LTC1274 Linear Technology, LTC1274 Datasheet - Page 10

• Manufacturer Part Number: LTC1274
• Description: 12-Bit/ 10mW/ 100ksps ADCs with 1uA Shutdown
• Manufacturer: Linear Technology

APPLICATIONS
CONVERSION DETAILS
The LTC1274/LTC1277 use a successive approximation
algorithm and an internal sample-and-hold circuit to con-
vert an analog signal to a 12-bit parallel output. The ADCs
are complete with a precision reference and an internal
clock. The control logic provides easy interface to micro-
processors and DSPs. (Please refer to the Digital Interface
section for the data format.)
Conversion start is controlled by the CS and CONVST
inputs. At the start of conversion the successive approxi-
mation register (SAR) is reset. Once a conversion cycle
has begun it cannot be restarted.
During conversion, the internal 12-bit capacitive DAC out-
put is sequenced by the SAR from the most significant bit
(MSB) to the least significant bit (LSB). Referring to
Figure 1, the A
nects to the sample-and-hold capacitor during the acquire
phase, and the comparator offset is nulled by the feedback
switch. In this acquire phase, a minimum delay of 2 s will
provide enough time 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 switch connects C
to ground (LTC1274) or A
analog input charge onto the summing junction. This input
charge is successively compared with the binary-weighted
LTC1274/LTC1277
TI I G DIAGRA S
10
W
U
NAP to CONVST Wake-Up Timing (LTC1277)
CONVST
RD
CS
NAP
IN
(LTC1274) or A
CS to RD Setup Timing
U
t
1
INFORMATION
t
U
3
W
IN
–
(LTC1277), injecting the
IN
+
LTC1274/77 • TD01
W
(LTC1277) input con-
LTC1274/77 • TD03
U
SAMPLE
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
A
bit data word) which represent the A
A
DYNAMIC PERFORMANCE
The LTC1274/LTC1277 have excellent high speed sam-
pling capability. FFT (Fast Fourier Transform) test tech-
niques are used to test the ADCs’ frequency response,
distortion and noise at the rated throughput. By applying
a low distortion sine wave and analyzing the digital output
IN
IN
+
+
A
– A
– A
IN
IN
IN
SAMPLE
–
HOLD
–
CONVST
(LTC1277) are loaded into the 12-bit output latches.
(LTC1277) input charge. The SAR contents (a 12-
REFRDY
SLEEP to REFRDY Wake-Up Timing
CS
SLEEP
C
CS to CONVST Setup Timing
Figure 1. LTC1274 A
SAMPLE
C
V
DAC
DAC
t
14
t
2
DAC
IN
SAMPLE
–
+
LTC1274/77 • TD02
COMPAR-
Input
ATOR
LTC1274/77 • TD04
SI
IN
IN
(LTC1274) or
(LTC1274) or
1274 • F01
12-BIT
LATCH
S
A
R