LTC2217 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC2217' PDF : 32 Pages View PDF
LTC2217
OPERATION
DYNAMIC PERFORMANCE
Signal-to-Noise Plus Distortion Ratio
The signal-to-noise plus distortion ratio [S/(N+D)] is the
ratio between the RMS amplitude of the fundamental input
frequency and the RMS amplitude of all other frequency
components at the ADC output. The output is band lim-
ited to frequencies above DC to below half the sampling
frequency (Nyquist Frequency).
Signal-to-Noise Ratio
The signal-to-noise (SNR) is the ratio between the RMS
amplitude of the fundamental input frequency and the RMS
amplitude of all other frequency components, except the
first five harmonics.
Total Harmonic Distortion
Total harmonic distortion is the ratio of the RMS sum of
all harmonics of the input signal to the fundamental itself.
The out-of-band harmonics alias into the frequency band
between DC and half the sampling frequency (Nyquist
Frequency). THD is expressed as:
⎛
THD = –20Log⎜⎜
⎝⎜
( ) V22 + V32 + V42 +…VN2
⎞
⎟
V1
⎟
⎠⎟
by the presence of another sinusoidal input at a different
frequency.
If two pure sine waves of frequencies fa and fb are applied
to the ADC input, nonlinearities in the ADC transfer function
can create distortion products at the sum and difference
frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc.
For example, the 3rd order IMD terms include (2fa + fb),
(fa + 2fb), (2fa - fb) and (fa - 2fb). The 3rd order IMD is
defined as the ratio of the RMS value of either input tone
to the RMS value of the largest 3rd order IMD product.
Spurious Free Dynamic Range (SFDR)
The ratio of the RMS input signal amplitude to the RMS
value of the peak spurious spectral component expressed
in dBc. SFDR may also be calculated relative to full scale
and expressed in dBFS.
Full Power Bandwidth
The Full Power bandwidth is that input frequency at which
the amplitude of the reconstructed fundamental is reduced
by 3dB from a full scale input signal.
Aperture Delay Time
The time from when a rising ENC+ equals the ENC–voltage
to the instant that the input signal is held by the sample-
and-hold circuit.
where V1 is the RMS amplitude of the fundamental fre-
quency and V2 through VN are the amplitudes of the second
through nth harmonics.
Intermodulation Distortion
If the ADC input signal consists of more than one spectral
component, the ADC transfer function nonlinearity can
produce intermodulation distortion (IMD) in addition to
THD. IMD is the change in one sinusoidal input caused
Aperture Delay Jitter
The variation in the aperture delay time from conversion
to conversion. This random variation will result in noise
when sampling an AC input. The signal-to-noise ratio term
due to the jitter alone will be:
SNRJITTER = – 20log (2π • fIN • tJITTER)
This formula states SNR due to jitter alone at any amplitude
in terms of dBc.
2217f
17
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