The SNR for a sinusoidal input signal can be described as:
SNR = 20
.
log
signal (rms)
noise (rms)
dB
An application note from the well‑known ADC manufacturer MAXIM
describes the formula used and explains how the SNR parameter
correlates with ADC performance: Defining and Testing Dynamic
Parameters in High-Speed ADCs, Part 1. It is available at
http://www.maxim-ic.com/appnotes.cfm/appnote_number/728.
Calculation of SNR
Often the peak-to-peak values of the signal and the noise are
mistakenly taken to calculate the SNR with the formula shown above.
However, this results in much lower performance than the A/D
converter really obtains. While the rms value of a sine wave can be
obtained by dividing its peak-to-peak value by 2.828, the peak-to-
peak value of a noise signal must be divided by approximately 6 … 8
to obtain its rms value (occasional extreme peaks must be ignored
using this equation). Furthermore, the peak-to peak noise
theoretically might be ±10 V (even if the inputs are shorted) without
decreasing the SNR, assuming that such high peaks appear rarely
enough.
Notes on calculation of SNR
Notes on Measuring Noise
It is often claimed that the noisy signal measured with the dSPACE
board looks much better when measured with an oscilloscope.
Objective
The reason is that you are using the wrong “voltage range” for the
dSPACE system!
dSPACE boards have an input range of ±10 V and you are looking at
signals with a voltage range of a few mV. That is similar to using a
ruler to measure 1/1,000 inches.
Reason
When you use your oscilloscope, you select an appropriate voltage
range for the signal you are watching. You should do the same with
the dSPACE system: You should adapt your input signal to the ±10 V
scale of the dSPACE board before converting it. Use external
amplifiers to ensure that the maximum amplitude range of the signal
to be measured is mapped to the full scale range of the ADC, which
is usually ±10 V.
Measures
s
ADC Performance of dSPACE Boards
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DS1103 Hardware Installation and Configuration November 2014
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