Delta sigma modulator help

[Isaacp]

Hi all,

I was hoping someone on this forum could help me out with a circuit I'm working on, it's a delta sigma modulator for a class D guitar amp.

I've started by doing some rough spice simulations (my usual process) but i cant seem to get a decent square wave out of it, I'm thinking the issue is with the integrator, my only reason for that is because this is the first time I've used one .

heres the schematic I have so far:



and here is the plots i'm getting



red: input signal
green: integrator output
Blue: Square wave output from that last comparator - looks identical to output from the flip flop just at 24vpp instead of 1vpp

Any chance someone with more experience then could jump in with a suggestion?

cheers,
Isaac

[phase_accurate]

First of all remove C2. DS looops must be DC coupled !

Then what is the output swing of the second comparator ? You might need some offset correction if this is not symmetrical to ground.

If you get this running the real problems will start: You will need to use a higher loop order if you want some reasonable signal quality.

Regards

Charles

[djoffe]

previous poster had good advice...to this, I would add a few things:
You don't need the comparator before the flip flop. In the real world, it would probably just cause havoc, assuming the +/-12 volt outputs drove into a 5 volt supplied flip flop...please correct me if I'm assuming too much about the parts.
The integrator has enough gain that the slicing of the flop at the 2.4 volt logic level won't create significant offset. Just connect your integrator straight into your flop.

Next, your time constants are kind of long, and your clock freq kind of low for audio.

Last, you might want to append a filter on the output of the final comparator to look at you results with the high freq hash removed...it will give you an idea of the fidelity at a glance.

good luck.

[Isaacp]

Wow, didn't respect a reply so quickly, I can see why this forums so popular. I'll have to make sure to check back more regularly.

Oops your right about the cap, i put it there while testing that integrator and forgot to pull it out, funnily enough though removing it had little effect :S (clue maybe?)

The output from the second comparator swings from -12 to 12V unloaded following the square wave out of the latch, but when i close the loop it swings from -12 to 0.8V. Increasing R3 and R8 makes it swing closer to the rails but it takes quite large values (~1Meg) to make an appreciable difference, but then the output from the integrator becomes tiny (~100mv swing) any theories?

sorry probably sounding ignorant at this point, just trying to get my head around the 1st order loop first then i should be able to troubleshoot for myself - or at least that's how most of my projects seem to go :P

Thanks for you help this far,
Isaac

[Andrew Eckhardt]

If that's a 74HC74 from 74HC.lib it doesn't act like it has input clamp diodes like a real part. Unless you arrange for a maximum of 6 volts drive and a 6 volt supply on the flip flop, a real part will either mess everything up or get messed up. For noise reasons I'd stay well below 5 volts at the logic part unless you must have the speed. 12 volt logic is usually slow anyway.

[adman]

Isaacp, can you specify the type of D-flipflop and its supply-voltage(s) ?

[phase_accurate]

Instead og a D-FF I usually use some sort of sample-and hold followed by a saturated gain-stage to simulate delta-sigma modulators.


OK it is not actually the real thing but it works quite well for sims.

Regards

Charles

[Isaacp]

Originally Posted by djoffe

You don't need the comparator before the flip flop. In the real world, it would probably just cause havoc, assuming the +/-12 volt outputs drove into a 5 volt supplied flip flop...please correct me if I'm assuming too much about the parts.

good point I'll give that a go

Originally Posted by djoffe

Next, your time constants are kind of long, and your clock freq kind of low for audio.

yeah, that was an attempt to more accurately see what was going on, but i'll fiddle and see if it has an effect. (i'm thinking that as long as the clock rate and input frequency are the same order of magnitude apart it should work? I have been wrong before though )

Originally Posted by djoffe

Last, you might want to append a filter on the output of the final comparator to look at you results with the high freq hash removed...it will give you an idea of the fidelity at a glance.

yeah good idea, although at this point its just a reminder that the circuits not working should be a nice moment when it starts to resemble a sine wave though.

@Andrew, Adman and Phase accurate
I was using the generic 'behavioural flip flop provided by ltspice; maybe a bad idea by the sounds of it? in any case the sample and hold followed by a comparator swinging between 0 and 5v sounds like an easier fix than trying to figure out how to add a new part to ltspice (another project for another time i think)

thanks again everyone for your help, ill try all of the above and post how it goes.

cheers,
Isaac

[djoffe]

I just threw something together that works, at least for simulation.
100uH and 1 uF is filter for 8 Ohm load. I grabbed some likely FETs to drive
the load. It's kind of unrealistic in that a real flop wouldn't drive the FETs
nicely without some buffering...the floating 5 volt supply as a level shifter
is also unrealistic, but the idea was to get you into the ballpark.

It has a gain of 5, and doesn't take too horribly long to simulate...the output looks a bit noisy, but about what you'd expect given the OSR and the low order (first) modulator.

Note...sigma_delta.txt is really an LT spice .asc file. Just rename it to be sigma_delta.asc, and you can simulate in LT Spice.

[Isaacp]

Thank you djoffe Just what I needed
turns out my original problem seemed to be the integrator clipping due to the signal from the first op amp - oh well at least its an easy fix, hopefully someone with the same issue as me will stumble across this and save themselves some frustration.
thanks for the help everyone, ill keep working away on a higher order loop and post here. If all goes well a prototype should be next... I have no idea how people just start building these things and have them work, oh well experience I guess, maybe one day I'll get there

Thanks again,
Isaac