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Old 4th March 2015, 10:34 AM   #291
zfe is offline zfe
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Quote:
Originally Posted by oneoclock View Post
Format: signature, samplerate, interpolationrate, type, numbercoefficients, multiplier

What is multiplier? Is the gain?
It is the number with which all filter coeficents are multiplied. So yes it is gain if >1. With the new version of mkrom.exe the multiplier can also be a float.
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Old 4th March 2015, 10:56 AM   #292
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Quote:
Originally Posted by oneoclock View Post
Format: signature, samplerate, interpolationrate, type, numbercoefficients, multiplier

What is multiplier? Is the gain?
Yes, the multiplier is gain.

I've found that you need to keep multiplier * max coefficient value = 1 or lower or the filter will clip.
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Old 4th March 2015, 07:51 PM   #293
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Then 8 gain , F1 and F2 is bad.
I can't edit and delete my proposed NOS filter to not download more people.
The correct NOS is proposed by spzzzzkt: 1021filtNewNOS.skr
I'll measure and hear.

Robertrowett have much reason.

Last edited by oneoclock; 4th March 2015 at 07:54 PM.
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Old 5th March 2015, 12:53 AM   #294
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Originally Posted by zfe View Post
So the "problem" is that RC is to high and the DAC needs several similar samples of the final sample rate of about 3MHz, so a FIR2 filter, to come near U_sig.
As we want to filter anyhow and are not designing a MHz AWG signal generator this is not really an issue, I think. But...

The easiest way to get a more text-book like output with the bypass filters, is to lower or remove C.
Very interesting.

When you say lower C what value would you propose? I think Søren has mentioned the filter cap is 1200pF. If you leave the resistors as is, wouldn't the value of the capacitor need to increase to lower the corner frequency?

I'd been looking at SMD film caps in this position but was dissuaded by the "for reflow only" warnings.

Add: I plugged the formula into excel and had a play, and I can see that you need to reduce the capacitor to increase the output level.

Last edited by spzzzzkt; 5th March 2015 at 01:06 AM.
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Old 5th March 2015, 03:04 AM   #295
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Panasonic 1200pf / 16v PPS in 0603 package should work ok here. Digikey has them for sure.

Not hard to solder, use a good solder & be quick!

Greg in Mississippi
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Old 5th March 2015, 03:19 AM   #296
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Quote:
Originally Posted by Greg Stewart View Post
Panasonic 1200pf / 16v PPS in 0603 package should work ok here. Digikey has them for sure.

Not hard to solder, use a good solder & be quick!

Greg in Mississippi
Thanks for the tip. I had thought that something like WBT silver solder would have low enough melting point to avoid dramas.
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Old 5th March 2015, 04:18 AM   #297
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Quote:
Originally Posted by spzzzzkt View Post
Very interesting.

When you say lower C what value would you propose? I think Søren has mentioned the filter cap is 1200pF. If you leave the resistors as is, wouldn't the value of the capacitor need to increase to lower the corner frequency?

I'd been looking at SMD film caps in this position but was dissuaded by the "for reflow only" warnings.

Add: I plugged the formula into excel and had a play, and I can see that you need to reduce the capacitor to increase the output level.
I was working on this earlier for the output stage thread. In it Søren said,

"625R Zout, capacitor is 1200 pf ceramic NP0, -3db is then at 212 Khz. NP0/C0G types are supposed to be fine, especially when their effect are way out of the audio band...."

When I calculate for 10,000pf substitution I get 25464.79 hz. My hope is that this will help with the aliasing in the NOS setting and avoid the time smear of the FIR OS. Of course, a .5uf PIO cap is much easier to come by so changing the resistor may also be in order.
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Old 5th March 2015, 07:21 AM   #298
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Quote:
Originally Posted by robertrowett View Post
I was working on this earlier for the output stage thread. In it Søren said,

"625R Zout, capacitor is 1200 pf ceramic NP0, -3db is then at 212 Khz. NP0/C0G types are supposed to be fine, especially when their effect are way out of the audio band...."

When I calculate for 10,000pf substitution I get 25464.79 hz. My hope is that this will help with the aliasing in the NOS setting and avoid the time smear of the FIR OS. Of course, a .5uf PIO cap is much easier to come by so changing the resistor may also be in order.
Where did you find 1200pF? I only saw
Quote:
Originally Posted by soekris View Post
...
And the 470 pF capacitor at the output of the R-2R resistor chain take care of anything coming though.
You can calculate the internal resistance from the measured data if you know the value of the capacitor.
625 Ohm is what you get from the DAC ladder alone as resistance (I measured R=5000 Ohm for a R in the R 2R network) .
With 470pF the internal calculates as about 1600 Ohm, with 1200pF it drops but is still above the ladder-allone resistance.

The value for replacing the capacitor depends on what you want to do.
If you want the bypass filter to have full output. you have to lower the value. If you want more soothing you have to increase.
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Old 5th March 2015, 08:10 AM   #299
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Quote:
Originally Posted by robertrowett View Post
I was working on this earlier for the output stage thread. In it Søren said,

"625R Zout, capacitor is 1200 pf ceramic NP0, -3db is then at 212 Khz. NP0/C0G types are supposed to be fine, especially when their effect are way out of the audio band...."

When I calculate for 10,000pf substitution I get 25464.79 hz. My hope is that this will help with the aliasing in the NOS setting and avoid the time smear of the FIR OS. Of course, a .5uf PIO cap is much easier to come by so changing the resistor may also be in order.
This is not good thinking. Increasing the capacitor to create a first order filter that low in frequency will have effects in the audio band.

Also the output Z of the R2R looks to be fixed and inherent in the DAC design. There is no way you will be able to make use of a 0.5uF capacitor here.

Last edited by gazzagazza; 5th March 2015 at 08:12 AM.
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Old 5th March 2015, 11:41 AM   #300
zfe is offline zfe
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OK, instead of guessing, we also can calculate the value of the capacitor.
A value to relay on is the resistance of the DAC ladder, I get the 625 Ohm as well my measuring the whole thing as well as by calculation based on one resistor. Moreover that is the number given by Soren.
The capacitor is only discharged through the DAC ladder. The time for loosing half the Voltage is about 520ns. Using U(t)= U (1-exp(-t/(R C)) we get for roughtly C=580pF.
Taking in account the inaccuracies of the measurement this makes 470pF the more plausible value.
Using the above formula again with that C, t=1/(64*44,1kHz) rise time and U(t)=0.745V, U=2V, we get as internal resistance of the DAC as voltage source (=DAC Ladder+ internal resistance before the ladder) something about 1600Ohm.
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