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Old 25th February 2013, 10:07 PM   #271
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Having trouble replying to PM's here Terry just in case it did not get to you.



Originally Posted by zenelectro
George,

Just to be clear, we are talking 844 in open loop
configuration as two separate elements, grounded
base IP stage and opem loop buffer?

I haven't been following thread closely, too much going
on.

Yes Terry, open open loop (no feedback) with 2.7k and 570pf parralelled to ground from TZ (pin 5) And then the signal comes off TZ to my 3rd oder LP filter. I don't use the 844's buffer, it sounds better without and I don't think the LP filter is loading down TZ.
Ccheers George

Terry
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Old 25th February 2013, 10:11 PM   #272
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Quote:
Originally Posted by jcx View Post
thinking wrong - Iout can happily go straight into gnd copper whithout hurting the chip
100% sure?
Must try it then, because if it work there, and also I can still have the 1st order at TZ (pin 5) then I might be able to ditch the 3rd order opamp filter.

Then I will have two -3db LP filters around 160khz that would be nice.

Comments anyone because I don't want to do damage to the dacs.

Cheers George
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Last edited by georgehifi; 25th February 2013 at 10:13 PM.
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Old 25th February 2013, 10:57 PM   #273
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Terry still having trouble answering your PM's are you getteing them. Here is the second one .

Terry[/QUOTE][/QUOTE]

Thanks George

I'm surprised you need the additional 3rd order LPF.

I've found in the past open loop I-V's if done well needed very little LP filtering.
Maybe it's an 844 specific thing

cheers

Terry[/QUOTE]



You can still see the reminance of the stair case noise on a full output 20khz sine wave if you don't use it, picture back on this post.
Using the AD844 as an I/V

Cheers George
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Old 26th February 2013, 01:45 AM   #274
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Quote:
Originally Posted by dirkwright View Post
I assumed normal filtering for all incoming and outgoing cables.
What's 'normal' depends on context. Normal filtering on cables in the audio world is none, whereas normal in the RF world is feedthrough capacitors and fully enclosed (i.e. faraday shielded) boxes with gaskets on the lids.

Quote:
What I said is still true: all those pretty black anodized boxes are basically worthless for shielding.
Yep - and the ones you showed are scarcely any better due to absence of cable filtering.

@qusp - is that 'OTT' as in 'over the top' or OTT as in Henry?
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Last edited by abraxalito; 26th February 2013 at 01:47 AM.
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Old 26th February 2013, 05:52 AM   #275
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Quote:
Originally Posted by dirkwright View Post
Yet another reason to put your gear inside of RFI resistant metal boxes. Most of those "pretty" boxes they sell to hobbyists do not shield RF because they are anodized aluminum. Here's what a more professional RFI resistant box looks like. This is from Par-Metal (I am only a satisfied customer) and contains a SV oscillator (not quite finished yet).
Par-Metal
Nice case ! But also from aluminum....and anodized it seems.
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Old 26th February 2013, 09:54 AM   #276
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Quote:
Originally Posted by AudioLapDance View Post
Hey George,

I wanted to give some 'pay back' for all the work you've done in the thread! Since you're not into Tina, feel free to request any sim.

...
Jeff if it's not too much trouble can you see how this SIM's up, also I'm very interested to see what the input (with & without the 100k) and output impedances are.
I may try this straight from the TZ of the AD844 next weekend
This is one of the ebay buffers from raindrophuey cost me $10 shipped all ready made up.

Cheers George
Attached Images
File Type: jpg fetbuffer 2 + dc offset adjust.jpg (42.4 KB, 209 views)
File Type: jpg fetbuffer2.jpg (124.4 KB, 208 views)
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Old 26th February 2013, 12:24 PM   #277
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No problem!

One way to quickly DC analyze a circuit is to see gain devices as high input impedance and low output impedance.

So input impedance (w 100k) would be 100k in parallel with 2 inputs in parallel

So 100k || 1M || 1M so like 90k

no 100k: 1M||1M = 500k

Output impedance 22||22 =10ohm

I'll sim it at lunch!

Cheers,
Jeff

PS My same buffers had significant DC offset (+50, +80mV) indicating that the input FETs aren't matched! And the seller didn't care ...

Last edited by AudioLapDance; 26th February 2013 at 12:28 PM.
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Old 26th February 2013, 12:30 PM   #278
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Quote:
Originally Posted by jean-paul View Post
Nice case ! But also from aluminum....and anodized it seems.
Actually, it is alodine, which remains electrically conductive while preventing the aluminum from corroding.

From their website:
"Series 14 rack-mount chassis have been carefully engineered and improved to provide both reliable EMI/RFI shielding and structural rigidity. Special treatment has been used for all panel contact areas to guarantee the shielding. Test data from our chassis reached 70dB electrical noise attenuation for varying frequencies. Our EMI/RFI shielded chassis are specially designed by our electrical engineering team with expertise in RFI shielding and testing. Par-Metal can custom manufacture, modify, and test the EMI/RFI shielded enclosures to meet your specifications. We can also save you time and money by delivering your custom order in lead times of 1 to 3 weeks."

and:
"Add suffix-x for your choice of color or finish:

-A alodine (EMI / RFI shielded)
-B black anodize (no EMI / RFI shielding)
-C clear anodize (no EMI / RFI shielding)
-E painted beige (mask for EMI / RFI shielding)
-G painted gray (mask for EMI / RFI shielding)
-N clear alodine (EMI / RFI shielding)
-P painted black (mask for EMI / RFI shielding)
-S black anodize front and gold alodine body"

And thanks, it is a very well made chassis.

It seems that using a steel chassis can cause problems for circuit boards that are too close to it, meaning that the box has to be bigger for a given board size.
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Old 26th February 2013, 12:34 PM   #279
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Quote:
Originally Posted by abraxalito View Post
What's 'normal' depends on context. Normal filtering on cables in the audio world is none, whereas normal in the RF world is feedthrough capacitors and fully enclosed (i.e. faraday shielded) boxes with gaskets on the lids.



Yep - and the ones you showed are scarcely any better due to absence of cable filtering.
I don't agree that "cable filtering", by which I assume you mean filtering the signal at the cable connector on the chassis, is a serious weakness nor a serious problem for well designed cables. I use Canare Star Quad cable, which is designed to dramatically reduce RF interference. See the attached document for reference.
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File Type: pdf Canare Star Quad story.pdf (66.3 KB, 36 views)
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Old 26th February 2013, 12:38 PM   #280
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Quote:
Originally Posted by georgehifi View Post
That's a big cap to ground that the PCM1704 will see, I'm too scared to try that, it could cook the 1704.

Cheers George
Below the -3dB point, the capacitor looks like an open circuit to the DAC. Above the -3dB point, it more and more looks like a short circuit. So, since the vast majority of the spectral energy coming out of the DAC is in the audio range, which is below the -3dB point, then only a small amount of high frequency "junk" becomes a short circuit for the DAC. I don't think that it will be a problem, but have not tested it in real life yet so I am not completely certain.
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