Datasheet

LTC2315-12
16
231512fa
For more information www.linear.com/LTC2315-12
APPLICATIONS INFORMATION
with a 12-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
distribution 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 0.33LSB. This corresponds to
a noise level of 73dB relative to a full scale voltage of 4.096V.
Figure 12. Histogram for 16384 Conversions
CODE
2047
COUNTS
2049
231512 F12
2048
2050 2051
σ = 0.33
0
10000
20000
30000
40000
60000
50000
Dynamic Performance
The LTC2315-12 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
LTC2315-12 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 between 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 LTC2315-12 maintains a
SINAD above 71dB up to the Nyquist input frequency of
2.5MHz.
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 LTC2315-12
maintains an ENOB above 11.7 bits up to the Nyquist input
frequency of 2.5MHz. (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 LTC2315-12 achieves a typical SNR of 73dB at a
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 LTC2315-12 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.