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sroochi
sroochi
12/22/2016 10:38:44 AM
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Newbie
Re: Rederivation and results
I realize there is more than one way to skin a cat.  Such is the case with DACs.  You can use fully digital  techniques to create precise voltages with respect to a reference by working out the on --off ratios mathematically and implementing them digitally -- with simple digital blocks.  

A simple way that I use, and uses less overall silicon is to work out binary area ratios of devices and use resistors, capacitors, and even devices -- components one have to use anyway to  create and implement any useful device/product.  For example, a simple string of resistors -- equal weighted resistors with a simple switching matrix can create without trims, and without complex error correction up to 10Bits (~1024 levels) and with minor complication to ~20Bits. One can even throw in error correction algorithms -- especially non-linearity correction--simply and in a very straight forward manner.  The correct design of the basic DAC can be fully monotonic as well so the only need for error correction is the inevitable non-linearity as the design goes beyond 16Bits.

Mixed signal analog-digital design is probably the most powerful design methodology of any that I personally know.  It can create machines of any complexity/intelligence.  By the way, all can be fully computer simulated so one knows for certain that whatever idea you may have created will work first time and every time.

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D Feucht
D Feucht
9/10/2014 11:43:12 PM
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Blogger
Rederivation and results
I reviewed the given derivation and noticed that the denominator was approximated by 1 instead of the exponential approximation. When it is used instead, then the result is

D*(1-D)*(Ts/tau) = 2^-n

and (Ts/tau) is not squared because the one in the denominator cancels one of them in the numerator. Then the result is that the x100 ratio for fs/fbw applies to 6 (not 8) bits and this number scales by 2 times per bit. For 7 bits it is x200 and for 5 bits, x50. These more conservative numbers show even more why this is not a fast DAC!

 

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Victor Lorenzo
Victor Lorenzo
9/1/2014 3:48:50 AM
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Blogger
Re: Two simple applications
@SunitaT0, according to customer's requirement, the electronics part is required to work without interruption for at least 10 years. The mechanical part allows routine maintenance (cleaning, oil, etc) once every five years.

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SunitaT0
SunitaT0
8/31/2014 11:43:44 PM
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Master
Re: Two simple applications
@ DaeJ, that is well explained in brief and concise format.

@ Victor Lorenzo, I was just wondering the maximum number of years an installation based on

your explanation would go for.

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SunitaT0
SunitaT0
8/31/2014 11:43:09 PM
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Master
Re: Selection of RC Value
Hi Dennis, I must admit that this information proved to be quite useful especially the derivation of the design equation. With everything lined out and well explained, modelling of a simple PWM DAC has been reduced to a simpler task.

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D Feucht
D Feucht
8/25/2014 11:49:37 AM
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Blogger
Re: Selection for RC value
Vasanjk,

First, there is no point in driving an RC integrator with a current source; the R in series with the source has no effect on circuit behavior because the current source is already an infinite resistance.

As I understand the advice you've been given, the scheme is to switch a current source into (let us say) a capacitor. This resuls in triange-waves instead of the exponential-waves in the article. It would be a good exercise for you or your advisor to work out the equations for this scheme; it is easier than the derivation presented in the article but you can use the article as a kind of template in doing it. (Then submit the result for publication, perhaps.)

I have not done this particular derivation but I think you'll find that the result is not much different than what is in the article. The reason is simple: the exponential ripple is intended to be small and the exponential waveforms are much less than a time constant. This makes them nearly linear, like the triangle-waves of the switched-current scheme. Consequently, I think you can apply the equations of the article as being closely approximate for the suggested scheme.

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vasanjk
vasanjk
8/24/2014 11:57:50 PM
User Rank
Master
Re: Selection for RC value
DF

That was an extensive reply. Thanks. By the way, one of my colleagues tells me to use a current source to drive the RC while the PWM waveform is used to switch the current source accordingly. He claims that output voltage linearity would be much improved. Any thoughts on this?

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D Feucht
D Feucht
8/22/2014 8:03:24 PM
User Rank
Blogger
Re: Selection for RC value
Vasanjk,

In my previous post, I was referring to the PWM frequency and its effect on the DAC output waveform ripple, though as you noted, the waveform dynamics are also affected by both PWM and DAC output waveform frequencies.

Although the article did not address DAC dynamics, the DAC will have the usual zero-order hold (ZOH) transfer function. The output waveform will be delayed by half the PWM period and as the waveform frequency increases, will roll off (magnitude will decrease) until it is zero at the Nyquist frequency of half the PWM frequency.

The PWM DAC is not known for its dynamic performance yet is a valuable means for minimizing circuitry when a static or near-0 Hz waveform is required, such as setting analog parameters.

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vasanjk
vasanjk
8/22/2014 12:44:18 AM
User Rank
Master
Re: Selection for RC value
DF

 

For a given set of RC values, if the frequency selected is higher than specified, would it have an effect on the response time of the resultant analog signal. Will there be a latency between the PWM change and analog signal.

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D Feucht
D Feucht
8/21/2014 10:45:23 PM
User Rank
Blogger
Re: Selection for RC value
For a switching frequency greater than that given by the table or equation, the ripple is less than an LSB. In other words, the result improves.

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