Photodiode amplifier noise

Either the amplifier has some significant equivalent noise voltage that you're bringing into play with the cap, or you're creating a great big peak in the noise spectrum.

Dunno which, I haven't played with photodiode amps in a _long_ time.

It's called thermal noise; or more properly Nyquist noise. The way I've understood it - it's noise created by electrons 'boiling' off of surfaces. The effect is actually temperature-dependent, and follows the same type of plot as thermal radiation. I'm not 100% sure on this next statement, but I believe the effect is more pronounced on capacitors (read: capacitors, semiconductor junctions, antennas) than any other circuit element because electrons boiling off of the dielectric must always end up in your circuit.

At RF this acts as a feedback voltage divider. Small capacitance from OUT to IN- (so it won't oscillate), big capacitance from IN- to ground. Lets say the ratio is 100. That's like having a 1K in the feedback and

-> drops to unity gain (at frequency), less noise.

Unless I've misunderstood you somehow.

The amp has inherent input noise. If you configure an opamp to have, say, a gain of 1000, you'll see 1000x its input noise at its output; it's amplifying its own noise. The input noise is usually measured in nanovolts per root Hertz (ie, per the square root of the measurement bandwidth) where 1 of same is a very, very good value.

Now connect the amp with a negative feedback resistor and a capacitor from the inverting input to ground. This is an inverting amplifier with a gain of Rf/Xc, where Xc is the impedance of the cap at any given frequency. Xc drops as F goes up, so the opamp front-end noise is amplified more and more at higher frequencies. That's the simple version.

The cap itself doesn't generate any noise.

The simple fix is to add another cap across Rf to limit the gain explosion, but there are better ways to do it.

Phil Hobbs analyzes this in detail in his electro-optics book.

John

I've just been bitten by the same bug. (ignoring the input capacitance to ground in an inverting amplifier.) OK it was a few months ago, but it led me to sand off the ground plane underneath my input and improve the response time by an order of magnitude. So the noise comes from the op-amp and the gain comes from the ratio of the capacitance from the inverting input to ground and the capacitance from the inverting input to the output.

George Herold

The feedback resistor and input capacitance together, besides attenuating the output signal also cause a 90 degree phase shift, which isn't a problem except at the high frequency end of the opamp's useful gain, where it also starts shifting the phase by close to 90 degrees. The result is a loop phase shift of nearly 180 degrees, or positive feedback. This results in a big peak in the gain seen by the input noise voltage at this high frequency. A good place to look is Phil Hobb's website

Paul Probert

You can also look at the data sheets on TI fast FET-input op amps - they still seem to be being written by the people who were working for Burr-Brown back when TI took them over.

who don't yet seem to have been properly indoctrinated about the TI corporate policy of ripping off the customers.

-- Bill Sloman, Nijmegen

-- Bill Sloman, Nijmegen

website

TI makes a lot of kick-ass parts. Their field guys call on us, and support is excellent.

They have the best fast 15-volt analog process on the planet; look up THS3201.

And check out THS4303.

And they are *still* shipping SN7400N.

Whose parts do you prefer? Philips? Siemens? Plessey? Ferranti? GEC? Intermetall?

John

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I started using the their analog parts with the TLC2001 for which the data sheet didn't mention the anomalously high inut capacitiance (at around 15pF). When this reared up and bit me I rang TI support, who didn't have a clue what the input capacitance might be and showed no willingness to find out.

If they've raised theor game sice then I'll be pleased, but surprised. Burr-Brown wasn't ever anything like as helpful as Analog Devices, but they never practised the active deceit that I'd run into with Texas Instruments in 1972.

Analog Devices, Linear Technology, Fairchild, National Semiconductor, Siemens (now Infineon) and Motorola (now ON=3DSemiconductor) but I'll use anything that I can be sure of getting my hands on. Complete and reliable data sheets are a big plus, but it is the device performance that is crucial - and good data sheets can save a lot of debugging time.

-- Bill Sloman, Nijmegen

website

Ooh, nifty part!

What's the story there?

website

Holy s*it! Who still uses plain vanilla 74xx parts?

website

If Cin wasn't specified, why did you assume it was suitable for your application? That's reckless. I always measure stuff like that when it matters.

All my dealings with B-B (pre-TI) were great. Their parts worked as aadvertised, they were available when the parts were first announced, and support was excellent. THings remained that way after TI acquired them.

There was a fiasco a year or so ago. The sales of the BB group ramped up and a lot of old test sets failed and/or couldn't be reproduced to keep up, or so I've heard. At any rate it became hard to get parts. I've been told the TI worked very hard to fix it, and it seems to be fixed. That sort of thing happens in the semi business.

Maxim is the real horror story.

Analog and Linear are excellent lately, although Analog has burned us a couple of times. National is fair, overall.

I've been trying to buy some parts from Linear Integrated Systems, the little fet/diode operation in Fremont. So far, they seem downright hostile to people who want to buy their stuff, sort of like Western Electric in the old days, reputed to have an anti-sales staff.

John

I have read around these parts that increasing bias will increase sensitivity and I would assume increase SNR, but Thats all I know. I would like to know more.

greg

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Get Phil Hobb's book if you want the low down on photo diode preamps. There is some good stuff on his web site if you can't afford the $200 price tag. You bias the PD to reduce capacitance (increase bandwidth). No effect on SNR unless you're running at low currents where the dark current might be an issue. Bias has no effect on sensitivity. One photon =3D one electron

George Herold

Thank you folks, that's helped get it clear in my mind what's really going on. And to answer some points -

- I'd already pored over Phil Hobbs' front ends article (it was mentioned here recently)

- I'm already using the OPA657. I noticed it gave much lower noise due, I think, to its low current noise across the large feedback resistor. It seems way better than the OPA847 which has

I made up some amplifiers using the AD795 after looking around. At least I thought they were pretty good.

greg

I never thought it would help sensitivity but I thought I heard at least on person mention it did.

I always wanted to jack up the voltage to detect gamma rays.

greg

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I've always regarded LIS as a hobby of the founder. I think it had a different name, or that is Hall's second jfet company. Is there something they make that Interfet can't provide?

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It was certainly reckless, but lots of cheap op amps don't specify Cin and it's usually close enough to a couple of pF. I'd actually checked to see if I'd need a feedback capacitor for an input capacitance in that ball-park. Since the TLC2001 was in a non-critical bit of the circuit, I didn't have any trouble solving the problem with a 4.7pF capacitor from output to inverting input.

It pissed me off because it was - in fact - quite a hairy circuit which I'd designed and laid out without any protoptyping, and all the tricky bits of the circuit worked exactly as I'd intended, except for this boring little integrator wrapped around the TLC2001. Most of the components were SMD so the 4.7pF stuck out like a sore thumb.

-- Bill Sloman, Nijmegen