LTC1400 LINER [Linear Technology], LTC1400 Datasheet - Page 7

• Manufacturer Part Number: LTC1400
• Description: Complete SO-8, 12-Bit, 400ksps ADC with Shutdown
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
Conversion Details
The LTC1400 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 200ns 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 output through the serial pin D
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
input charge. The SAR
SAMPLE
W
S1
–
+
COMP
OUT
LTC1400 • F01
.
U
D
OUT
S
A
R
Dynamic Performance
The LTC1400 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 LTC1400 FFT plot.
Signal-to-Noise Ratio
The signal-to-noise plus distortion ratio [S/(N + D)] is the
ratio between the RMS amplitude of the fundamental input
frequency to the RMS amplitude of all other frequency
components at the A/D output. The output is band limited
to frequencies from DC to half the sampling frequency.
Figure 2a shows a typical spectral content with a 400kHz
sampling rate and a 100kHz input. The dynamic perfor-
mance is excellent for input frequencies up to the Nyquist
limit of 200kHz as shown in Figure 2b.
Effective Number of Bits
The effective number of bits (ENOBs) is a measurement of
the effective resolution of an ADC and is directly related to
the S/(N + D) by the equation:
N
S N D
Figure 2a. LTC1400 Nonaveraged, 4096 Point FFT
Plot with 100kHz Input Frequency in Bipolar Mode
/
–100
–110
–120
– 60
–10
–20
–30
–40
–50
–70
–80
–90
0
6 02
0
.
f
f
SINAD = 72.5dB
THD = – 82dB
SAMPLE
IN
20
= 94.824kHz
– .
1 76
40
= 400kHz
60
FREQUENCY (kHz)
80 100
120
140
160
LTC1400 • F02a
180
LTC1400
200
7