CS5101A CS5102A
Calibration
the track mode. After allowing a short time for
acquisition, the device will be ready for another
conversion.
The ability of the CS5101A or the CS5102A to
convert accurately to 16-bits clearly depends on
the accuracy of its comparator and DAC. Each
device utilizes an "auto-zeroing" scheme to null
errors introduced by the comparator. All offsets
are stored on the capacitor array while in the
track mode and are effectively subtracted from
the input signal when a conversion is initiated.
Auto-zeroing enhances power supply rejection at
frequencies well below the conversion rate.
In contrast to systems with separate track-and-
holds and A/D converters, a sampling clock can
simply be connected to the HOLD input. The
duty cycle of this clock is not critical. The HOLD
input is latched internally by the master clock, so
it need only remain low for 1/f + 20 ns, but no
clk
longer than the minimum conversion time minus
two master clocks or an additional conversion cy-
cle will be initiated with inadequate time for
acquisition. In Free Run mode, SCKMOD =
OUTMOD = 0, the device will convert at a rate
of CLKIN/80, and the HOLD input is ignored.
To achieve 16-bit accuracy from the DAC, the
CS5101A and CS5102A use a novel self-calibra-
tion scheme. Each bit capacitor shown in
Figure 1 actually consists of several capacitors in
parallel which can be manipulated to adjust the
overall bit weight. An on-chip micro controller
precisely adjusts each capacitor with a resolution
of 18 bits.
As with any high-resolution A-to-D system, it is
recommended that sampling is synchronized to
the master system clock in order to minimize the
effects of clock feedthrough. However, the
CS5101A and CS5102A may be operated entirely
asynchronous to the master clock if necessary.
The CS5101A and CS5102A should be reset
upon power-up, thus initiating a calibration cycle.
The device then stores its calibration coefficients
in on-chip SRAM. When the CS5101A and
CS5102A are in power-down mode (SLEEP
low), they retain the calibration coefficients in
memory, and need not be recalibrated when nor-
mal operation is resumed.
Tracking the Input
Upon completing a conversion cycle the
CS5101A and CS5102A immediately return to
the track mode. The CH1/2 pin directly controls
the input switch, and therefore directly deter-
mines which channel will be tracked. Ideally, the
CH1/2 pin should be switched during the conver-
sion cycle, thereby nullifying the input mux
switching time, and guaranteeing a stable input at
the start of acquisition. If, however, the CH1/2
control is changed during the acquisition phase,
adequate coarse charge and fine charge time must
be allowed before initiating conversion.
OPERATION OVERVIEW
Monolithic design and inherent sampling archi-
tecture make the CS5101A and CS5102A
extremely easy to use.
Initiating Conversions
When the CS5101A or the CS5102A enters track-
ing mode, it uses an internal input buffer
amplifier to provide the bulk of the charge on the
capacitor array (coarse-charge), thereby reducing
the current load on the external analog circuitry.
Coarse-charge is internally initiated for 6 clock
cycles at the end of every conversion. The buffer
A falling transition on the HOLD pin places the
input in the hold mode and initiates a conversion
cycle. The charge is trapped on the capacitor ar-
ray the instant HOLD goes low. The device will
complete conversion of the sample within 66
master clock cycles, then automatically return to
DS45F2
13
CIRRUS [ CIRRUS LOGIC ]
