, the feedback capacitor value could be selected that gives the best response for that photodiode. I was planning on using an on-board switch like TS12A4514 in order to minimize the effect of the switches on the circuit.
I'm still a little green with this stuff, so I'm sure there are things I'm not thinking of here.
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