| Author |
Message |
Joel Kolstad
Guest
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 Posted:
Thu Feb 10, 2005 8:41 am Post subject:
Frequency response of LDRs |
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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 Vactec.com) and Clairex
doesn't seem to make these any more.
Thanks,
---Joel Kolstad
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Tony Williams
Guest
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| Posted:
Thu Feb 10, 2005 9:39 am Post subject:
Re: Frequency response of LDRs |
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In article <KsKdnRbcBaRblZbfRVn-sA@comcast.com>,
Joel Kolstad <JKolstad71HatesSpam@Yahoo.Com> wrote:
| Quote: | 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 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.
--
Tony Williams. |
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Keith Wootten
Guest
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| Posted:
Thu Feb 10, 2005 3:04 pm Post subject:
Re: Frequency response of LDRs |
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In message <KsKdnRbcBaRblZbfRVn-sA@comcast.com>, Joel Kolstad
<JKolstad71HatesSpam@Yahoo.Com> writes
| Quote: | 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 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://www.aicl.com.tw/cds/p6-12.htm is a good guide. Look at the
'Light History Effect' section.
Cheers
--
Keith Wootten
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Guest
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| Posted:
Thu Feb 10, 2005 4:40 pm Post subject:
Re: Frequency response of LDRs |
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Have a look at the Fairchild H11F1 and H11F3 parts.
http://www.fairchildsemi.com/ds/H1/H11F1.pdf
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 |
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Ian
Guest
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| Posted:
Thu Feb 10, 2005 7:18 pm Post subject:
Re: Frequency response of LDRs |
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|
"Joel Kolstad" <JKolstad71HatesSpam@Yahoo.Com> wrote in message
news:KsKdnRbcBaRblZbfRVn-sA@comcast.com...
| Quote: | 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 Vactec.com) and Clairex
doesn't seem to make these any more.
Thanks,
---Joel Kolstad
|
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 |
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Joel Kolstad
Guest
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| Posted:
Thu Feb 10, 2005 9:20 pm Post subject:
Re: Frequency response of LDRs |
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<bill.sloman@ieee.org> wrote in message
news:1108032036.727058.300680@f14g2000cwb.googlegroups.com...
| Quote: | 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.
|
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 |
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Jim Thompson
Guest
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| Posted:
Thu Feb 10, 2005 9:25 pm Post subject:
Re: Frequency response of LDRs |
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On Thu, 10 Feb 2005 07:20:12 -0800, "Joel Kolstad"
<JKolstad71HatesSpam@Yahoo.Com> wrote:
| Quote: | bill.sloman@ieee.org> wrote in message
news:1108032036.727058.300680@f14g2000cwb.googlegroups.com...
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.
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
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food. |
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John Larkin
Guest
|
| Posted:
Thu Feb 10, 2005 10:37 pm Post subject:
Re: Frequency response of LDRs |
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|
On Thu, 10 Feb 2005 07:20:12 -0800, "Joel Kolstad"
<JKolstad71HatesSpam@Yahoo.Com> wrote:
| Quote: | bill.sloman@ieee.org> wrote in message
news:1108032036.727058.300680@f14g2000cwb.googlegroups.com...
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.
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.
|
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 |
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Joel Kolstad
Guest
|
| Posted:
Thu Feb 10, 2005 10:43 pm Post subject:
Re: Frequency response of LDRs |
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Hi Jim,
"Jim Thompson" <thegreatone@example.com> wrote in message
news:75vm01pvja84fhjh3ikmdld661pbuuva6v@4ax.com...
| Quote: | If it's uP-controlled, why not use a ladder of R's switched with
analog switches?
|
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 |
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John Larkin
Guest
|
| Posted:
Thu Feb 10, 2005 10:43 pm Post subject:
Re: Frequency response of LDRs |
|
|
On Thu, 10 Feb 2005 08:25:39 -0700, Jim Thompson
<thegreatone@example.com> wrote:
| Quote: | On Thu, 10 Feb 2005 07:20:12 -0800, "Joel Kolstad"
JKolstad71HatesSpam@Yahoo.Com> wrote:
bill.sloman@ieee.org> wrote in message
news:1108032036.727058.300680@f14g2000cwb.googlegroups.com...
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.
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
|
AKA "multiplying DAC"
John |
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Jim Thompson
Guest
|
| Posted:
Fri Feb 11, 2005 12:22 am Post subject:
Re: Frequency response of LDRs |
|
|
On Thu, 10 Feb 2005 08:43:50 -0800, John Larkin
<jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote:
| Quote: | On Thu, 10 Feb 2005 08:25:39 -0700, Jim Thompson
thegreatone@example.com> wrote:
On Thu, 10 Feb 2005 07:20:12 -0800, "Joel Kolstad"
JKolstad71HatesSpam@Yahoo.Com> wrote:
bill.sloman@ieee.org> wrote in message
news:1108032036.727058.300680@f14g2000cwb.googlegroups.com...
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.
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
AKA "multiplying DAC"
John
|
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
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food. |
|
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|
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Pooh Bear
Guest
|
| Posted:
Fri Feb 11, 2005 6:10 am Post subject:
Re: Frequency response of LDRs |
|
|
Joel Kolstad wrote:
| Quote: | 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 Vactec.com) and Clairex
doesn't seem to make these any more.
|
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
?
http://www.silonex.com/audiohm/index.html
Graham |
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Roger Lascelles
Guest
|
| Posted:
Fri Feb 11, 2005 12:13 pm Post subject:
Re: Frequency response of LDRs |
|
|
"Joel Kolstad" <JKolstad71HatesSpam@Yahoo.Com> wrote in message
news:KsKdnRbcBaRblZbfRVn-sA@comcast.com...
| Quote: | 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 Vactec.com) and Clairex
doesn't seem to make these any more.
Thanks,
---Joel Kolstad
|
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.
Roger |
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Roger Lascelles
Guest
|
| Posted:
Fri Feb 11, 2005 12:24 pm Post subject:
Re: Frequency response of LDRs |
|
|
"Roger Lascelles" <invalidl@invalid.invalid> wrote in message
news:3730orF593q01U1@individual.net...
| Quote: |
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.
Roger
|
Have a look at :
http://optoelectronics.perkinelmer.com/content/RelatedLinks/analogoptoisolat
orapplicationnotes.pdf
i.e.
http://tinyurl.com/57tw4
to see discussion of distortion "Keep voltage across LDR to 1.0V or less"
Roger |
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