ADC12D1600

Hi Sherman,

The datasheet does contain a general section of definitions: look for the Specification Definitions.  This section does contain a simple definition of Noise Power Ratio (NPR), but it looks like you are looking for more details.  Please see the attached presentation for a more detailed overview of NPR on the GSPS ADCs.

5775.ADC1xD1x00 NPR for E2E.pdf

Let me know if you have further questions.

Kind regards,

Marjorie

Marjorie,

Thanks for the response and the information on NPR testing. My question concerns how to interpert an inportant technical specification: RMS LOADING LEVEL. I tried to attacha screenchot to this email , but looks like attachments are not allowed. I have re-stated my question.  Any information would be appreciated

Here is a screen shot  of the TI (former National) ADC12D1600 high speed A/D, page 1 and  page 41.

At the bottom of page 41 is a graph of NPR vs “RMS LOADING LEVEL (dB).  My question:  the X axis is labeled “RMS LOADING LEVEL (dB)”.  “dB” in this case is a relative term. What is the reference for the RMS LOADING LEVEL ?  Is it relative to one of the Table 8 (Page 20) analog parameters?  If so, which one and shat is the power of the input signal in dBm. If the input signal is stated in volts peak to peak, what is the impedance (50 ohm, 100 ohm??) so that the input signal power can be calculated?

 Thanks for a quick response.

Sherman

Hi Sherman,

I think I understand your question this time... let me know this response answers your actual question:

First, let us consider the power in a full-scale sine wave. The power is:

P [Watts] = [ (Vpp/2)^2 ] / [2 * Rin, diff ]

P [dBm] = 10 * log10( P / 0.001 ) 

For this example, Vpp = 0.8V; Rin, diff = 100 ohm.  Therefore,

P = -0.97 dBm

What does this equate to in dBFS?  This is a bit of a debate... since this is the maximum amplitude sine wave which can be input to the ADC without clipping, some folks like to label this '0dBFS' for a sine wave.  However, it is actually -3dBFS since the rms power which is in a sine wave is 3dB less than that in a full-scale DC signal.

Now, let's examine the RMS noise loading level.  From p.9 of the pdf document:

2 * Vo = 0.8 Vpp

Vo = 0.4 Vp

Assuming the input is a FS sine wave, sigma = 0.8 / 2 / sqrt(2) = 0.28

k = Vo / sigma = 1.414

1/k = 0.707

RMS noise loading level = 20 * log10(k) = -3 dBFS

So, the RMS noise loading level is in units of dBFS, with the understanding that a full-scale sine wave is -3dBFS, not 0dBFS, as it is sometimes considered to be for an ADC.

Kind regards,

Marjorie 

Marjorie, 

Thanks for the additional information. I am beginning to see the relationships, just a couple of additional comments if you do not mind.

So, for the NPR Vs RMS NOISE LOADING LEVEL graph the X axis is labeled “Vrms LOADING LEVEL (dB)” represents a DC voltage applied to the converter input?  The X axis “VRMS LOADING LEVEL” value of 0 dB would correspond to -3dBFS for a sine wave (according to your explanation, more on that later).  From the graph, the peak NPR occurs at a X axis VRMS LOADING LEVEL of -12 dB and a corresponding NPR value of ~ 49 dB.  So the equivalent power in the sine wave to produce max NPR is 15 dB below the sine wave value to produce FS.

In your example, a sine wave of 0.8 Vpp has a RMS power of -0.97 dBm, corresponding to -3dBFS. The sine wave input to produce max NPR would have a power of -0.97 dBm – 15dB = -15.97 dBm.  Corresponding to 0.025 millwatt , Vrms = 0.503 millivolt rms or 1.4 millVolt pp.

You made the statement, “”However, it is actually -3dBFS since the rms power which is in a sine wave is 3dB less than that in a full-scale DC signal”. The definition of RMS voltage for a sine wave is “ a 1 volt RMS sine wave has the identical heating power as 1 volt DC.” 1 volt rms sine wave applied to a 1 ohm resistance produces 1 watt of heat (or power) and 1volt DC applied to 1 ohm resistance produces 1 watt of heat (or power).  Why the 3dB difference in your statement?  Is it in the definition of  “… a full scale DC signal”?

 Your comments are appreciated.

One non technical question about the forum.  Is there a way to have the forum notify me by email when a reply is posted? I thought I set that up, but I do not seem to be receiving e-mail notices of your posts.

Thanks again for your time

Sherman

Marjorie,

Thank you for the reply. A little bit of housekeeping first ---- you posted on May 31. I received notice of the post on June 5, apparently after  "JimBrinkhurst84999 has marked your post RE: ADC12D1600 in the High Speed Data Converters Forum as answered"  Do not know what that really means, but it appears that need to veryify all of the posts.

Your answer comes a little closer. Here are some additional details about my question.  We do testing of ADC's using both a CW sign wave, continuous noise (AWGN) and notched noise for NPR tests. I an familiar with the notched noise NPR set up - Aglient has a signal generator that can produce true notched AWGN rather than using tones. We use  swept sign wave input to measure frequency response and two or three sign wave to measure distortion. The question is " What RMS power level (as measured by a true RMS power meter) is to be used for the different input signals, so that the ADC is accurately characterized?  Your example of  notched noise mentioned DC to the Nyquest frequency.  For illustration purposes, lets say the RSM power in your signal is -10 dBm, and this signal is applied to the ADC input.  Could a -10 dBm sign wave at 1/2 the Nyquest frequency be used to measure frequency response, or two tones, each tone  -13 dBm at 1/2 Nyquest +/- 5 MHz, be used to measure two tone distortion? Or would some correction factor, say the -3dB(FS) factor you mentioned in an earlier post on 27 April need to be applied?  The object is to have the three different test signals (RMS power?) operate the ADC input at the same point so that the performance is  accurately characterized.

Thanks for your time.

Sherman

Hi Sherman,

Thanks for the chat on Friday - I think it was good to speak in person as sometimes writing emails just doesn't get the message across!  Let me know if you have further questions.

Kind regards,

Marjorie