Delta sigma modulator help

[Isaacp]

ok, I think I'm starting to get the hang of this. I've managed to put together a second order loop which is slow and pretty temperamental with regards to changing components - especially the caps and resisters controlling the integrators, but its progress nevertheless
here's the schematic

and here's the output


The next step is to fiddle with component values and opamps/comparators to get a sense of which bits affect what, the goal being to make it a little more stable and of course faster, any suggestions as to where to start?

oh and one other thing, I've read that opamp integrators cant be used in loops above second order, so I suppose i really have two questions. First how would you implement an integrator without an opamp? (i'm guessing it'd be some kind of rc filter) and second is it worthwhile for a guitar amp that is likely to have distortion dialled in, one way or another regardless?

cheers,
Isaac

[phase_accurate]

I would start by removing the error:

Using just one feedback resistor for both integrators (i.e. R6) doesn't work. Both need an own feedback resistor !

Play a little with them first (and also the caps). We will show you how to determine a noise transfer function later on.

Regards

Charles

[Isaacp]

Sure, like this?

actually looking at it now, i can see it why it should be there, so thankyou for preventing future headaches (or am i wrong - should it be in series with R6?)
noise transfer function sound interesting, I look forward to it. until then I'll keep fiddeling, still having trouble getting the speed up, but it seems like a much smaller problem problem compared to where I was two days ago

thanks again,
Isaac

[terminator12]

Originally Posted by Isaacp

oh and one other thing, I've read that opamp integrators cant be used in loops above second order,

Not at all. People have managed easily going upto 3rd and 4th order. Above that, compensating becomes a nightmare. If you refer the IEEE or elsevier journals you can very frequently see people using 3rd and 4th order loops.
As far as sigma delta modulation is concerned, it is better to go for higher order loops to increase the randomness of the quantizer. It is always easy and reliable (stability-wise) to extract the coefficients from the Richard-Shreier's toolbox(MATLAB).

Originally Posted by Isaacp

so I suppose i really have two questions. First how would you implement an integrator without an opamp? (i'm guessing it'd be some kind of rc filter) and second is it worthwhile for a guitar amp that is likely to have distortion dialled in, one way or another regardless?
cheers,
Isaac

1) Impossible. Integrators can never be realized without an active element. (passive filters can never have a pole at the origin of the s-plane)
2) could you rephrase please.

[Isaacp]

Originally Posted by terminator12

Not at all. People have managed easily going upto 3rd and 4th order. Above that, compensating becomes a nightmare. If you refer the IEEE or elsevier journals you can very frequently see people using 3rd and 4th order loops.
As far as sigma delta modulation is concerned, it is better to go for higher order loops to increase the randomness of the quantizer. It is always easy and reliable (stability-wise) to extract the coefficients from the Richard-Shreier's toolbox(MATLAB).

unfortunately I don't have access to any of the above, i subscription to IEEE might be worthwhile though provided its not stupidly expensive...

Originally Posted by terminator12

1) Impossible. Integrators can never be realized without an active element. (passive filters can never have a pole at the origin of the s-plane)

sorry must have been late when I was replying I had this in mind:
RC circuit - Wikipedia, the free encyclopedia I realise now that would only work with a single time constant - not a varying signal - Fourier analysis never was my strong point .

Originally Posted by terminator12

2) could you rephrase please.

again I blame tiredness :P my question was, is it worth going to all this trouble to reduce distortion and noise for a guitar amp that is going to have at least a little distortion added intentionally? I haven't had an opportunity to listen to the kind of distortion produced by a low order delta sigma loop, so I don't really know how it sounds.

thank you everyone for your patience I'm trying to learn all this as I go.
Isaac

[Isaacp]

Just did another simulation, up to third order now, using opamp integrators so in conclusion Terminator12, your right and I'm wrong as to be expected
anyway here's the schematic:

and the output:

if your wondering about why its such a small voltage, its because its taken through a high impedance tap from the output of the flip-flop to avoid effecting the circuit. Looks nice and clean though

[terminator12]

Isaacp, what is the reason behind putting those comparators in-between the integrators? I think it would work better without them. Do you have any concrete reason behind using them?