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Signal chain basics: Noise in ADCs

Posted: 15 Sep 2015 ?? ?Print Version ?Bookmark and Share

Keywords:analogue-to-digital converter? ADC? noise? Noise spectral density? ADS54J60?

The signal-to-noise ratio (SNR) specification for ADCs is the sum of all noise sources in the ADC relative to the input signal power. SNR is often expressed as an effective number of bits, where the noise is compared to quantisation noise for a theoretical resolution using

SNR(dB)=1.76+6.02ENOB (1)

For high-speed ADCs, this is usually dominated by white noise, and so dividing the SNR by the Nyquist bandwidth can be a good approximation of the white noise. For example, the ADS54J60 has an SNR of 71 dBFS with a 100MHz input, resulting in C159 dBFS/Hz NSD, similar to what is shown in figure 2. However, for frequencies within 5MHz of DC or the input signal, this is not accurate due to the 1/f and phase noise.

Figure 2: ADC at 1 GSPS with 105MHz Input showing 1/f, phase and white noise (note: spur level not accurate).

Returning now to the question of the lowest noise ADC, let us compare two different ADCs: the ADS1672 (24bit, 625 kSPS) versus the ADS54J60. SNR for the 24bit ADC is 102 dB at 625 kSPS which translates to a NSD of C158 dBFS/Hz, compared to the NSD of the 16bit ADC of C159 dBFS/Hz. Although the 16bit ADC has 31 dB worse total noise, because it is spread out over a much wider bandwidth, NSD is actually lower than the 24bit ADC. However, for low frequency (Please join us next time when we will discuss the importance of input common-mode range for industrial transceivers.

About the author
Robert Keller is a Systems Manager in TI's High-Speed Products group. He has 14 years of experience supporting high-speed products in wireless infrastructure communication, test and measurement and military systems. He received a B.A. in Physics and Mathematics from Washington University, St. Louis, Missouri, and a Ph.D. in Applied Physics from Stanford University. He has 10 US patents in networking and sensor applications.

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