Raj_Singh

Joined Nov 26, 2023
31
I have built a one bit Sigma Delta ADC. So, I am now trying to increase the resolution of the ADC by using a digital filter which consists of a Binary counter connected to a PIPO shift register.
The red graph is the output of the counter and green one is the output of a shift register. How do I calculate the resolution or the number of bits from the below graph?
The clock frequency of the Sigma delta is 18.8kHz.

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MisterBill2

Joined Jan 23, 2018
18,464
For starters, THAT IS A VERY GOOD QUESTION!! For earlier ADCs it was easy, because the resolution is based on the total number of steps between minimum and maximum. It is not so very simple in an SD DAC. . In the example it appears that there is no difference in resolution because the effective ON/OFF ratio does not seem to change.
I suggest running a simulation and see how much the actual filtered output changes per one-bit change in the command value.

Raj_Singh

Joined Nov 26, 2023
31
For starters, THAT IS A VERY GOOD QUESTION!! For earlier ADCs it was easy, because the resolution is based on the total number of steps between minimum and maximum. It is not so very simple in an SD DAC. . In the example it appears that there is no difference in resolution because the effective ON/OFF ratio does not seem to change.
I suggest running a simulation and see how much the actual filtered output changes per one-bit change in the command value.
I have update the clock pulse and now I am getting an output like this. So the red graph is the output of the sigma delta, the green graph is the output of the counter and the blue graph is the output of the shift register. Also, could you please explain what is ON/OFF Ratio and how can I find it from my graph

Ian0

Joined Aug 7, 2020
9,803
Isn’t the resolution inversely proportional to frequency?

MisterBill2

Joined Jan 23, 2018
18,464
OK, an explanation of ON/OFF ratio, as applied to any PWM circuit, and probably to a sigma-delta DAC. It is the ratio of digital output "high" to digital output "low"over one complete cycle of the varied output. Also it would be the ration of pulses present to pulses not present in a single cycle of a scheme that varies the number of pulses inan interval rather than the width of a single pulse. Not that simple to generate as I discovered when trying to generate digital AM as an option. In fact it becomes quite complex, trying to generate an AM broadcast signal digitally, without a processor.