How to determine SFDR, SNR, SNDR of ADC with LTSpice

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kickbeer

Joined: 07 Nov 2008
Posts: 160
Helped: 1

05 Sep 2009 16:41 sfdr importance in adc

Hello,

The standard SFDR is 48dBc. What does it mean, when SFDR is higher than 48dBc or lower than that value?

Thank you very much

Regards,
Kickbeer

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JoannesPaulus

Joined: 19 Mar 2008
Posts: 261
Helped: 44
Location: USA

05 Sep 2009 18:14 sndr adc

SFDR is defined as the ratio of the RMS value of the carrier frequency to the RMS value of the next largest noise or harmonic distortion component.

kickbeer

Joined: 07 Nov 2008
Posts: 160
Helped: 1

05 Sep 2009 19:59 noise sfdr

so, does that mean the higher the SFDR, the better it is?? Because the higher the noise, SFDR will be lower

JoannesPaulus

Joined: 19 Mar 2008
Posts: 261
Helped: 44
Location: USA

06 Sep 2009 2:43 snr for adc

kickbeer wrote:

so, does that mean the higher the SFDR, the better it is??

Correct. SFDR is a measure o linearity.

kickbeer wrote:

Because the higher the noise, SFDR will be lower

I do not know what you mean.

kickbeer

Joined: 07 Nov 2008
Posts: 160
Helped: 1

06 Sep 2009 10:57 adc simulation sndr matlab

I'm simulating my ADC with LTSpice. I took this from an article:
"To determine a system’s SFDR, apply a sine wave to your ADC input and use an appropriate sample rate.Then run an FFT and examine the spectral output. You can see the fundamental frequency and “spurs” from harmonics of the test signal and system components. I recommend you make this test at several sine-wave frequencies and sample rates."

Do you know, how many times(minimum) should we apply sine-wave with different frequencies?

Petre Petrov

Joined: 05 Aug 2009
Posts: 51
Helped: 5
Location: Sofia

06 Sep 2009 12:06 ltspice maxim

May you should take at least 20 experiments with different values.
The requirements about the experiments/simulations depend on the test goals.

Also here you have some explanations:
http://en.wikipedia.org/wiki/Spurious-free_dynamic_range
http://www.maxim-ic.com/glossary/definitions.mvp/term/sfdr/gpk/268
http://www.maxim-ic.com/tools/calculators/index.cfm/calc_id/maxim_sinad

But the term and the source seem to me controversial.

Could you please give more data about your paper and test goals?

kickbeer

Joined: 07 Nov 2008
Posts: 160
Helped: 1

06 Sep 2009 12:38 wiki sfdr

Petre Petrov wrote:

I just have designed a 8-bit folding and interpolating ADC. In order to check the static performance, i have put a ramp input signal and found out that there is no missing codes. Now i want to know test the dynamic performance for example the SFDR at simulation level. I really don't have idea how to record all the data and use it in matlab. I don't know if it is easy or not because i am matlab beginner. I think all the article in Maxim website are for fabricated chip. I really need help because i am stuck at this point. Thy very much

JoannesPaulus

Joined: 19 Mar 2008
Posts: 261
Helped: 44
Location: USA

06 Sep 2009 19:51 rms ltspice

you need to figure out how your simulator can write text data to a file (I have never used LTSpice so I cannot help you here) then you can use matlab to read the text file in and create a data vector that will be processed by an fft function to give you the adc output spectrum.
Once you have the spectrum, you can calculate the SNR, THD, SFDR, and so forth...
The learning curve might be a little steep but knowing the basics is fundamental in circuit design...

ozgura2

Joined: 02 Sep 2009
Posts: 3
Helped: 2
Location: Turkey

09 Sep 2009 17:54 ideal sfdr of a dac

You will do a transient simulation, assume sampling with a 500MHz clock, fsampling is 2ns. For 8-bit performance 256 data points is sufficient for FFT. Do a minimum 256*2ns (512ns) transient simulation. Input sinus frequency should be (127/256)*500M. 256 is FFT points 127 is a prime number. For this case input frequenccy is near Nyquist frequency, you can decrease it. After tran sim use Cadence's dft calculator function. If you are using other tools there should be fft function similar.

kickbeer

Joined: 07 Nov 2008
Posts: 160
Helped: 1

10 Sep 2009 15:30 sndr from dft

ozgura2 wrote:

Is there no need to connect my ADC with ideal DAC or something else? Can i just use my ADC alone? Look at this http://cmosedu.com/cmos2/hw_MSD/Ch5_MSD_hw.pdf

Added after 6 minutes:

JoannesPaulus wrote:

Hi Joannes,

Have you ever worked with Matlab before? i have just found a Matlab Code to convert .raw waveform file into compatible matlab data. i have successfully converted the .raw data(output of my ADC) into matlab compatible data. but now, i don't know how to use the data to calculate parameter like SNR, SNDR, and SFDR..I really have no idea at the moment

ozgura2

Joined: 02 Sep 2009
Posts: 3
Helped: 2
Location: Turkey

10 Sep 2009 19:25 sfdr of adc

I use binary weighting of digital output data at Cadence calculator like (BIT<7>*2^7+BIT<6>*2^6...) Of course you can use an ideal DAC also. DFT will sample discrete points of this summed data.

kickbeer

Joined: 07 Nov 2008
Posts: 160
Helped: 1

10 Sep 2009 20:56 ltspice for chip design

ozgura2 wrote:

I use binary weighting of digital output data at Cadence calculator like (BIT<7>*2^7+BIT<6>*2^6...) Of course you can use an ideal DAC also. DFT will sample discrete points of this summed data.

Can you show me the snapshot of your simulation result? And what type of ADc you have designed?