8
LTC1603
1603f
APPLICATIONS INFORMATION
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CONVERSION DETAILS
The LTC1603 uses a successive approximation algorithm
and internal sample-and-hold circuit to convert an analog
signal to a 16-bit parallel output. The ADC is complete with
a sample-and-hold, 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 the conversion the successive
approximation register (SAR) resets. Once a conversion
cycle has begun it cannot be restarted.
During the conversion, the internal differential 16-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
IN
+
and A
IN
–
inputs are
acquired during the acquire phase and the comparator
offset is nulled by the zeroing switches. In this acquire
phase, a duration of 480ns will provide enough time for the
sample-and-hold capacitors to acquire the analog signal.
During the convert phase the comparator zeroing switches
open, putting the comparator into compare mode. The
input switches connect the C
SMPL
capacitors to ground,
transferring the differential analog input charge onto the
Figure 1. Simplified Block Diagram
summing junctions. This input charge is successively
compared with the binary-weighted charges supplied by
the differential capacitive DAC. Bit decisions are made by
the high speed comparator. At the end of a conversion, the
differential DAC output balances the A
IN
+
and A
IN
–
input
charges. The SAR contents (a 16-bit data word) which
represent the difference of A
IN
+
and A
IN
–
are loaded into
the 16-bit output latches.
DIGITAL INTERFACE
The A/D converter is designed to interface with micropro-
cessors as a memory mapped device. The CS and RD
control inputs are common to all peripheral memory
interfacing. A separate CONVST is used to initiate a con-
version.
Internal Clock
The A/D converter has an internal clock that runs the A/D
conversion. The internal clock is factory trimmed to achieve
a typical conversion time of 3.3µs and a maximum conver-
sion time of 3.8µs over the full temperature range. No
external adjustments are required. The guaranteed maxi-
mum acquisition time is 480ns. In addition, a throughput
time (acquisition + conversion) of 4µs and a minimum
sampling rate of 250ksps are guaranteed.
3V Input/Output Compatible
The LTC1603 operates on ±5V supplies, which makes the
device easy to interface to 5V digital systems. This device
can also talk to 3V digital systems: the digital input pins
(SHDN, CS, CONVST and RD) of the LTC1603 recognize
3V or 5V inputs. The LTC1603 has a dedicated output
supply pin (OV
DD
) that controls the output swings of the
digital output pins (D0 to D15, BUSY) and allows the part
to talk to either 3V or 5V digital systems. The output is
two’s complement binary.
Power Shutdown
The LTC1603 provides two power shutdown modes, Nap
and Sleep, to save power during inactive periods. The Nap
mode reduces the power by 95% and leaves only the
digital logic and reference powered up. The wake-up time
from Nap to active is 200ns. In Sleep mode all bias
–
+
COMP
A
IN
+
C
SMPL
HOLD
SAMPLE
A
IN
–
C
SMPL
+C
DAC
+V
DAC
–C
DAC
–V
DAC
HOLD
HOLD
SAMPLE
HOLD
SAR
OUTPUT
LATCHES
16
D15
D0
1603 F01
•
•
•
ZEROING SWITCHES
