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ADuC7019/20/21/22/24/25/26/27/28
Rev. B | Page 38 of 92
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
different 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 VREF when
operating in single-ended or pseudo differential mode. In fully
differential mode, the input signal must be balanced around a
common-mode voltage (VCM) in the 0 V to AVDD range with a
04
95
5-
01
1
AVDD
VCM
0
2VREF
Figure 37. 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
Single or continuous conversion modes can be initiated in the
software. An external CONVSTART 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 VREF. 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 VREF = 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
04
95
5-
01
2
O
UT
P
UT
CO
DE
VOLTAGE INPUT
1111 1111 1111
1111 1111 1110
1111 1111 1101
1111 1111 1100
0000 0000 0011
1LSB
0V
+FS – 1LSB
0000 0000 0010
0000 0000 0001
0000 0000 0000
1LSB =
FS
4096
Figure 38. 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 VIN+ and VIN– pins (that is, VIN+ – VIN–).
The maximum amplitude of the differential signal is therefore,
–VREF to +VREF p-p (that is, 2 × VREF). This is regardless of the
common mode (CM). The common mode is the average of the
two signals, for example, (VIN+ + VIN–)/2, and is therefore, the
voltage that the two inputs are centered on. This results in the
span of each input being CM ± VREF/2. This voltage has to be set
up externally and its range varies with VREF (see the Driving the The output coding is twos complement in fully differential mode
with 1 LSB = 2 VREF/4096, or 2 × 2.5 V/4096 = 1.22 mV when
VREF = 2.5 V. The output result is ±11 bits, but this is shifted by
one 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). The ideal
input/output transfer characteristic is shown in
Figure 39.04
95
5-
013
O
U
T
P
UT
CO
DE
VOLTAGE INPUT (VIN+ – VIN–)
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
–VREF + 1LSB
+VREF – 1LSB
0LSB
1LSB =
2 × VREF
4096
SIGN
BIT
Figure 39. ADC Transfer Function in Differential Mode