LTC2240-10_15 Datasheet, PDF (14/28 Pages) Linear Technology – 10-Bit, 170Msps ADC

LTC2240-10
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 first five harmonics and DC.
2fa + fb, 2fb + fa, 2fa – fb and 2fb – fa. The intermodula-
tion distortion is defined 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.
Full Power Bandwidth
The full power bandwidth is that input frequency at which
the amplitude of the reconstructed fundamental is reduced
by 3dB for a full scale input signal.
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⎛⎝⎜ V22 + V32 + V42 + ...Vn2 / 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 fifth.
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
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.
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)
CONVERTER OPERATION
As shown in Figure 1, the LTC2240-10 is a CMOS pipelined
multi-step converter. The converter has five pipelined ADC
stages; a sampled analog input will result in a digitized
value five cycles later (see the Timing Diagram section). For
optimal performance the analog inputs should be driven
differentially. The encode input is differential for improved
common mode noise immunity. The LTC2240-10 has two
phases of operation, determined by the state of the dif-
ferential ENC⁺/ENC^– input pins. For brevity, the text will
refer to ENC⁺ greater than ENC^– as ENC high and ENC⁺
less than ENC^– as ENC low.
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