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Photodiodes 1. Photodiode Variations 1.
6939386 Photodiode Amplifiers Op Amp Solutions J Graeme 1996 WW
PIN Photodiodes 1. Your email address will not be published. Nice talk about project. Hi Craig, Nice post and information on IRis! Thanks so much for the info on this as well as everything on Analog Zoo — great stuff!
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Sincerely, Marshall Cummings. Leave a Reply Cancel reply Your email address will not be published. Harrison, Chapter 6. Therefore, every time you compute the response and the noise, and change any factor, the computations may change considerably.
There's no simple or obvious way to compute or optimize the performance. The performance, in terms of response or bandwidth, in terms of peaking or overshoot, and in terms of noise or SNR, is an extremely complicated, nonlinear, and highly interacting function of:.
Jerry and I certainly agree on that.
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But I also have worked on this general problem many times over the years and have several suggestions that go be-yond Jerry's book. More on this later. Most bipolar op amps have much higher current noise than FETs. It's a rare case when an op amp with bipolar input transistors is better, except when R S is very low or resistive or in cases where the input is capacitive but the bandwidth is narrow. C You usually want to avoid an op amp with large input capacitance. Unfortunately, most data sheets don't properly specify the op amp's input capacitances, neither differential-mode nor common-mode.
But it's fair to assume that most "low-noise" op amps have a larger input capacitance than ordinary op amps. You may want to ask the manufacturer, or you might just decide to measure it yourself. So if you want to get low noise, you must optimize very carefully. Specifically, begin by computing the im-pedance Z S of your sensor at the maximum frequency of interest:.
For a good amplifier, the voltage noise and the current noise times Z S should both be as small as you can get.
If one of these noises is much larger than the other, then you're probably far off optimum. E If you have any choice of what sensor you employ, try to find a lower-capacitance sensor. Furthermore, make a low-capacitance layout between the sensor and the op amp. If you want to get fast response, low noise, or wide bandwidth, Jerry's book offers some pretty good advice. More on that later. Because some are better than others, I bet you can use Paul Grohe's selector guide to find some low-noise op amps.
See www. Also, Jerry neglected to mention that you can design your own op amp with better, lower voltage noise and better bandwidth. But you can "roll your own" surprisingly easily and accomplish even better performance for a specified application!
Photodiode Amplifiers: OP AMP Solutions
I'm not proposing that you design a complete op amp, but it's simple to just add a new low-noise front end ahead of a suitable op amp. The basic idea is to add a couple of good low-noise FETs in front of an existing op amp. Most op amps don't operate the front-end transistors as rich as the output. Yet in a case like this, there's no reason at all not to run more current through the front end than in the rest of the op amp. What's the voltage noise of this amplifier? When you're designing an op amp, remember this: adding gain is one of the cheapest things you can add.
You on-ly need to be careful about how to give that gain away—to roll it off. In this case, it's easy. The R1-C1 network in Figure 5 just rolls off the gain for a fairly smooth frequency response. We can roll off the amplifier's gain simply in two swoops. The low-frequency gain is rolled off by R X and C X. Then after the gain rolls off flatly, we roll it off some more by R Y and C Y.
This isn't exactly rocket science. We just want to make it a practical design. But this is a whole system design.
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