LTC2247 Linear Technology, LTC2247 Datasheet - Page 13

• Manufacturer Part Number: LTC2247
• Description: (LTC2246 - LTC2248) 65/40/25Msps Low Power 3V ADCs
• Manufacturer: Linear Technology

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APPLICATIO S I FOR ATIO
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 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.
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:
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 applied
to the ADC input, nonlinearities in the ADC transfer func-
tion can create distortion products at the sum and differ-
ence 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 intermodulation
distortion is defined as the ratio of the RMS value of either
THD = 20Log (√(V2
U
2
+ V3
U
2
+ V4
2
W
+ . . . Vn
2
)/V1)
U
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
spurious 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 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
CONVERTER OPERATION
As shown in Figure 1, the LTC2248/LTC2247/LTC2246 is
a CMOS pipelined multistep converter. The converter has
six pipelined ADC stages; a sampled analog input will
result in a digitized value five cycles later (see the Timing
Diagram section). For optimal AC performance the analog
inputs should be driven differentially. For cost sensitive
applications, the analog inputs can be driven single-ended
with slightly worse harmonic distortion. The CLK input is
single-ended. The LTC2248/LTC2247/LTC2246 has two
phases of operation, determined by the state of the CLK
input pin.
Each pipelined stage shown in Figure 1 contains an ADC,
a reconstruction DAC and an interstage residue amplifier.
In operation, the ADC quantizes the input to the stage and
the quantized value is subtracted from the input by the
LTC2248/LTC2247/LTC2246
JITTER
= –20log (2π • f
IN
• t
JITTER
)
13
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