Datasheet

9
LTC1418
APPLICATIONS INFORMATION
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Referring to Figure 1, the A
IN
+
and A
IN
–
inputs are con-
nected to the sample-and-hold capacitors (C
SAMPLE
) dur-
ing the acquire phase and the comparator offset is nulled by
the zeroing switches. In this acquire phase, a minimum
delay of 1µs will provide enough time for the sample-and-
hold capacitors to acquire the analog signal. During the
convert phase the comparator zeroing switches open,
putting the comparator into compare mode. The input
switches the C
SAMPLE
capacitors to ground, transferring
the differential analog input charge onto the summing
junction. This input charge is successively compared with
the binary weighted charges supplied by the differential
capacitive DAC. Bit decisions are made by the high speed
comparator. At the end of a conversion, the differential
DAC output balances the A
IN
+
and A
IN
–
input charges. The
SAR contents (a 14-bit data word) which represent the
difference of A
IN
+
and A
IN
–
are loaded into the 14-bit
output latches.
DYNAMIC PERFORMANCE
The LTC1418 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 distortion sine
wave and analyzing the digital output using an FFT algo-
rithm, the ADC’s spectral content can be examined for
frequencies outside the fundamental. Figure 2 shows a
typical LTC1418 FFT plot.
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 from above DC and below half the sampling
frequency. Figure 2a shows a typical spectral content with
a 200kHz sampling rate and a 10kHz input. The dynamic
performance is excellent for input frequencies up to and
beyond the Nyquist limit of 100kHz.
FREQUENCY (kHz)
01030507090
AMPLITUDE (dB)
0
–20
–40
–60
–80
–100
–120
20 40 60 80
1418 F02a
100
f
SAMPLE
= 200kHz
f
IN
= 9.9609375kHz
SFDR = 99.32
SINAD = 82.4
Figure 2a. LTC1418 Nonaveraged, 4096 Point FFT,
Input Frequency = 10kHz
FREQUENCY (kHz)
01030507090
AMPLITUDE (dB)
0
–20
–40
–60
–80
–100
–120
20 40 60 80
1418 F02b
100
f
SAMPLE
= 200kHz
f
IN
= 97.509765kHz
SFDR = 94.29
SINAD = 81.4
Figure 2b. LTC1418 Nonaveraged, 4096 Point FFT,
Input Frequency = 97.5kHz
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 200kHz the LTC1418 maintains near ideal ENOBs
up to the Nyquist input frequency of 100kHz (refer to
Figure 3).