AMC7891_15 Datasheet, PDF (27/41 Pages) Texas Instruments – Analog Monitor and Control Circuit with 10-Bit, Multi-Channel ADC and Four DACs, Temperature Sensor, and 12 GPIOs

AMC7891
www.ti.com
SBAS518A – AUGUST 2011 – REVISED DECEMBER 2011
TEMPERATURE SENSOR OPERATION (SECONDARY ADC)
The AMC7891 contains an on-chip temperature sensor used to measure the device temperature. The
temperature sensor is continuously monitoring, and new readings are automatically available every cycle. The
analog temperature reading is converted by a secondary ADC that runs in the background at a lower speed than
the primary ADC.
The temperature measurement relies on the characteristics of a semiconductor junction operating at a fixed
current level. The forward voltage of the diode (V_BE) depends on the current passing through it and the ambient
temperature. The change in V_BE when the diode operates at two different currents (a low current of I_LOW and a
high current of I_HIGH, is shown in Equation 1:
V_{BE_HIGH} – V_{BE_LOW} = ηkT/q × ln(I_HIGH/I_LOW)
(1)
Where:
k is Boltzmann’s constant.
q is the charge of the carrier.
T is the absolute temperature in Kelvins (K).
η is the ideality of the transistor as sensor.
I_LOW
I_HIGH
SW1
SW2
Mux
Diode
Temperature
Sensor
LPF and Signal
Conditioning
Secon ADC
and Signal
Processing
temp _data
register
Figure 30. Integrated Temperature Sensor
The temperature sensor can be disabled by clearing to ‘0’ the temp_en bit (TEMP_config register, bit 3). When
disabled, the sensor is not converted. The AMC7891 continuously monitors the temperature sensor in the
background, leaving the user free to perform conversions on the primary ADC. When one monitor cycle finishes,
a signal passes to the control logic to automatically initiate a new conversion.
The analog sensing signal is preprocessed by a low-pass filter and signal conditioning circuitry, and then
digitized by the secondary ADC. The resulting digital signal is further processed by the digital filter and
processing unit. The final result is stored as a 12-bit value in the TEMP_data register as tempdata[11:0]. The
format of the final result is in twos complement, as shown in Table 4. Note that the device measures the
temperature from –40°C to 150°C.
If a data transfer is in progress between the TEMP_data register and the AMC Shift Register, the TEMP_data
register is frozen until the data transfer is complete.
Copyright © 2011, Texas Instruments Incorporated
Product Folder Link(s): AMC7891
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