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PDF SPT7734 Data sheet ( Hoja de datos )
| Número de pieza | SPT7734 | |
| Descripción | 8-BIT / 40 MSPS / 175 mW A/D CONVERTER | |
| Fabricantes | Fairchild Semiconductor | |
| Logotipo |  |
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FEATURES
• Monolithic 40 MSPS Converter
• 175 mW Power Dissipation
• On-Chip Track-and-Hold
• Single +5 V Power Supply
• TTL/CMOS Outputs
• 5 pF Input Capacitance
• Low Cost
• Tri-State Output Buffers
• High ESD Protection: 3,500 V Minimum
• Selectable +3 V or +5 V Logic I/O
SPT7734
8-BIT, 40 MSPS,175 mW A/D CONVERTER
APPLICATIONS
• All High-Speed Applications Where
Low Power Dissipation is Required
• Video Imaging
• Medical Imaging
• Radar Receivers
• IR Imaging
• Digital Communications
GENERAL DESCRIPTION
The SPT7734 is a 8-bit monolithic, low cost, ultralow power
analog-to-digital converter capable of minimum word rates
of 40 MSPS. The on-chip track-and-hold function assures
very good dynamic performance without the need for exter-
nal components. The input drive requirements are mini-
mized due to the SPT7734's low input capacitance of only
5 pF.
Power dissipation is extremely low at only 175 mW typical at
40 MSPS with a power supply of +5.0 V. The digital outputs
are +3 V or +5 V, and are user selectable. The SPT7734 has
incorporated proprietary circuit design and CMOS process-
ing technologies to achieve its advanced performance. In-
puts and outputs are TTL/CMOS compatible to interface with
TTL/CMOS logic systems. Output data format is straight
binary.
The SPT7734 is available in 28-lead SOIC and 32-lead small
(7 mm square) TQFP packages over the commercial tem-
perature range.
BLOCK DIAGRAM
1:16
AIN Mux
CLK In
Enable
Data
Vali
d
P1
Timing
and
Control
P2
...
P15
P16
Ref
In
ADC Section 1
T/H
Auto-
Zero
CMP
ADC Se...ction 2
ADC Section 15
ADC Section 16
T/H
Auto-
Zero
CMP
9-Bit
SAR
9
DAC
...
9-Bit
SAR
9
DAC
Reference Ladder
VREF
9
9
...
9
9
9-Bit
16:1
Mux/
Error
Correction
D8 Overrange
D7 (MSB)
D6
D5
D4
D3
D2
D1
DØ (LSB)
1 page  
TYPICAL INTERFACE CIRCUIT
Very few external components are required to achieve the
stated device performance. Figure 1 shows the typical inter-
face requirements when using the SPT7734 in normal circuit
operation. The following sections provide descriptions of the
major functions and outline critical performance criteria to
consider for achieving the optimal device performance.
Figure 1 - Typical Interface Circuit
Ref In
(+4 V)
VIN
CLK IN
VRHF
VRHS
D8
VRLS
VRLF
VIN
VCAL
SPT7734
D0
CLK EN
DAV
AVDD AGND DGND* DVDD
Interfacing
Logics
FB1
FB2
+D5
Enable/Tri-State
+A5 (Enable = Active Low)
+A5 AGND
FB3
DGND
+D5
+
*To reduce the possibility of latch-up, avoid
10 µF
connecting the DGND pins of the ADC to the
+5 V +5 V digital ground of the system.
Analog Analog
RTN
+
10 µF
+5 V
Digital
RTN
+5 V
Digital
NOTES: 1) FB3 is to be located as closely to the device as possible.
2) There should be no additional connections to the right of FB1 and FB2.
3) All capacitors are 0.1 µF surface-mount unless otherwise specified.
4) FB1, FB2 and FB3 are 10 µH inductors or ferrite beads.
POWER SUPPLIES AND GROUNDING
Fairchild suggests that both the digital and the analog supply
voltages on the SPT7734 be derived from a single analog
supply as shown in figure 1. A separate digital supply should
be used for all interface circuitry. Fairchild suggests using
this power supply configuration to prevent a possible latch-
up condition on power up.
OPERATING DESCRIPTION
The general architecture for the CMOS ADC is shown in the
block diagram. The design contains 16 identical successive
approximation ADC sections, all operating in parallel, a 16-
phase clock generator, an 9-bit 16:1 digital output multi-
plexer, correction logic, and a voltage reference generator
which provides common reference levels for each ADC section.
The high sample rate is achieved by using multiple SAR ADC
sections in parallel, each of which samples the input signal in
sequence. Each ADC uses 16 clock cycles to complete a
conversion. The clock cycles are allocated as follows:
Table II - Clock Cycles
Clock
1
2
3
4
5-15
16
Operation
Reference zero sampling
Auto-zero comparison
Auto-calibrate comparison
Input sample
9-bit SAR conversion
Data transfer
The 16 phase clock, which is derived from the input clock,
synchronizes these events. The timing signals for adjacent
ADC sections are shifted by one clock cycle so that the analog
input is sampled on every cycle of the input clock by exactly
one ADC section. After 16 clock periods, the timing cycle
repeats. The latency from analog input sample to the corre-
sponding digital output is 12 clock cycles.
• Since only 16 comparators are used, a huge power savings
is realized.
• The auto-zero operation is done using a closed loop
system that uses multiple samples of the comparators
response to a reference zero.
• The auto-calibrate operation, which calibrates the gain
of the MSB reference and the LSB reference, is also
done with a closed loop system. Multiple samples of the
gain error are integrated to produce a calibration voltage for
each ADC section.
• Capacitive displacement currents, which can induce sam-
pling error, are minimized since only one comparator
samples the input during a clock cycle.
• The total input capacitance is very low since sections of the
converter which are not sampling the signal are isolated
from the input by transmission gates.
VOLTAGE REFERENCE
The SPT7734 requires the use of a single external voltage
reference for driving the high side of the reference ladder. It
must be within the range of 3 V to 5 V. The lower side of the
ladder is typically tied to AGND (0.0 V), but can be run up to
2.0 V with a second reference. The analog input voltage
range will track the total voltage difference measured be-
tween the ladder sense lines, VRHS and VRLS.
Force and sense taps are provided to ensure accurate and
stable setting of the upper and lower ladder sense line
voltages across part-to-part and temperature variations. By
using the configuration shown in figure 2, offset and gain
errors of less than ±2 LSB can be obtained.
SPT7734
5 1/27/98
5 Page | ||
| Páginas | Total 9 Páginas | |
| PDF Descargar | [ Datasheet SPT7734.PDF ] | |
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