AMC7812B Datasheet, PDF (38/92 Pages) TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS – 12-Bit Analog Monitoring and Control Solution with Multichannel ADC, DACs, and Temperature Sensors

AMC7812B
SBAS625A – SEPTEMBER 2013 – REVISED SEPTEMBER 2013
www.ti.com
Filtering
Figure 88(a) and Figure 88(b) show the connection of recommended NPN or PNP transistors, respectively.
Remote junction temperature sensors are usually implemented in a noisy environment. Noise is most often
created by fast digital signals, and can corrupt measurements. The AMC7812B has a built-in 65-kHz filter on the
D+ and D– inputs to minimize the effects of noise. However, a bypass capacitor placed differentially across the
inputs of the remote temperature sensor can make the application more robust against unwanted coupled
signals. If filtering is required, the capacitance between D+ and D– should be limited to 330 pF or less for
optimum measurement performance. This capacitance includes any cable capacitance between the remote
temperature sensor and the device.
2N3904
D+
2N3906
D+
D-
D-
(a) NPN
(b) PNP
Figure 88. Remote Temperature Sensor Using Transistor
Series Resistance Cancellation
Parasitic resistance (in series with the remote diode) to the D+ and D– inputs of the device is caused by a variety
of factors, including printed circuit board (PCB) trace resistance and trace length. This series resistance appears
as a temperature offset in the remote sensor temperature measurement, and causes more than 0.45°C error per
ohm. The device implements a technology to automatically cancel out the effect of this series resistance, thus
providing a more accurate result without requiring user characterization of this resistance. With this technology,
the device is able to reduce the effects of series resistance to typically less than 0.0075°C per ohm. The
resistance cancellation is disabled when the RC bit in the temperature configuration register is cleared ('0').
Reading Temperature Data
Temperature is always read as 12-bit data. When the conversion finishes, the temperature is sent to the
corresponding temperature-data register. However, if a data transfer is in progress between the temperature-data
register and the AMC shift register, the temperature-data register is frozen until data transfer completes.
Conversion Time
The conversion time depends on the type of sensor and configuration, as shown in Table 4.
Table 4. Conversion Times
TEMPERATURE SENSOR
Local sensor is active, remote sensors are disabled or in power-down
One remote sensor is active and RC = 0, local sensor and one remote sensor are disabled
or in power-down
One remote sensor is active and RC = 1, local sensor and one remote sensor are disabled
or in power-down
One remote sensor and local sensor are active and RC = 0, one remote sensor is disabled
or in power-down
One remote sensor and local sensor are active and RC = 1, one remote sensor is disabled
or in power-down
Two remote sensors are active and RC = 0, local sensor is disabled or in power-down
Two remote sensors are active and RC = 1, local sensor is disabled or in power-down
All sensors are active and RC is '0'
All sensors are active and RC is '1'
MONITORING
CYCLE TIME (ms)
15
44
93
59
108
88
186
103
201
PROGRAMMABLE
DELAY RANGE (s)
0.48 to 3.84
1.40 to 11.2
2.97 to 23.8
1.89 to 15.1
3.45 to 27.65
2.81 to 22.5
5.95 to 47.6
3.92 to 26.38
6.43 to 51.45
38
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