Connect with us

Photodiode amplifier noise

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

Scroll to continue with content
  1. Nemo

    Nemo Guest

    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.

    http://focus.ti.com/lit/ds/symlink/opa657.pdf

    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 Larter

    Bob Larter Guest

    Ooh, nifty part!
    What's the story there?
     
  7. Bob Larter

    Bob Larter Guest

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

    GregS Guest

    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. Ecnerwal

    Ecnerwal Guest

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

    Insert:

    15 Design something that uses it

    if feeling really masochistic.
     
  10. ....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. Nemo

    Nemo Guest

    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.)

    Concerning the "favourite manufacturer" thread:

    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
    had run out.)
     
  13. GregS

    GregS Guest

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

    greg
     
  14. GregS

    GregS Guest

     
  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 Arthur

    James Arthur Guest

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

    Cheers,
    James Arthur
     
  18. What wrongness have you seen? The commercial stuff for research
    applications looks pretty decent to me.



    Best regards,
    Spehro Pefhany
     
  19. Nothing can touch them on noise performance, although differential
    input costs noise and power consumption.


    Best regards,
    Spehro Pefhany
     
  20. James Arthur

    James Arthur Guest

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

    Cheers,
    James Arthur
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-