Datasheet

Data Sheet ADuC7019/20/21/22/24/25/26/27/28/29
Rev. F | Page 45 of 104
ADC CIRCUIT OVERVIEW
The analog-to-digital converter (ADC) incorporates a fast,
multichannel, 12-bit ADC. It can operate from 2.7 V to 3.6 V
supplies and is capable of providing a throughput of up to
1 MSPS when the clock source is 41.78 MHz. This block
provides the user with a multichannel multiplexer, a differential
track-and-hold, an on-chip reference, and an ADC.
The ADC consists of a 12-bit successive approximation converter
based around two capacitor DACs. Depending on the input
signal configuration, the ADC can operate in one of three modes.
 Fully differential mode, for small and balanced signals
 Single-ended mode, for any single-ended signals
 Pseudo differential mode, for any single-ended signals,
taking advantage of the common-mode rejection offered
by the pseudo differential input
The converter accepts an analog input range of 0 V to V
REF
when
operating in single-ended or pseudo differential mode. In fully
differential mode, the input signal must be balanced around a
common-mode voltage (V
CM
) in the 0 V to AV
DD
range with a
maximum amplitude of 2 V
REF
(see Figure 48).
04955-011
AV
DD
V
CM
V
CM
V
CM
0
2V
REF
2V
REF
2V
REF
Figure 48. Examples of Balanced Signals in Fully Differential Mode
A high precision, low drift, factory calibrated, 2.5 V reference is
provided on-chip. An external reference can also be connected as
described in the Band Gap Reference section.
Single or continuous conversion modes can be initiated in the
software. An external
CONV
START
pin, an output generated from
the on-chip PLA, or a Timer0 or Timer1 overflow can also be
used to generate a repetitive trigger for ADC conversions.
A voltage output from an on-chip band gap reference propor-
tional to absolute temperature can also be routed through the
front-end ADC multiplexer, effectively an additional ADC channel
input. This facilitates an internal temperature sensor channel
that measures die temperature to an accuracy of 3°C.
TRANSFER FUNCTION
Pseudo Differential and Single-Ended Modes
In pseudo differential or single-ended mode, the input range
is 0 V to V
REF
. The output coding is straight binary in pseudo
differential and single-ended modes with
1 LSB = FS/4096, or
2.5 V/4096 = 0.61 mV, or
610 μV when V
REF
= 2.5 V
The ideal code transitions occur midway between successive
integer LSB values (that is, 1/2 LSB, 3/2 LSB, 5/2 LSB, … ,
FS − 3/2 LSB). The ideal input/output transfer characteristic
is shown in Figure 49.
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OUTPUT CODE
VOLTAGE INPUT
1111 1111 1111
1111 1111 1110
1111 1111 1101
1111 1111 1100
0000 0000 0011
1LSB0V +FS – 1LSB
0000 0000 0010
0000 0000 0001
0000 0000 0000
1LSB =
FS
4096
Figure 49. ADC Transfer Function in Pseudo Differential or Single-Ended Mode
Fully Differential Mode
The amplitude of the differential signal is the difference between
the signals applied to the V
IN+
and V
IN–
input voltage pins (that
is, V
IN+
− V
IN–
). The maximum amplitude of the differential
signal is, therefore, –V
REF
to +V
REF
p-p (that is, 2 × V
REF
). This is
regardless of the common mode (CM). The common mode is
the average of the two signals, for example, (V
IN+
+ V
IN–
)/2, and
is, therefore, the voltage that the two inputs are centered on.
This results in the span of each input being CM V
REF
/2. This
voltage has to be set up externally, and its range varies with V
REF
(see the Driving the Analog Inputs section).
The output coding is twos complement in fully differential mode
with 1 LSB = 2 V
REF
/4096 or 2 × 2.5 V/4096 = 1.22 mV when
V
REF
= 2.5 V. The output result is ±11 bits, but this is shifted by 1
to the right. This allows the result in ADCDAT to be declared as a
signed integer when writing C code. The designed code
transitions occur midway between successive integer LSB values
(that is, 1/2 LSB, 3/2 LSB, 5/2 LSB, … , FS − 3/2 LSB). e ideal
input/output transfer characteristic is shown in Figure 50.
04955-013
OUTPUT CODE
VOLTAGE INPUT (V
IN
+ – V
IN
–)
0 1111 1111 1110
0 1111 1111 1100
0 1111 1111 1010
0 0000 0000 0010
0 0000 0000 0000
1 1111 1111 1110
1 0000 0000 0100
1 0000 0000 0010
1 0000 0000 0000
–V
REF
+ 1LSB +V
REF
– 1LSB0LSB
1LSB =
2 × V
REF
4096
SIGN
BIT
Figure 50. ADC Transfer Function in Differential Mode