Analog Devices ADT7460 Manual page 18

ADT7460
If a discrete transistor is used, the collector is not grounded, and
it should be linked to the base. If a PNP transistor is used, the
base is connected to the D− input and the emitter to the D+
input. If an NPN transistor is used, the emitter is connected to
the D− input, and the base to the D+ input. Figure 26 and
Figure 27 show how to connect the ADT7460 to an NPN or
PNP transistor for temperature measurement. To prevent
ground noise from interfering with the measurement, the more
negative terminal of the sensor is not referenced to ground but
is biased above ground by an internal diode at the D− input.
To measure ΔV , the sensor is switched between operating
BE
currents of I and N × I. The resulting waveform is passed
through a 65 kHz low-pass filter to remove noise and to a
chopper stabilized amplifier that performs the functions of
amplification and rectification of the waveform to produce a dc
voltage proportional to ΔV
BE
ADC to give a temperature output in 10-bit, twos complement
format. To further reduce the effects of noise, digital filtering is
performed by averaging the results of 16 measurement cycles. A
remote temperature measurement takes nominally 25.5 ms. The
results of remote temperature measurements are stored in
10-bit, twos complement format, as illustrated in Table 12. The
extra resolution for the temperature measurements is held in
the Extended Resolution Register 2 (Reg. 0x77). This gives
temperature readings with a resolution of 0.25°C.
2N3904
NPN
Figure 26. Measuring Temperature by Using an NPN Transistor
2N3906
PNP
Figure 27. Measuring Temperature by Using a PNP Transistor
. This voltage is measured by the
ADT7460
D+
D–
ADT7460
D+
D–
Table 12. Temperature Data Format
Temperature
−128°C
−125°C
−100°C
−75°C
−50°C
−25°C
−10°C
0°C
+10.25°C
+25.5°C
+50.75°C
+75°C
+100°C
+125°C
+127°C
1 Bold denotes 2 LSBs of measurement in the Extended Resolution Register 2
(Reg. 0x77) with 0.25°C resolution.
Table 13. Temperature Measurement Registers
Register
0x25
0x26
0x27
0x77
Table 14. Extended Resolution Temperature Measurement
Register Bits (Addr = 0x77)
Bit Mnemonic
<7:6> TDM2
<5:4> LTMP
<3:2> TDM1
Reading Temperature from the ADT7460
It is important to note that temperature can be read from the
ADT7460 as an 8-bit value (with 1°C resolution) or as a 10-bit
value (with 0.25 C resolution). If only 1°C resolution is required,
the temperature readings can be read back at any time and in no
particular order.
If the 10-bit measurement is required, this involves a 2-register
read for each measurement. The extended resolution register
(Reg. 0x77) should be read first. This causes all temperature
reading registers to be frozen until all temperature reading
registers have been read from. This prevents an MSB reading
from being updated while its two LSBs are being read, and vice
versa.
Rev. C | Page 18 of 52
Digital Output (10-Bit)
1000 0000 00
1000 0011 00
1001 1100 00
1011 0101 00
1100 1110 00
1110 0111 00
1111 0110 00
0000 0000 00
0000 1010 01
0001 1001 10
0011 0010 11
0100 1011 00
0110 0100 00
0111 1101 00
0111 1111 00
Description
Remote 1 temperature
Local temperature
Remote 2 temperature
Extended Resolution 2
Description
Remote 2 temperature LSBs
Local temperature LSBs
Remote 1 temperature LSBs
1
Default
0x80
0x80
0x80
0x00