OPERATING DESCRIPTION
The general architecture for the CMOS ADC is shown in the
block diagram. The design contains two sets of eight identical
successive approximation ADC sections, all operating in par-
allel, a 16-phase clock generator, an 11-bit 8:1 digital output
multiplexer, correction logic, and a voltage reference genera-
tor 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 I – Clock Cycles
Clock
1
2
3
4
5–15
16
Operation
Reference zero sampling
Auto-zero comparison
Auto-calibrate comparison
Input sample
11-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 two clock cycles so that the
analog input is sampled on every other cycle of the input
clock by exactly one ADC section. After 16 clock periods,
the timing cycle repeats. The sample rate for the configura-
tion is one-half of the clock rate, e.g., for a 40 MHz clock
rate, the input sample rate is 20 MHz. The latency from ana-
log input sample to the corresponding digital output is 12
clock cycles.
• Since only sixteen comparators are used, a huge power
savings is realized.
• The auto-zero operation is done using a closed loop sys-
tem that uses multiple samples of the comparator's re-
sponse 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 per
V
IN
input 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 iso-
lated from the input by transmission gates.
SPT7852
5
1/12/00
CADEKA [ CADEKA MICROCIRCUITS LLC. ]
