
8
LTC1279
TEST CIRCUITS
Load Circuits for Output Float Delay
Load Circuits for Access Timing
3k
CL
DBN
DGND
A) HIGH-Z TO VOH (t8)
AND VOL TO VOH (t6)
CL
DBN
3k
5V
B) HIGH-Z TO VOL (t8)
AND VOH TO VOL (t6)
DGND
1279 TC01
3k
10pF
DBN
DGND
A) VOH TO HIGH-Z
10pF
DBN
3k
5V
B) VOL TO HIGH-Z
DGND
1279 TC02
CS to RD Setup Timing
SHDN to CONVST Wake-Up Timing
CS to CONVST Setup Timing
TI I G DIAGRA S
WU
W
APPLICATIONS INFORMATION
WU
U
CONVERSION DETAILS
The LTC1279 uses a successive approximation algorithm
and an internal sample-and-hold circuit to convert an
analog signal to a 12-bit parallel output. The ADC is
complete with a precision reference and an internal clock.
The control logic provides easy interface to microproces-
sors 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
output is sequenced by the SAR from the most significant
bit (MSB) to the least significant bit (LSB). Referring to
Figure 1, the AIN input connects 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 160ns will provide enough
VDAC
1279 F01
+
–
CDAC
DAC
SAMPLE
HOLD
CSAMPLE
S
A
R
12-BIT
LATCH
COMPAR-
ATOR
SAMPLE
SI
AIN
Figure 1. AIN Input
t3
SHDN
CONVST
1279 TD03
t1
CS
RD
1279 TD01
t2
CS
CONVST
1279 TD02
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 switches CSAMPLE to
ground, injecting the analog input charge onto the sum-
ming junction. This input charge is successively com-