AMC7812SPAPR Datasheet, PDF (35/92 Pages) Texas Instruments – 12-Bit Analog Monitoring and Control Solution with Multichannel ADC, DACs, and Temperature Sensors

AMC7812
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SBAS513E – JANUARY 2011 – REVISED SEPTEMBER 2013
SECONDARY ADC/TEMPERATURE SENSOR OPERATION
The AMC7812 contains one local and two remote temperature sensors. The temperature sensors continuously
monitor the three temperature inputs, and new readings are automatically available every cycle. The on-chip
integrated temperature sensor (shown in Figure 86) is used to measure the device temperature, and two remote
diode sensor inputs are used to measure the two external temperatures. All analog signals are converted by the
secondary ADC that runs in the background at a lower speed. The measurement relies on the characteristics of
a semiconductor junction operation 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:
( ) ^VBE_HIGH
-
V_{BE_LOW}
=
hkT
q
ln
I_HIGH
I_LOW
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.
(1)
I_LOW
I_HIGH
SW1
SW2
Mux
Diode
Temperature
Sensor
LPF and Signal
Conditioning
Second ADC
and Signal
Processing
Local
Temperature
Registers
Figure 86. Integrated Local Temperature Sensor
The remote sensing transistor can be a discrete small-signal type transistor or substrate transistor built within the
microprocessor. This architecture is shown in Figure 87. An internal voltage source biases the D– terminal above
ground to prevent the ground noise from interfering with the measurement. An external capacitor (up to 330pF)
may be placed between D+ and D– to further reduce noise interference.
I_LOW
I_HIGH
SW1
SW2
D+
Mux
LPF and Signal
Conditioning
D-
V_BIAS
Remote
Temperature
Registers
Second ADC
and Signal
Processing
Figure 87. Remote Temperature Sensor
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