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Old 30th December 2012, 06:17 AM   #1371
1audio is offline 1audio  United States
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Default AC level detector

Here is one could do the job:
http://www.thatcorp.com/datashts/THA..._Datasheet.pdf 80 KHz max. Most of the current devices are focusing on 800 MHz to 2 GHz.

Attached is the simplest peak detector I could think up that would work. The LM339 is an open collector comparator. The input circuit is very like an opamp and what happens here is the the output follows the input until it goes positive. Since its open collector the output is then limited by the charge on the cap and the pull up resistor. Because its a moderately fast comparator the internal circuitry doesn't latch up or slow down when there is significant differential on the inputs. I have used this in several applications in the past and it worked well. And you have three more sections of the LM339 to do useful things with.
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Old 30th December 2012, 07:10 AM   #1372
davada is offline davada  Canada
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Quote:
Originally Posted by 1audio View Post
Here is one could do the job:
http://www.thatcorp.com/datashts/THA..._Datasheet.pdf 80 KHz max. Most of the current devices are focusing on 800 MHz to 2 GHz.

Attached is the simplest peak detector I could think up that would work. The LM339 is an open collector comparator. The input circuit is very like an opamp and what happens here is the the output follows the input until it goes positive. Since its open collector the output is then limited by the charge on the cap and the pull up resistor. Because its a moderately fast comparator the internal circuitry doesn't latch up or slow down when there is significant differential on the inputs. I have used this in several applications in the past and it worked well. And you have three more sections of the LM339 to do useful things with.
"Here is one could do the job:
http://www.thatcorp.com/datashts/THA..._Datasheet.pdf 80 KHz max."

If I used enough of this sort of thing in cascade I should be able to push the ripple frequency right out of band. But it would cost a small fortune.

This one is a possibility.

Thanks,
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Old 30th December 2012, 10:46 AM   #1373
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Quote:
Originally Posted by davada View Post
"Here is one could do the job:
http://www.thatcorp.com/datashts/THA..._Datasheet.pdf 80 KHz max."

If I used enough of this sort of thing in cascade I should be able to push the ripple frequency right out of band. But it would cost a small fortune.

This one is a possibility.

Thanks,
That's an IC version of the old DBX rms detector. When I visited DBX in 1977 they showed me a "blessed" one that came out with 120dB range that was enshrined with a huge analog readout meter as their "lab standard".
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Old 30th December 2012, 04:18 PM   #1374
davada is offline davada  Canada
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Originally Posted by scott wurcer View Post
That's an IC version of the old DBX rms detector. When I visited DBX in 1977 they showed me a "blessed" one that came out with 120dB range that was enshrined with a huge analog readout meter as their "lab standard".

Hi Scott,

I was thinking that the ripple out of a squaring rms detector, like the AD536 or the ones you sent me, is double the frequency of the input. They accept dc so if I put a number of them in series it should multiply the ripple moving it up the bandwidth. Less filtering would be needed or better smoothing for the same amount of filtering. We don't need accuracy just repeatability and it should cover the bandwidth of interest. But these IC's are expensive. Is there another device that will do the same at a lesser cost.

This is building on an idea Dick Moore gave me some time ago.

Edit;

Come to think of it these devices double the frequency of AC. The output of the rms detector is dc with a ripple component that is not ac so it won't work.
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Old 30th December 2012, 04:37 PM   #1375
RNMarsh is offline RNMarsh  United States
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First think of the very Best Way.... for best results. Then pay the cost to do it Best Way. Dont throw away the best way because it cost more. Too much consideration to an IC cost, other parts cost. Thx-RNMarsh
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Old 30th December 2012, 04:46 PM   #1376
davada is offline davada  Canada
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First think of the very Best Way.... for best results. Then pay the cost to do it Best Way. Dont throw away the best way because it cost more. Too much consideration to an IC cost, other parts cost. Thx-RNMarsh
Good advise Rick.

Okay Infinite sample and hold it is then. ADC to MDAC. No ripple.
So what if I have to give it a long time constant. The output will be pure.

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Old 30th December 2012, 04:53 PM   #1377
RNMarsh is offline RNMarsh  United States
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Ok then lets do it that way. The best way for best performance.


[BTW--Did you look at the AD8436 (90dB dynamic range). An input amp with approp scaling will extend it]

Thx-RNMarsh

Last edited by RNMarsh; 30th December 2012 at 05:00 PM.
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Old 30th December 2012, 05:17 PM   #1378
davada is offline davada  Canada
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Originally Posted by RNMarsh View Post
Ok then lets do it that way. The best way for best performance.


[BTW--Did you look at the AD8436 (90dB dynamic range). An input amp with approp scaling will extend it]

Thx-RNMarsh
Yes I have looked at that but the package turned me off. It's the same as all other rms detectors. We still have a ripple component at the 2nd H to deal with.

ISH has no ripple at all or droop. There is a some glitch from MDAC switches but the integrator
in the proportional controller takes this out.
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Old 30th December 2012, 05:46 PM   #1379
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by davada View Post
ISH has no ripple at all or droop. There is a some glitch from MDAC switches but the integrator
in the proportional controller takes this out.
OK. Then that is the way to proceed. Thx-RNMarsh
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Old 30th December 2012, 05:58 PM   #1380
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Quote:
ISH has no ripple at all or droop. There is a some glitch from MDAC switches but the integrator in the proportional controller takes this out.
If the controller has proportional action (and it needs this to be fast settling) there is by definition no (significant) low pass filtering. So glitches surely contribute some distortion.

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