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Rarely Asked Questions – Issue 87, November 2012 Apples and Oranges—Correlating Specifications Requires Common Test Conditions
A: In Switching Bits, we covered why input referred noise causes high-speed ADC outputs to switch even though the analog input is static. Input referred noise and SNR are related, but only if you compare them under the same test conditions. An ADC’s SNR can be used to calculate the equivalent rms input noise as
where the input full scale range (FSR) is specified in LSBs and the SNR is specified in dBFS (decibels relative to full scale). As an example, from the grounded input histogram shown in the AD9434 12-bit, 500-MSPS ADC data sheet (Figure 25), the input referred noise is 1.24 LSB rms. From the specifications table, the typical SNR varies from 65.9 dBFS to 63.5 dBFS for 30‑MHz to 450‑MHz inputs. Which number should we use for our calculation? We want to compare the SNR to the grounded input histogram result. For modern single-supply ADCs the inputs are open, not grounded, so they float to the midpoint of the analog power supply. SNR is not measured at dc, so choose the lowest input frequency, 30 MHz. Using the equation, we can calculate the input noise as
Hmm… 1.47 ≠ 1.24… what’s the problem? Well, we can consider a few factors to help reduce the discrepancy. First, the SNR test is performed with a 30-MHz input signal at a –1 dBFS level, while there is no signal for the grounded input histogram test. Will this make a difference? The jitter induced noise at the output of an ADC can be estimated as
where _{input} = 1.29 LSBs, which is
only 0.05 LSB off from the measured input referred noise with the grounded
input histogram, close enough to say we have successfully correlated the
two specifications. Alternatively, perform an FFT of the AD9434 with no
input signal. From the FFT you can calculate the SNR to be 67.5 dBFS.
Using this result to calculate the input referred noise gives 1.22 LSBs,
within 0.02 LSB of the histogram result! It takes a little massaging, but
you can successfully relate SNR and input referred noise for high-speed
ADCs.I invite you to comment on ADC Noise in the Analog Dialogue Community on EngineerZone.
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