CS5101A CS5102A
GENERAL DESCRIPTION
array share a common node at the comparator’s
input. As shown in Figure 1, their other terminals
are capable of being connected to AGND, VREF,
or AIN (1 or 2). When the device is not calibrat-
ing or converting, all capacitors are tied to AIN.
Switch S1 is closed and the charge on the array,
tracks the input signal.
The CS5101A and CS5102A are 2-channel, 16-
bit A/D converters. The devices include an
inherent sample/hold and an on-chip analog
switch for 2-channel operation. Both channels
can thus be sampled and converted at rates up to
50 kHz each (CS5101A) or 10 kHz each
(CS5102A). Alternatively, each of the devices
can be operated as a single channel ADC operat-
ing at 100 kHz (CS5101A) or 20 kHz
(CS5102A).
When the conversion command is issued, switch
S1 opens. This traps the charge on the compara-
tor side of the capacitor array and creates a
floating node at the comparator’s input. The con-
version algorithm operates on this fixed charge,
and the signal at the analog input pin is ignored.
In effect, the entire DAC capacitor array serves
as analog memory during conversion much like a
hold capacitor in a sample/hold amplifier.
Both the CS5101A and CS5102A can be config-
ured to accept either unipolar or bipolar input
ranges, and data is output serially in either binary
or 2’s complement coding. The devices can be
configured in 3 different output modes, as well as
an internal, synchronous loopback mode. The
CS5101A and CS5102A provide coarse
charge/fine charge control, to allow accurate
tracking of high-slew signals.
The conversion consists of manipulating the free
plates of the capacitor array to VREF and AGND
to form a capacitive divider. Since the charge at
the floating node remains fixed, the voltage at
that point depends on the proportion of capaci-
tance tied to VREF versus AGND. The
successive-approximation algorithm is used to
find the proportion of capacitance, which when
connected to the reference will drive the voltage
at the floating node to zero. That binary fraction
of capacitance represents the converter’s digital
output.
THEORY OF OPERATION
The CS5101A and CS5102A implement the suc-
cessive approximation algorithm using a charge
redistribution architecture. Instead of the tradi-
tional resistor network, the DAC is an array of
binary-weighted capacitors. All capacitors in the
Fine
AIN
+
-
Coarse
Fine
S1
VREF
AGND
C
C/2
C/4
C/32,768
C/32,768
+
-
-
+
Bit 15
MSB
Bit 14
Bit 13
Bit 0
LSB
Dummy
Coarse
Fine
C
= C + C/2 + C/4 + C/8 + ... C/32,768
tot
+
-
Coarse
Figure 1. Coarse Charge Input Buffers and Charge Redistribution DAC
12
DS45F2
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