9
LTC1278
compare mode. The input switch switches C
SAMPLE
to
ground, injecting the analog input charge onto the sum-
ming junction. This input charge is successively com-
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 LTC1278 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. Figure 2
shows a typical LTC1278 FFT plot.
a 500kHz sampling rate and a 100kHz input. The dynamic
performance is excellent for input frequencies up to the
Nyquist limit of 250kHz.
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 500kHz the LTC1278 maintains very good ENOBs up
to the Nyquist input frequency of 250kHz. Refer to Figure 3.
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 2 shows a typical spectral content with
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:
THD = 20log
√V
2
2
+ V
3
2
+ V
4
2
... + V
N
2
V
1
where V
1
is the RMS amplitude of the fundamental fre-
quency and V
2
through V
N
are the amplitudes of the
second through Nth harmonics. THD versus input
INPUT FREQUENCY (Hz)
10k
0
EFFECTIVE NUMBER OF BITS
S/(N+D) (dB)
3
5
7
10
100k 1M 2M
LT1278 G4
1
4
6
9
12
11
62
56
74
68
8
2
f
SAMPLE
= 500kHz
NYQUIST
FREQUENCY
Figure 3. Effective Bits and Signal-to-Noise + Distortion vs
Input Frequency
Figure 2. LTC1278 Nonaveraged, 4096 Point FFT Plot
APPLICATIONS INFORMATION
WUU
U
FREQUENCY (Hz)
0
–120
AMPLITUDE (dB)
–100
–80
–60
–40
–20
0
50k 100k 150k 200k
LTC1278 F2
250k
f
SAMPLE
= 500kHz ±5V
f
IN
= 97.045kHz
