Datasheet

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SBAS314B − APRIL 2004 − REVISED JANUARY 2009
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8
OVERVIEW
The ADS1222 is an A/D converter comprised of a
delta-sigma modulator followed by a digital filter. A mux
allows for one of two 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 MUX, as shown in
Table 1. The ADS1222 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 ADS1222 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 MUX
DIGITAL PIN SELECTED ANALOG INPUTS
MUX POSITIVE INPUT NEGATIVE INPUT
0 AINP1 AINN1
1 AINP2 AINN2
Mux
AINP
V
IN
AINN
Temp
Sensor
Digital
Filter
and
Serial
Interface
∆Σ
Modulator
VREFP
Σ
VREFN
Buffer
AINP1
AINN1
AINP2
AINN2
+
−
CLK
DRDY/DOUT
SCLK
TEMPEN
MUX BUFEN
2
V
REF
2V
REF
Σ
Figure 13. Conceptual Diagram of the ADS1222