Frequency response of LDRs

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.

----------------- Bill Sloman, Nijmegen

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,

---Joel

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

...Jim Thompson

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.

John

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.

---Joel

AKA "multiplying DAC"

John

In message , Joel Kolstad writes

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.

Cheers

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.

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

as

filter?

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.

Regards Ian

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 ?

Graham

as

filter?

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.

1. 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.

2. 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.

Roger

operation.

controlled

tried

Have a look at :

i.e.

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

Roger