LTC1404 LINER [Linear Technology], LTC1404 Datasheet - Page 10

• Manufacturer Part Number: LTC1404
• Description: Complete SO-8, 12-Bit, 600ksps ADC with Shutdown
• Manufacturer: LINER [Linear Technology]

LTC1404
APPLICATIONS
Conversion Details
The LTC1404 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 easy inter-
face to microprocessors and DSPs through 3-wire con-
nections.
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 acquired phase and the comparator
offset is nulled by the feedback switch. In this acquire
phase, it typically takes 160ns 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
connect C
charge onto 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 output balances the A
contents (a 12-bit data word) which represent the input
voltage, are presented through the serial pin D
10
A
IN
SAMPLE
HOLD
SAMPLE
IN
input connects to the sample-and-hold
C
to ground, injecting the analog input
SAMPLE
DAC
U
Figure 1. A
INFORMATION
U
V
C
DAC
DAC
IN
IN
input charge. The SAR
Input
SAMPLE
W
S1
–
+
COMP
OUT
U
D
1404 F01
OUT
S
A
R
.
Dynamic Performance
The LTC1404 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 LTC1404 FFT plot.
Figure 2a. LTC1404 Nonaveraged, 4096 Point FFT
Plot with 100kHz Input Frequency in Bipolar Mode
Figure 2b. LTC1404 Nonaveraged, 4096 Point FFT
Plot with 300kHz Input Frequency in Bipolar Mode
–100
–110
–120
–100
–110
–120
–10
–20
–30
–40
–50
– 60
–70
–80
–90
–10
–20
–30
–40
–50
– 60
–70
–80
–90
0
0
0
0
30
30
60 90 120 150 180 210 240 270 300
60 90 120 150 180 210 240 270 300
FREQUENCY (kHz)
FREQUENCY (kHz)
f
f
SINAD = 72dB
THD = – 88dB
f
f
SINAD = 71dB
THD = – 84dB
SAMPLE
IN
SAMPLE
IN
= 298.681kHz
= 99.169kHz
= 600kHz
= 600kHz
1404 F02a
1404 F02b