Datasheet

Remote
Temperature
Registers
LPFandSignal
Conditioning
Mux
SW1
I
LOW
SW2
I
HIGH
V
BIAS
D+
SecondADC
andSignal
Processing
D-
Diode
Temperature
Sensor
SW1
I
LOW
SecondADC
andSignal
Processing
LPFandSignal
Conditioning
Local
Temperature
Registers
Mux
SW2
I
HIGH
V V =-
BE_HIGH BE_LOW
ln
hkT
q
()
I
I
HIGH
LOW
AMC7812
www.ti.com 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:
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)
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.
Figure 87. Remote Temperature Sensor
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