Datasheet

LTC2314-14
16
231414fa
For more information www.linear.com/LTC2314-14
APPLICATIONS INFORMATION
transition noise associated with a 14-bit resolution ADC
which can be measured with a fixed DC signal applied
to the input of the ADC. The resulting output codes are
collected over a large number of conversions. The shape
of the distribution of codes will give an indication of the
magnitude of the transition noise. In Figure 12, the distri
-
bution of output codes is shown for a DC input that has
been digitized 16,384 times. The distribution is Gaussian
and the RMS code transition noise is 0.7LSB. This cor
-
responds to a noise level of 77.5dB relative to a full scale
voltage of 4.096V.
Figure 12. Histogram for 16384 Conversions
231414 F12
CODE
8194
COUNTS
8196 8197
8195
8198 8199 8200
σ = 0.7
0
2000
1000
3000
4000
5000
7000
6000
Dynamic Performance
The LTC2314-14 has excellent high speed sampling
capability. Fast Fourier Transform (FFT) techniques are
used to test the ADC’s frequency response, distortion and
noise at the rated throughput. By applying a low distortion
sine wave and analyzing the digital output using an FFT
algorithm, the ADC’s spectral content can be examined
for frequencies outside the applied fundamental. The
LTC2314-14 provides guaranteed tested limits for both
AC distortion and noise measurements.
Signal-to-Noise and Distortion Ratio (SINAD)
The signal-to-noise and distortion ratio (SINAD) is the ratio be
-
tween the RMS amplitude of the fundamental input frequency
and the RMS amplitude of all other frequency components
at the A/D output. The output is band-limited to frequencies
from above DC and below half the sampling frequency. Figure
14 shows the LTC2314-14 maintains a SINAD above 74dB
up to the Nyquist input frequency of 2.25MHz.
Effective Number of Bits (ENOB)
The effective number of bits (ENOB) is a measurement of
the resolution of an ADC and is directly related to SINAD
by the equation where ENOB is the effective number of
bits of resolution and SINAD is expressed in dB:
ENOB = (SINAD – 1.76)/6.02
At the
maximum sampling
rate of 5MHz, the LTC2314-14
maintains an ENOB above 12 bits up to the Nyquist input
frequency of 2.25MHz. (Figure 14)
Signal-to-Noise Ratio (SNR)
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. Figure 13 shows
that the LTC2314-14 achieves a typical SNR of 77.5dB at
a 4.5MHz sampling rate with a 500kHz input frequency.
Total Harmonic Distortion (THD)
Total Harmonic Distortion (THD) 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 (f
SMPL
/2).
THD is expressed as:
THD=20log
V2
2
+ V3
2
+ V4
2
+ V
N
2
V1
where V1 is the RMS amplitude of the fundamental fre-
quency and
V2 through V
N
are the amplitudes of the second
through Nth harmonics. THD versus Input Frequency is
shown in the Typical Performance Characteristics section.
The LTC2314-14 has excellent distortion performance up
to the Nyquist frequency.
Intermodulation Distortion (IMD)
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.