Datasheet

8
LTC1409
APPLICATIONS INFORMATION
WUU
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CONVERSION DETAILS
The LTC1409 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 the conversion the successive
approximation register (SAR) is reset. Once a conversion
cycle has begun it cannot be restarted.
During the conversion, the internal differential 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
IN
and –A
IN
inputs are
connected to the sample-and-hold capacitors (C
SAMPLE
)
during the acquire phase and the comparator offset is
nulled by the zeroing switches. In this acquire phase, a
minimum delay of 150ns 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
SAMPLE
capacitors to ground,
transferring the differential analog input charge onto the
summing junction. This input charge is successively com-
pared 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 DACs output balances the +A
IN
and –A
IN
input
charges. The SAR contents (a 12-bit data word) which
represents the difference of +A
IN
and –A
IN
are loaded into
the 12-bit output latches.
DYNAMIC PERFORMANCE
The LTC1409 has excellent high speed sampling capabil-
ity. FFT (Fast Four 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 FFT algorithm,
the ADC’s spectral content can be examined for frequen-
cies outside the fundamental. Figure 2 shows typical
LTC1409 plots.
Figure 1. Simplified Block Diagram
COMP

+C
SAMPLE


–C
DAC
•
•
•
D11
D0
ZEROING SWITCHES
HOLD
HOLD
+A
IN
–A
IN

+C
DAC

–C
SAMPLE
12
LTC1409 • F01
+
–
SAR
OUTPUT
LATCHES
+V
DAC
–V
DAC
HOLD
HOLD
Figure 2b. LTC1409 Nonaveraged, 4096 Point FFT,
Input Frequency = 375kHz
FREQUENCY (kHz)
0
AMPLITUDE (dB)
100 200
300
400
LT1409 • F02b
0
–20
–40
–60
–80
–100
–120
50 150
250
350
f
SAMPLE
= 800kHz
f
IN
= 375kHz
SFDR = 89dB
SINAD = 72.5dB
Figure 2a. LTC1409 Nonaveraged, 4096 Point FFT,
Input Frequency = 100kHz
FREQUENCY (kHz)
0
AMPLITUDE (dB)
100 200
300
400
LT1409 • F02a
0
–20
–40
–60
–80
–100
–120
50 150
250
350
f
SAMPLE
= 800kHz
f
IN
= 97.45kHz
SFDR = 89.1dB
SINAD = 73.1dB