LTC2228/LTC2227/LTC2226
15
222876fb
APPLICATIONS INFORMATION
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 fundamen-
tal input frequency and the RMS amplitude of all other
frequency components at the ADC output. The output is
band limited to frequencies above DC to below half the
sampling frequency.
Signal-to-Noise Ratio
The signal-to-noise ratio (SNR) is the ratio between the
RMS amplitude of the fundamental input frequency and
the RMS amplitude of all other frequency components
except the fi rst fi ve harmonics and DC.
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. THD
is expressed as:
THD = 20Log (√(V2
2
+ V3
2
+ V4
2
+ . . . Vn
2
)/V1)
where V1 is the RMS amplitude of the fundamental fre-
quency and V2 through Vn are the amplitudes of the second
through nth harmonics. The THD calculated in this data
sheet uses all the harmonics up to the fi fth.
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
by the presence of another sinusoidal input at a different
frequency.
If two pure sine waves of frequencies fa and fb are ap-
plied 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. The 3rd order intermodulation products are
2fa + fb, 2fb + fa, 2fa – fb and 2fb – fa. The intermodula-
tion distortion is defi ned as the ratio of the RMS value of
either input tone to the RMS value of the largest 3rd order
intermodulation product.
Spurious Free Dynamic Range (SFDR)
Spurious free dynamic range is the peak harmonic or spuri-
ous noise that is the largest spectral component excluding
the input signal and DC. This value is expressed in decibels
relative to the RMS value of a full-scale input signal.
Input Bandwidth
The input bandwidth is that input frequency at which the
amplitude of the reconstructed fundamental is reduced
by 3dB for a full-scale input signal.
Aperture Delay Time
The time from when CLK reaches mid-supply to the in-
stant that the input signal is held by the sample-and-hold
circuit.
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 due
to the jitter alone will be:
SNR
JITTER
= –20log (2π • f
IN
• t
JITTER
)
