
Data Sheet
ADuC7023
| Page 29 of 96
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 two
different modes: fully differential mode (for small and balanced
signals) or single-ended mode (for any single-ended signals).
The converter accepts an analog input range of 0 V to VREF when
operating in single-ended 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 maximum amplitude of
08
67
5-
0
12
AVDD
VCM
0
2VREF
Figure 19. 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. This temperature channel can be
selected as an ADC input. This facilitates an internal temperature
sensor channel that measures die temperature.
TRANSFER FUNCTION
Single-Ended Mode
In single-ended mode, the input range is 0 V to VREF. The
output coding is straight binary in single-ended mode 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
086
75-
0
13
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 20. ADC Transfer Function in 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 on which the two inputs are centered. 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 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 the ADCDAT MMR 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
086
75
-014
O
UT
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 21. ADC Transfer Function in Differential Mode
Rev. E