Datasheet

TEMPERATURE SENSOR VOLTAGE REFERENCE INPUTS
AVDD
Zeff =1.5MW
VREFP VREFN
AVDD
ESD
Protection
16pF
GND * 100mV t
(
VREFP, VREFN
)
t VDD ) 100mV
AINP
AINP1
AINN1
AINP2
AINN2
8I 1I
1X 8X
AINN
TEMPEN
AVDD
MUX
ADS1225
ADS1226
SBAS346C – MAY 2006 – REVISED JANUARY 2009 ....................................................................................................................................................... www.ti.com
(VREFP, VREFN)
Internal diodes provide temperature-sensing
capability. By setting the TEMPEN pin high, the The voltage reference used by the modulator is
selected analog inputs are disconnected and the generated from the voltage difference between
inputs to the A/D converter are connected to the VREFP and VREFN: V
REF
= VREFP – VREFN. The
anodes of two diodes scaled to 1x and 64x in current reference inputs use a structure similar to that of the
and size inside the MUX, as shown in Figure 24 . By analog inputs. A simplified diagram of the circuitry on
measuring the difference in voltage of these diodes, the reference inputs is shown in Figure 25 . The
temperature changes can be inferred from a baseline switches and capacitors can be modeled with an
temperature. Typically, the difference in diode effective impedance of 1.5M Ω .
voltages is 106mV at +25 ° C, with a temperature
coefficient of 360 µ V/ ° C. A similar structure is used in
the MSC1210 for temperature measurement. For
more information, see TI application report SBAA100 ,
Using the MSC121x as a High-Precision Intelligent
Temperature Sensor, available for download at
www.ti.com .
Figure 25. Simplified Reference Input Circuitry
ESD diodes protect the reference inputs. To prevent
these diodes from turning on, make sure the voltages
on the reference pins do not go below GND by more
than 100mV, and likewise, do not exceed VDD by
100mV. This limitation is shown in Equation 2 :
(2)
For best performance, bypass the voltage reference
inputs with a 0.1 µ F capacitor between VREFP and
VREFN. Place the capacitor as close as possible to
the pins.
Figure 24. Measurement of the Temperature
Sensor in the Input
The differential voltage reference inputs and the wide
range of operation (V
REF
can support up to AVDD)
make ratiometric measurements easy to implement.
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