LTC1278-4IN Linear Technology, LTC1278-4IN Datasheet - Page 9

• Manufacturer Part Number: LTC1278-4IN
• Description: IC A/DCONV SAMPLNG W/SHTDN 24DIP
• Manufacturer: Linear Technology

Specifications of LTC1278-4IN

• Number Of Bits: 12
• Sampling Rate (per Second): 400k
• Data Interface: Parallel
• Number Of Converters: 1
• Power Dissipation (max): 150mW
• Voltage Supply Source: Dual ±
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Through Hole
• Package / Case: 24-DIP (0.300", 7.62mm)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

APPLICATIONS
compare mode. The input switch switches C
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
data word) which represent the A
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.
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
Figure 2. LTC1278 Nonaveraged, 4096 Point FFT Plot
–100
–120
–20
–40
–60
–80
0
0
IN
input charge. The SAR contents (a 12-bit
50k
U
FREQUENCY (Hz)
100k
INFORMATION
U
f
f
SAMPLE
IN
150k
= 97.045kHz
= 500kHz 5V
IN
W
200k
are loaded into the
LTC1278 F2
250k
SAMPLE
U
to
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:
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.
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:
where V
quency and V
second through Nth harmonics. THD versus input
N = [S/(N + D) – 1.76]/6.02
THD = 20log
Figure 3. Effective Bits and Signal-to-Noise + Distortion vs
Input Frequency
1
is the RMS amplitude of the fundamental fre-
12
11
10
9
8
7
6
5
4
3
2
1
0
10k
f
2
SAMPLE
through V
V
2
= 500kHz
2
INPUT FREQUENCY (Hz)
+ V
100k
3
2
FREQUENCY
N
NYQUIST
+ V
V
are the amplitudes of the
1
4
2
... + V
1M
LT1278 G4
N
LTC1278
2M
2
74
68
62
56
9