9
LTC1279
APPLICATIONS INFORMATION
WUU
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pared with the binary-weighted charges supplied by the
capacitive DAC. Bit decisions are made by the high speed
comparator. At the end of a conversion, the DAC output
balances the A
IN
input charge. The SAR contents (a 12-bit
data word) which represent the A
IN
are loaded into the
12-bit output latches.
DYNAMIC PERFORMANCE
The LTC1279 has excellent high speed sampling capabil-
ity. FFT (Fast Fourier Transform) test techniques are used
to test the ADC’s frequency response, distortion and
noise at the rated throughput. By applying a low distor-
tion sine wave and analyzing the digital output using an
FFT algorithm, the ADC’s spectral content can be exam-
ined for frequencies outside the fundamental. Figures 2a
and 2b show typical LTC1279 FFT plots.
Figure 2b. LTC1279 Nonaveraged, 4096 Point FFT Plot
with 300kHz Input Frequency
Signal-to-Noise Ratio
The Signal-to-Noise plus Distortion Ratio [S/(N + D)] is the
ratio between the RMS amplitude of the fundamental input
frequency to the RMS amplitude of all other frequency
components at the A/D output. The output is band limited
to frequencies above DC and below half the sampling
frequency. Figure 2a shows a typical spectral content with
a 600kHz sampling rate and a 100kHz input. The dynamic
performance is excellent for input frequencies up to the
Nyquist limit of 300kHz as shown in Figure 2b.
Effective Number of Bits
The Effective Number of Bits (ENOBs) is a measurement of
the resolution of an ADC and is directly related to the
S/(N + D) by the equation:
N = [S/(N + D) – 1.76]/6.02
where N is the Effective Number of Bits of resolution and
S/(N + D) is expressed in dB. At the maximum sampling
rate of 600kHz the LTC1279 maintains very good ENOBs up
to the Nyquist input frequency of 300kHz. Refer to Figure 3.
Total Harmonic Distortion
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. THD is
expressed as:
Figure 3. Effective Bits and Signal/(Noise + Distortion) vs
Input Frequency
FREQUENCY (Hz)
10k
EFFECTIVE NUMBER OF BITS
SIGNAL/(NOISE + DISTORTION) (dB)
100k 1M 5M
1279 G03
12
11
10
9
8
7
6
5
4
3
2
1
0
74
68
62
56
50
f
SAMPLE
= 600kHz
NYQUIST
FREQUENCY
FREQUENCY (kHz)
0
AMPLITUDE (dB)
50
100 150 200
1279 F02a
250 300
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
SAMPLE
= 600kHz
f
IN
= 97.705kHz
FREQUENCY (kHz)
0
AMPLITUDE (dB)
50
100 150 200
1279 F02
250 300
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–110
–120
f
SAMPLE
= 600kHz
f
IN
= 292.822kHz
Figure 2a. LTC1279 Nonaveraged, 4096 Point FFT Plot
with 100kHz Input Frequency
