Datasheet

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SBAS286C − JUNE 2003 − REVISED JANUARY 2009
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8
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
IN
= (AINP – AINN) against the differential
reference V
REF
= (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
REF
. The digital filter receives the modulator
signal and provides a low-noise digital output. A 2-wire
serial interface indicates conversion completion and
provides the user with the output data.
ANALOG INPUTS (AINPx, AINNx)
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,
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.
Table 1. Input Channel selection with
MUX0 and MUX1
DIGITAL PINS SELECTED ANALOG INPUTS
MUX1 MUX0 POSITIVE INPUT NEGATIVE INPUT
0 0 AINP1 AINN1
0 1 AINP2 AINN2
1 0 AINP3 AINN3
1 1 AINP4 AINN4
Mux
AINP
V
IN
AINN
Temp
Sensor
Digital
Filter
and
Serial
Interface
∆Σ
Modulator
VREFP
Σ
VREFN
Buffer
MUX0
AINP1
AINN1
AINP2
AINN2
AINP3
AINN3
AINP4
AINN4
+
−
CLK
DRDY/DOUT
SCLK
TEMPEN
MUX1 BUFEN
2
V
REF
2V
REF
Σ
Figure 13. Conceptual Diagram of the ADS1224