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SBAS286A − JUNE 2003 − REVISED MARCH 2004
as shown in Table 1. The ADS1224 accepts differential
input signals, but can also measure unipolar signals.
When measuring unipolar (or single-ended signals)
with respect to ground, connect the negative input
(AINNx) to ground and connect the input signal to the
positive input (AINPx). Note that when the ADS1224 is
configured this way, only half of the converter full-scale
range is used since only positive digital output codes
are produced. An input buffer can be selected to
increase the input impedance of the A/D converter with
the BUFEN pin.
OVERVIEW
The ADS1224 is an A/D converter comprised of a
delta-sigma modulator followed by a digital filter. A mux
allows for one of four input channels to be selected. A
buffer can also be selected to increase the input
impedance. The modulator measures the differential
input signal V = (AINP – AINN) against the differential
IN
REF
reference V
= (VREFP – VREFN). Figure 13 shows
a conceptual diagram of the device. The differential
reference is scaled internally so that the full-scale input
range is 2V
. The digital filter receives the modulator
REF
signal and provides a low-noise digital output. A 2-wire
serial interface indicates conversion completion and
provides the user with the output data.
Table 1. Input Channel selection with MUX0 and
MUX1
DIGITAL PINS
SELECTED ANALOG INPUTS
POSITIVE INPUT NEGATIVE INPUT
AINP1 AINN1
ANALOG INPUTS (AINPx, AINNx)
MUX0
MUX1
0
0
1
1
0
1
0
1
The input signal to be measured is applied to the input
pins AINPx and AINNx. The positive internal input is
generalized as AINP, and the negative internal input is
generalized as AINN. The signal is selected though the
input mux, which is controlled by pins MUX0 and MUX1,
AINP2
AINP3
AINP4
AINN2
AINN3
AINN4
TEMPEN
VREFP VREFN
−
+
Temp
Sensor
Σ
CLK
VREF
AINP1
AINN1
2
2VREF
Digital
Filter
and
Serial
Interface
AINP2
AINP
SCLK
VIN
AINN2
Mux
∆Σ
Modulator
Buffer
Σ
AINP3
AINN
DRDY/DOUT
AINN3
AINP4
AINN4
MUX0 MUX1
BUFEN
Figure 13. Conceptual Diagram of the ADS1224
8
BB [ BURR-BROWN CORPORATION ]
