usernamer
Newbie level 6
Hi all,
I have some questions about ENOB measurement and improvement, I know it is a long post and many questions but I do not have an idea how to go on, I would be really grateful if someone could answer even to only a part of the questions.
I have to design a 9 bit accuracy amplifier with a gain of 2 for an ADC, and supply voltage is 800 mV.
The closed loop amplifier is a capacitive reset switched capacitor gain circuit, where clock frequency is 100 MHz so the only instant of the output waveform that matters is the one right before the amplification phase ends. See figure
only difference is that I am using a fully differential version of it.
The amplifier is a two stage folded cascode.
For the simulation I am using spectrum in Cadence virtuoso, which calculates automatically ENOB and SINAD. So I would need to get ENOB=9 and the way I use DFT is as follows:
My differential input signal is a sinusoid at f=50 MHz, and my DFT takes samples at F=2f=100 MHz at the end of each amplification phase (this is the way the following stage is supposed to behave) and the interval of the DFT is an integer number of periods of my input signal.
In the figure below the markers show 2 of the points where I am sampling right before the reset phase (red output, yellow input differential signals)
Also I am using strobeperiod to have uniform raw data points.
Here come some of my doubts:
Also as far as I know, the only ways now to improve ENOB would be to reduce noise or distortion:
Thanks for helping
I have some questions about ENOB measurement and improvement, I know it is a long post and many questions but I do not have an idea how to go on, I would be really grateful if someone could answer even to only a part of the questions.
I have to design a 9 bit accuracy amplifier with a gain of 2 for an ADC, and supply voltage is 800 mV.
The closed loop amplifier is a capacitive reset switched capacitor gain circuit, where clock frequency is 100 MHz so the only instant of the output waveform that matters is the one right before the amplification phase ends. See figure
only difference is that I am using a fully differential version of it.
The amplifier is a two stage folded cascode.
For the simulation I am using spectrum in Cadence virtuoso, which calculates automatically ENOB and SINAD. So I would need to get ENOB=9 and the way I use DFT is as follows:
My differential input signal is a sinusoid at f=50 MHz, and my DFT takes samples at F=2f=100 MHz at the end of each amplification phase (this is the way the following stage is supposed to behave) and the interval of the DFT is an integer number of periods of my input signal.
In the figure below the markers show 2 of the points where I am sampling right before the reset phase (red output, yellow input differential signals)
Also I am using strobeperiod to have uniform raw data points.
Here come some of my doubts:
- first not only the ENOB I get is lower than 9 but it also depends on the initial phase of my input sinusoid, so it is higher if the sampling occurs at the peaks of the waveform, why is that?
- It also depends on the frequency of input signal (I do not understand why since for each input signal anyway has a frequency equal or less than half of the sampling frequency so the DFT should "see" the correct signal regardless of its frequency). Again why does this happen? My hypothesis: it is because at 50MHz DFT does not "see" the higher harmonics (odd multiples) due to distortion, becuase it is sampling at 100 MHz, is it this the reason?
- I know that one should choose a number of samples N=2^n (a power of 2) so the algorithm is faster, but it is still correct even though I do not choose a power of two, isn't it? It should just be slower?
- I get the following warning: "The function values at from and to are not equal." but since the waveform is noisy I suppose it is not possible that first and last values are equal, is there a way to fix this warning?
Also as far as I know, the only ways now to improve ENOB would be to reduce noise or distortion:
- To reduce noise which has root mean square vn, in the feedback amplfier I have vn^2=γkT/C (where C=load capacitance and γ a constant parameter) so I cannot do much if I cannot increase C, is it correct?
- To reduce distortion I should improve linearity so I could inject higher signals and thus get better SINAD, but how can this be done? Or where could I find such information?
Thanks for helping