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SBAS286A − JUNE 2003 − REVISED MARCH 2004
TEMPERATURE SENSOR
VOLTAGE REFERENCE INPUTS
(VREFP, VREFN)
The voltage reference used by the modulator is
generated from the voltage difference between VREFP
On-chip diodes provide temperature-sensing capabili-
ty. By setting the TEMPEN pin high, the selected analog
inputs are disconnected and the inputs to the A/D
converter are connected to the anodes of two diodes
scaled to 1x and 64x in current and size inside the mux,
as shown in Figure 18. By measuring the difference in
voltage of these diodes, temperature changes can be
inferred from a baseline temperature. Typically, the
difference in diode voltages is 106mV at 25°C, with a
temperature coefficient of 360µV/°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.
and VREFN: V
= VREFP – VREFN. The reference
REF
inputs use a structure similar to that of the analog
inputs. A simplified diagram of the circuitry on the
reference inputs is shown in Figure 19. The switches
and capacitors can be modeled with an effective
impedance of:
tsample
ǒ Ǔń16pF + 500kW
2
where f
= 2MHz.
CLK
VREFP
VREFN
TEMPEN
AVDD
AVDD
AVDD
ESD
Protection
8I
1I
Self Gain Cal
16pF
AINP
AINN
(1)
Ω
Zeff = 500k
AINN
AINP
1X
8X
(1) fCLK = 2MHz
AINP1
AINN1
AINP2
AINN2
AINP3
AINN3
AINP4
AINN4
Figure 19. 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 AVDD by
100mV:
GND – 100mV < (VREFP, VREFN) < AVDD + 100mV
During self gain calibration, all the switches in the input
multiplexer are opened, VREFN is internally connected to
AINN, and VREFP is connected to AINP. The input buffer
may be disabled or enabled during calibration. When the
buffer is disabled, the reference pins will be driving the
circuitry shown in Figure 9 during self gain calibration,
resulting in increased loading. To prevent this additional
loading from introducing gain errors, make sure the
circuitry driving the reference pins has adequate drive
capability. When the buffer is enabled, the loading on the
reference pins will be much less, but the buffer will limit the
MUX0
MUX1
Figure 18. Measurement of the Temperature
Sensor in the Input Multiplexer
10
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