LTC1401 Linear Technology, LTC1401 Datasheet - Page 7

• Manufacturer Part Number: LTC1401
• Description: Complete SO-8/ 12-Bit/ 200ksps ADC with Shutdown
• Manufacturer: Linear Technology

APPLICATIONS
Conversion Details
The LTC1401 uses a successive approximation algorithm
and an internal sample-and-hold circuit to convert an
analog signal to a 12-bit serial output based on a precision
internal reference. The control logic provides an easy
interface to microprocessors and DSPs through serial
3-wire connections.
A rising edge on the CONV input starts a conversion. At the
start of a conversion the successive approximation regis-
ter (SAR) is reset. Once a conversion cycle has begun, it
cannot be restarted.
During conversion, the internal 12-bit capacitive DAC
output is sequenced by the SAR from the most significant
bit (MSB) to the least significant bit (LSB). Referring to
Figure 1, the A
capacitor during the acquire phase and the comparator
offset is nulled by the feedback switch. In this acquire
phase, it typically takes 315ns for the sample-and-hold
capacitor to acquire the analog signal. During the convert
phase, the comparator feedback switch opens, putting the
comparator into the compare mode. The input switches
C
the summing junction. This input charge is successively
compared 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
SAMPLE
A
IN
to ground, injecting the analog input charge onto
SAMPLE
HOLD
IN
input connects to the sample-and-hold
C
SAMPLE
DAC
U
Figure 1. A
INFORMATION
U
V
C
DAC
DAC
IN
Input
SAMPLE
W
S1
–
+
COMP
LTC1401 • F01
U
D
OUT
S
A
R
output balances the A
12-bit data word) which represent the input voltage, are
presented through the serial pin D
Dynamic Performance
The LTC1401 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 2a shows a
typical LTC1401 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 DC to half the sampling frequency.
Figure 2a shows a typical spectral content with a 200kHz
sampling rate and a 50kHz input. The dynamic perfor-
mance is excellent for input frequencies up to the Nyquist
limit of 100kHz as shown in Figure 2b.
Figure 2a. LTC1401 Nonaveraged, 4096 Point FFT
Plot with 50kHz Input Frequency
–100
–110
–120
– 60
–10
–20
–30
–40
–50
–70
–90
–80
0
0
f
f
SINAD = 68.5dB
THD = –72.4dB
V
T
SAMPLE
IN
CC
A
10
= 49.853516kHz
= 25 C
= 3V
20
= 200kHz
IN
30
FREQUENCY (kHz)
input charge. The SAR contents (a
40 50
60
70
OUT
80
LTC1401 • F02a
.
90
LTC1401
100
7