# Photodiode amplifier noise

Discussion in 'Electronic Design' started by Nemo, Apr 29, 2009.

1. ### NemoGuest

I'm embarrassed to ask this but -

I have a high gain (transconductance) amp with a photodiode, biased to
be a few hundred pF, on its input. I was tweaking it to optimise the
noise levels and found the noise of the amp was way less than the noise
of (the amp with the diode on the front). So I replaced the diode with a
simple capacitor... and I still get extra noise when I fit it. It does
not appear to be noise from the bias supply; the capacitance can be to
ground and I still see the extra noise appearing.

The texts I've read say there's noise generated by Cin, and it certainly
seems like there is. I can understand it acts as a potential divider on
the amp's gain and attenuates high frequency signals. What I don't get
is why it generates noise without a signal present? What is the physical
mechanism which creates this noise?

2. ### Guest

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.

3. ### Guest

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

4. ### Guest

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.

5. ### Guest

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=Semiconductor) 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.

6. ### Bob LarterGuest

Ooh, nifty part!
What's the story there?

7. ### Bob LarterGuest

Holy shit! Who still uses plain vanilla 74xx parts?

8. ### GregSGuest

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

9. ### EcnerwalGuest

10 Hey look, a neat part!
20 Discontinued.
30 GOTO 10

Insert:

15 Design something that uses it

if feeling really masochistic.

10. ### John DevereuxGuest

....And all your friends and family are still getting their mailers 10
years later

11. ### Guest

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 =
one electron

George Herold

12. ### NemoGuest

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 <1nV/rt Hz voltage noise but
higher current noise.

So I guess I'm thoroughly indoctrinated in the local groupthink!

(Actually I suspect there's something odd about the OPA847. It seems
very difficult to get stable, even using the Texas recommended circuit
for a mere x20 gain.)

For high end analog parts, cost no object, I would look first at Texas,
Analog and Linear Tech.

For cost-effective ones I would look at National and OnSemi but they
don't seem to make really leading edge stuff these days.

For cost-effective consumer parts I would also look at SGS Thomson and
Philips. However their product life cycles are only a few years so I
tend to avoid them, despite being European myself. My impression is that
European companies tend to specialise in niches I don't play in, such as
mobile phones, or produce cheap commodity parts in the Far East. I would
like to use more European parts but they just don't suit my
requirements.

Maxim have interesting parts but again, they tend to burn the users with
sudden shortages. I have heard they are fabless (not fab-u-lous) and so
if they run short of something, it can be months before the next batch
is cooked up. Whatever the reason, I found them risky to use in the
past.

Texas USED to be terrible at supporting designers, and I used to avoid
them. However about 10 years ago I realised they were essentially a
different company from the bad old days, much more customer-focused. And
had leading edge parts again. They are a delight to use now. Let's hope
Maxim evolves that way too.

I use plenty of other suppliers but those are the first ones I check for
a new requirement.

I avoid Far Eastern parts because I have been burned several times by
their incredibly short life cycles. However, they are becoming more
interesting. The Japanese have always been worth using for very low
power stuff; I recall using a Hitachi micro once which was withdrawn
without the European distributor telling us. Hitachi were horrified at
this treatment of a customer and bent over backwards to support us and
somehow provided us with the things for the rest of our product's life.
I've also had good experiences with a Korean sensor manufacturer, who
was investing in new product development during the same period the
established players were getting rid of all their own technical staff
and could no longer support their products. (The Korean company was
founded and run by engineer brothers, where the major established
Western ones had been bought by venture capitalists who were trying to
pare down costs... eventually they sold the concern to a major sensor
manufacturer who tried to use their now near-monopoly to force customers
to pay top dollar for increasingly shoddy wares. This simply forced
customers to look for new suppliers, or make their own as the patents

13. ### GregSGuest

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

greg

15. ### Guest

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?

16. ### Guest

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.

<snip>

17. ### James ArthurGuest

www.vishay.com/docs/70248/70248.pdf

Cheers,
James Arthur

18. ### Spehro PefhanyGuest

What wrongness have you seen? The commercial stuff for research
applications looks pretty decent to me.

Best regards,
Spehro Pefhany

19. ### Spehro PefhanyGuest

Nothing can touch them on noise performance, although differential
input costs noise and power consumption.

Best regards,
Spehro Pefhany

20. ### James ArthurGuest

I may have a wacky idea. What's your signal bandwidth (if you can say)?

Cheers,
James Arthur