Differential Nonlinearity
Differential nonlinearity (DNL) is the difference between
an actual step width and the ideal value of 1 LSB. A DNL
error specification of 1 LSB or less guarantees no missing
codes and a monotonic transfer function.
Aperture Jitter
Aperture jitter (t
AJ
) is the sample-to-sample variation in
the time between the samples.
Aperture Delay
Aperture delay (t
AD
) is the time defined between the
falling edge of CNVST and the instant when an actual
sample is taken.
Signal-to-Noise Ratio
For a waveform perfectly reconstructed from digital sam-
ples, signal-to-noise ratio (SNR) is the ratio of the full-
scale analog input (RMS value) to the RMS quantization
error (residual error). The theoretical minimum analog-to-
digital noise is caused by quantization error, and results
directly from the ADC’s resolution (N bits):
SNR = (6.02 x N + 1.76)dB
In reality, there are other noise sources besides quantiza-
tion noise, including thermal noise, reference noise, clock
jitter, etc. Therefore, SNR is computed by taking the ratio
of the RMS signal to the RMS noise, which includes all
spectral components minus the fundamental, the first five
harmonics, and the DC offset.
Signal-to-Noise Plus Distortion
Signal-to-noise plus distortion (SINAD) is the ratio of the
fundamental input frequency’s RMS amplitude to the
RMS equivalent of all other ADC output signals:
SINAD(dB) = 20 x log (Signal
RMS
/ Noise
RMS
)
Effective Number of Bits
Effective number of bits (ENOB) indicates the global
accuracy of an ADC at a specific input frequency and
sampling rate. An ideal ADC’s error consists of quantiza-
tion noise only. With an input range equal to the full-scale
range of the ADC, calculate the ENOB as follows:
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the RMS
sum of the first five harmonics of the input signal to the
fundamental itself. This is expressed as:
where V
1
is the fundamental amplitude, and V
2
through V
5
are the amplitudes of the 2nd- through 5th-
order harmonics.
Spurious-Free Dynamic Range
Spurious-free dynamic range (SFDR) is the ratio of the
RMS amplitude of the fundamental (maximum signal
component) to the RMS value of the next largest distor-
tion component.
Full-Power Bandwidth
Full-power bandwidth is the frequency at which the
input signal amplitude attenuates by 3dB for a full-scale
input.
Full-Linear Bandwidth
Full-linear bandwidth is the frequency at which the sig-
nal to noise plus distortion (SINAD) is equal to 68dB.
Intermodulation Distortion (IMD)
Any device with nonlinearities creates distortion prod-
ucts when two sine waves at two different frequencies
(f1 and f2) are input into the device. Intermodulation
distortion (IMD) is the total power of the IM2 to IM5
intermodulation products to the Nyquist frequency rela-
tive to the total input power of the two input tones, f1
and f2. The individual input tone levels are at -7dBFS.
The intermodulation products are as follows:
• 2nd-order intermodulation products (IM2): f
1
+ f
2
,
f
2
- f
1
• 3rd-order intermodulation products (IM3): 2f
1
- f
2
,
2f
2
- f
1
, 2f
1
+ f
2
, 2f
2
+ f
1
• 4th-order intermodulation products (IM4): 3f
1
- f
2
,
3f
2
- f
1
, 3f
1
+ f
2
, 3f
2
+ f
1
• 5th-order intermodulation products (IM5): 3f
1
- 2f
2
,
3f
2
- 2f
1
, 3f
1
+ 2f
2
, 3f
2
+ 2f
1
