ETTC'2003 - SEE

2. AC SPECIFICATIONS - HOW EFFECTIVE ARE YOUR BITS?
AC specifications are often ignored in FTI data-sheets, in particular with respect to noise and
harmonic distortion. Furthermore, there are other terms such as non-linearity and missing
codes that, while important, can be incorporated into other specifications. Table 2.1 shows
15 specifications (for an ADC with B bits) that should, arguably, be quoted in a data-sheet.
This paper suggest replacing all 15 with a single ENOB* specification as defined by the IEEE.
PARAMETER DESCRIPTION FORMULA
(For B-bits)
Resolution The average difference
between two consecutive
readings.
q = FSR / 2B (V)
INL / DNL Integral Non Linearity or The maximum deviation
Differential Non Linearity from and a straight line
approximation /
maximum step size
Cross-talk Noise induced by a sine wave = RMS (induced) /
in any other channel
Signal
Codes / Missing
codes
Quantization
noise power
Jitter noise
power
The number of different
outputs from the ADC
The noise introduced because
of limited resolution.
The noise introduced by
uncertainty (±Tj ) in sampling
"DC Noise" RMS or maximum value of
deviation from average when
DC signal applied
2 B
σ 2 = q 2 / 12 (V 2 )
= 20 log (π T j F s / √2)
(dB)
= RMS (Sig. - Mean)
= MAX (|Sig. - Mean|)
(V)
THD Total Harmonic Distortion = Sum of power of first
N (8) harmonics of input
signal
SNR Signal to Noise Ratio = Ratio of signal to all
noise not at DC or
harmonics
SFDR Spurious Free Dynamic Range = Difference between
signal and highest
power at any frequency
(not DC) (dB)
SINAD Signal In Noise And Distortion = 1.76 + 6.02B (dB)
= √(SNR2 + THD2 )
COMMENTS
Causes harmonic
distortion
Bandwidth of crosstalk
signal must be
specified
Assuming bandwidth
= cutoff = ¼
sampling rate (Fs)
Low RMS does not
mean low MAX (or
high MAX does not
mean low RMS)
If circuit is linear
there is no harmonic
distortion
Is not a measure of
non-linearity
Good SFDR only
means no big spikes
in FFT
Input amplitude must
be specified
ENOB *Effective Number Of Bits = B Assumes all noise is
quantization noise
Table 2-1: Possible Datasheet Specifications for an ADC Module
Replacing all of these terms with a single ENOB specification has the following advantages:
• It is intuitive (ENOB = 14 means the ADC acts like an ideal/perfect 14-bit ADC)
• The test is well defined in IEEE 1057 and IEEE 1241. To include the crosstalk
specification, signals can be induced in adjacent channels
• It covers other specifications (e.g. high THD => low ENOB and SINAD = 1.76 + 6.02
x ENOB dB)