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Old 14th November 2010, 06:58 PM   #11
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Excellent. Thanks for drawing my attention to this LTC1968

w
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Old 15th November 2010, 12:10 AM   #12
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I too will be interested in the PCB when you have it ready to go. Do you have any plans for a DC to Log conversion ?
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Old 15th November 2010, 02:44 AM   #13
grenert is offline grenert  United States
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Quote:
Originally Posted by wakibaki View Post
Excellent. Thanks for drawing my attention to this LTC1968

w
If you're looking for instant gratification, Linear sells an LTC1968 development board:
Linear Technology - DC427B - LTC1966, LTC1967, LTC1968 Evaluation Kit
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Old 18th November 2010, 05:38 AM   #14
Frex is offline Frex  France
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Hello,

Mother of Pearl, after some reflexion i think too that adding a logarythmic output can be interesting.
I've made some tests with a AD8307 and the results are very good.
I don't know yet how i will add it in the original design...
Maybe a switch in front panel for choosing with LOG or LIN output, or an other separate output...

I continue to do other tests about that and i work too to rebuild the front-end of RMS-DC converter.
I think add log converter before RMS-DC converter, in AC coupling mode (10Hz-1MHz).
DC measurement in LOG mode is problematic with the AD8307 and also not very useful in practical use.
I'll post soon a new synoptic.

Frex.
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Old 18th November 2010, 12:40 PM   #15
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Hi:
Having thought about this for some time I too think that putting the log converter first makes sense.
Also, for my purposes, it would be very useful to be able to output the logged, full wave rectified absolute value of the signal. It would be useful for loudspeaker transient, or "ring down," analysis using cosine-shaped 5 cycle tone bursts.
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Old 18th November 2010, 12:48 PM   #16
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Mother of PEARL View Post
............ it would be very useful to be able to output the logged, full wave rectified absolute value of the signal. It would be useful for loudspeaker transient, or "ring down," analysis using cosine-shaped 5 cycle tone bursts.
could you refer me/us to a paper/thread that explains what you are proposing?
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Old 18th November 2010, 01:11 PM   #17
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Hi:
This Google pulls many hits:

linkwitz shaped tone burst testing - Google Search

and this page shows exactly what I'm on about:

Stored-energy3a

It's funny to see that someone else has done exactly the work I did on this back in the mid-80s.
Now, if someone would come up with a "normalizing" time base for a 'scope's external input that would keep one, or five, or fifteen cycles the same width on the screen while frequency is varied that would be just ducky. Note that it's not that hard to do so as the "burst generator" Linkwitz & Co. designed so long ago starts with a continuous tone, and with a little glue logic, a zero crossing detector and a 5-steps-up/5-steps-down attenuator generates a very useful signal that has some interesting higher, odd order spectral components that are supremely useful for measuring what I call "resolving power."
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Old 27th November 2010, 01:27 PM   #18
Frex is offline Frex  France
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Arrow News from the projects.

Hello,

These days i've working on the ERMSDCv2 design.
The final schematic is very close, and all my tests results are concluant.

I've finally add, as requested by "Mother of Pearl" an logarythmic output.
This one used an AD8307 logarythmic amplifier from Analog-Devices.
The log output allow to measure extremly low signal level, down to 50uV(-85dBV).

To know more, you can see the new synoptic of the design below ;

Click the image to open in full size.

You can download it as pdf HERE.

The x1/x10 swtich has dissapear on this new version(2.1).
I prefer keeping low signal level to ensure good signal integrity in all measurement bandwith (0-1MHz) and staying away from supply rails.

The input sensivity level of the RMS converter is 350mVrms or 3.5Vrms with standard 10X scope probe.
For the log converter, the max input level is 1Vrms (0dBV) with direct connexion to input or 10Vrms ( 20dBV) with 10X scope probe.
Because input impedance is well know, 1MOhms 10pF, using X100 scope probe is also possible to extend input voltage range.

The input include a compensated divider network wich can be also use (or not) depending if you prefer an highest sensitivity level.
If you prefer use external scope probe, the network is shorted with a jumper.

The full schematics of this new version can be downloaded HERE. (pdf file 250ko). IT IS NOT THE FINAL VERSION !

I've doing many measurements on the RMS converter section and on the LOG section of the design.
You can see all results below;

1/ LTC1968 RMS voltmeter error vs input level (signal sine wave at 10kHz).


Click the image to open in full size.


2/ LTC1968 RMS voltmeter error vs input frequency (signal sine wave at 250mV).


Click the image to open in full size.


3/ AD8307 LOG converter passband, 10Hz-10Mz view (signal sine wave at 250mV).


Click the image to open in full size.


4/ AD8307 LOG converter passband, zoomed view 100Hz-1MHz (signal sine wave at 250mV).


Click the image to open in full size.


5/ AD8307 LOG converter linearity vs input level (signal sine wave at 10kHz).


Click the image to open in full size.


5/ AD8307 LOG converter dB error vs input level (signal sine wave at 10kHz).


Click the image to open in full size.


Note: All measurements where done with input network disable (x1).

As you can see, results aren't ridiculous.
When seeing RMS error graph, I think it could be better tuned.


Now schematics is near to be fully validate for me, and i think to report all of these modifications in the final PCB work.

Don't hesitate if you have any question about the design.

Mother of PEARL, i think it will be more interesting for you to add the external circuitry you need for your own need, adding all of that in the ERMSDC design does not seem to me very relevant.

Frex.
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Old 5th December 2010, 08:55 AM   #19
Frex is offline Frex  France
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Post Some news.

Hello,

I still work on the ERMSDCV2 design and improvements.
I'm very happy with last results.

Because LOG output has been added, i modify the input sensitivity of the RMS/LOG input.
For the log output, input can be up to about 1.5Vrms ( 3.5dBV) and can measure level as low as about 25V(-92dBV).
The dynamic range of the LOG converter is very impressive, and it's because i've
decided to add it on the final design.
You must note that the minimal sensitivity of the LOG converter are directly related on the noise of the analog input stage.
To obtain the noise floor og -94dBV, i use an input stage with OPA627 and AD817.
First OPAMP (OPA627) is FET input low noise.It allow high input impedance with very low noise contribution.
The 2nd OPAMP AD817 is more noisy but have greater bandwidth for doing ~1.8MHz 2nd order low pass filtering.


I've also modified the RMS/DC design.
Now, the max input level (before input OPAMP saturate) is about 2.5Vrms.
The input/output gain is now 1(instead of 2).
So, without input probe it can handle 2.5Vrms (7.1Vpkpk) and 25Vrms(71Vpkpk) with a 10X scope probe.
The input stage has been improved using FETs instead of 1N4148 diodes for over-voltage protection.

The overall precision of the RMS converter is very good in all bandwidth and with any waveform type (sine,square,triangular).

The new PCB is now ready, i'm checking all gerber files.
I'll post final schematics and more documents in my next post.
I open also a new wiki, for interested DIYers who would want build one.
To soon.

Frex.
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Old 8th December 2010, 03:30 AM   #20
carpin is offline carpin  Netherlands
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