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Frequency response of LDRs

Discussion in 'Electronic Design' started by Joel Kolstad, Feb 10, 2005.

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  1. Joel Kolstad

    Joel Kolstad Guest

    I'm thinking of using some of the combined LEDs/light dependent resistors as
    'voltage controlled resistors' in an op-amp circuit. Would anyone care to
    take a stab at what sort of frequency response I can get with such an
    approach? I.e., does something like a CdS cell LDR have significant
    parasitics that will seriously degrade the performance of an op-amp filter?

    I'm looking at items such as Silonex (e.g., NSL32). I couldn't find the
    Vactec web site (didn't look much, but it's not and Clairex
    doesn't seem to make these any more.

    ---Joel Kolstad
  2. I don't know the specifics, but they're certainly fine in the tens of
    kilohertz region. The problems are more to do with the 'Light History
    Effect' where their resistance depends not only on their current
    illumination, but also on their past illumination, so making predictable
    settings difficult.

    Http:// is a good guide. Look at the
    'Light History Effect' section.

  3. I used them many years ago, in an analogue multiplier.
    Clairex? did a pair of cells in one can. I used one
    light source and servo'd the control voltage on one
    cell. The other cell then controlled the signal channel.

    ISTR that they were fairly slow at low light levels,
    presumably due to the high resistance and stray capacitance.
  4. Guest

    Have a look at the Fairchild H11F1 and H11F3 parts.

    Wost case turn-on and turn-off times are listed as 25usec, but the
    resistive element is a FET rather than some horrible lump of CdS.
  5. Ian

    Ian Guest

    I've seen these used in agc loops in signal generators, and I've used them
    with no adverse effects over the audio band. I don't recollect capacitance
    specs, I think they are pretty low. There were no other parasitics apparent
    down to at least the 0.005% level

    However (you knew that was coming!) beware of the variations in response
    speed, and the often dramatic slow down if you want to use them at low
    illuminations. Response speed can vary a lot between manufacturers, and
    ISTR is not well specified.

    A trick used in the sig gen was to use a matched pair of the LDR's, with
    the same LED illuminating both devices. One of them was set up in a DC
    loop (no AC signals) and driven to track the AGC signal. The matching of
    the devices then meant that the other LDR resistance was pretty similar.

  6. Joel Kolstad

    Joel Kolstad Guest

    I was under the impression that one of the benefits of an LDR using CdS was
    that it's much, much more linear than a FET?

    In my case the resistance value will change very slowly (i.e., a
    microcontroller is going to set the gain of an op-amp a few times per day or
    something when someone wants a bigger/smaller signal) so I don't care how
    quickly the LDR can be 'modulated' or if there's 'history' (so long as the
    history's effect dies out within some seconds), just what the parasitics on
    the CdS part are that'll slow down the op-amp's loop response.

    Thanks for the help,
  7. Jim Thompson

    Jim Thompson Guest

    If it's uP-controlled, why not use a ladder of R's switched with
    analog switches?

    ...Jim Thompson
  8. John Larkin

    John Larkin Guest

    As far as the resistance goes, CdS and CdSe parts are very fast, the
    important parasitic for most applications being shunt C, which can be
    mostly compensated for. The actual resistive mechanism is very fast
    and very linear. The change in resistance vs illumination is slow and
    varies between device types, with CdSe being faster than CdS.

    Some of the fastest electrical samplers use laser-illuminated
    photoresistors. Radiation-damaged GaAs (adds recombination centers)
    can be light modulated in about a picosecond.

  9. Joel Kolstad

    Joel Kolstad Guest

    Hi Jim,

    Since I'm trying to minimize parasitics. This is part of the 'can you build
    an active 30MHz filter with a GHz op-amp?' idea.

  10. John Larkin

    John Larkin Guest

    AKA "multiplying DAC"

  11. Jim Thompson

    Jim Thompson Guest

    You might have to roll your own to get the steps you want. A recent
    chip I did had 0.25dB gain steps.

    ...Jim Thompson
  12. Pooh Bear

    Pooh Bear Guest

    They'll be fine for audio for sure. I've used loads ( tens of thousands ) of
    them over the years. Silonex parts are very nice btw. I did get some Vactec
    parts to evaluate but they're much more expensive and the product seems to have
    been passed around several owners in the last few years. I think that line
    really exists mainly for military devices now.

    I recall finding another supplier called Hamamatsu Photonics a long time back -
    also more expensive.

    For volume manufacture the Chinese make a criminally simply constructed part
    that performs quite well actually. It's actually a pcb with a standard led and
    CdS cell in a light tight package. I seem to recall that the brand name is Eagle
    but you'll probably only end up using it ( as I do now ) if your sub-contractor
    is manufacturing in China too.

    What frequency did you have in mind ?

    I'd stick with Silonex for small quantities. Did you find this bit of their site

  13. I've just used an LDR to stabilise a Wein Bridge oscillator at 600 KHz.

    To reduce the effects of capacitance, I suggest you keep the LDR in its 1K
    ohm region, not 10s of K ohm. Many LDRs will go down to 200 ohm.

    Some gotchas on LDRs :

    1. Drift over lifespan. You won't get the same ohms for the same
    illumination over time. If the illuminator is a LED, it too will age with
    time. Drift by a factor of 10 is quite on the cards. I have seen severely
    drifted parts after only one year operation.

    2. Life. I have measured LDRs as open circuit after 10 years of operation.
    I think life is decreases with equipment temperature. Life may depend on
    encapsulation quality, etc.

    3. Distortion. I worked with a guy who built broadcast voltage controlled
    mixers, and he would not use LDRs, because they produced 0.1% THD. He tried
    the Clarex, Vactrec parts. If the LDR was fading between off and hard on,
    minor distortion during the fade might not matter, but radio stations will
    not buy gear with 0.1% distortion spec. Distortion will depend on the
    voltage across the LDR.

  14. Have a look at :


    to see discussion of distortion "Keep voltage across LDR to 1.0V or less"

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