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How to calculate R&C in Sigma-Delta Modulator

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ahmed saad

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Hi
I'm trying to use Sigma-Delta modulator , and I using Altium Design to simulate it , but its response is not good
I think I need to change the r & c values but I don't know the equation
my I/P signal is Sin wave 80 KHz .
DeltaSigma Sch.pngDeltaSigma Sim.png
 

For integrator dimensioning of first order SD, you should review text books, e.g. Schreier.

Presently it's not a problem of integrator values, the circuit can't work because Ref2 is out of range. It's also unlikely that the circuit will work with 1 MHz clock and a very slow OP like LM358.
 

The inputs of an LM358 do not work if they have a voltage closer than 1.5V from the positive supply voltage. Your second opamp's (+) input voltage is the same as the positive supply voltage. Since it is an inverting opamp then its output will always be saturated as high as it can go.

The lousy old LM358 was designed for low power so it has trouble with frequencies above 2kHz (poor slew rate).
 

I Forgot changing V.ref2 DeltaSigma Sim.png
now it is 2.5 V
i'm using LM324 not LM358
I'm still need to know how to calculate R & C
& Why flip flop output not as a normal digital signal ?
 
Last edited:

Should the feedback be direct or controlled with the Clock?
 

Should the feedback be direct or controlled with the Clock?
Click to expand...
The post #1 schematic is showing basically a correct time discrete first order SD modulator.

- I just noticed that the feedback polarity which is wrong. There must be overall negative feedback, flip the + and - input of the comparator or take the feedback voltage from Qbar.
 

I did stimulation for both connections
I get this resultsDeltaSigma Fb Q.pngDeltaSigma Fb Qpar.pngDeltaSigma Fb Qpar flip com.png

witch one is right?
 
Last edited:

The second is right regarding feedback sign, but the test setup has other problems, e.g. input voltage range.
 

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