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)
VRHF
VRHS
VRLS
VRLF
VIN
VIN
VCAL
CLK IN
CLK
DAV
AVDD
AGND
DGND* DVDD
D8
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
SPT7734
D0
Interfacing
Logics
EN
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.
FB1
FB2
+D5
+A5
Enable/Tri-State
(Enable = Active Low)
FB3
+A5
AGND
DGND
+D5
+
10 µF
+5 V
Analog
+5 V
Analog
RTN
*To reduce the possibility of latch-up, avoid
connecting the DGND pins of the ADC to the
digital ground of the system.
+
10 µF
+5 V
Digital
RTN
+5 V
Digital
• 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, V
RHS
and V
RLS
.
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
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
CADEKA 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. CADEKA 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.
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
