ADM1034
http://onsemi.com
14
Temperature Measurement System
Internal Temperature Measurement
The ADM1034 contains an on-chip band gap temperature
sensor. The on-chip ADC performs conversions on the
sensors output, outputting the data in 13-bit format. The
resolution of the local temperature sensor is 0.03125癈.
Table 8 shows the format of the temperature data MSBs.
Table 9 shows the same for the LSBs. To ensure accurate
readings, read the LSBs first. This locks the current LSBs
and MSBs until the MSBs are read. They then start to update
again. (Reading only the MSBs does not lock the registers.)
Temperature updates to the look-up table take place in
parallel; so fan speeds may be updated even if the MSBs are
locked.
Table 8. TEMPERATURE DATA FORMAT
(LOCAL TEMPERATURE AND REMOTE
TEMPERATURE HIGH BYTES)
Temperature (5C)
Digital Output
64癈
0000 0000
40癈
0001 1000
32癈
0010 0000
2癈
0011 1110
1癈
0011 1111
0癈
0100 0000
1癈
0100 0001
2癈
0100 0010
10癈
0100 1010
20癈
0101 0100
50癈
0111 0010
75癈
1000 1011
100癈
1010 0100
125癈
1011 1101
150癈
1101 0110
191癈
1111 1111
Table 9. LOCAL AND REMOTE SENSOR EXTENDED
RESOLUTION
Extended Resolution (5C)
Temperature Low Bits
0.0000
00000
0.03125
00001
0.0625
00010
0.125
00100
0.250
01000
0.375
01100
0.500
10000
0.625
10100
0.750
11000
0.875
11100
Temperature (癈) = (MSB 64癈) + (LSB x 0.03125)
Example: MSB = 0101 0100 = 84d
LSB = 11100 = 28
Temperature 癈 = (84 64) + (28 x 0.03125) = 20.875
Remote Temperature Measurement
The ADM1034 can measure the temperature of two
external diode sensors or diode-connected transistors, which
are connected to Pins 9 and 10 and Pins 11 and 12. These pins
are dedicated temperature input channels. The series
resistance cancellation (SRC) feature can automatically
cancel out the effect of up to 1 kW of resistance in series with
the remote thermal diode.
The forward voltage of a diode or diode-connected
transistor, operated at a constant current, exhibits a negative
temperature coefficient of about 2 mV/癈. Unfortunately,
the absolute value of V
BE
varies from device to device, and
individual calibration is required to null this out. Therefore,
the technique is unsuitable for mass production.
Figure 26. Measuring Temperature by Using Discreet
Transistors
ADM1034
ADM1034
D+
D
D+
D
2N3904
2N3906
The ADM1034 operates at three different currents to
measure the change in V
BE
. Figure 27 shows the input signal
conditioning used to measure the output of an external
temperature sensor. It also shows the external sensor as a
substrate transistor, provided for temperature monitoring on
some microprocessors. The external sensor could work
equally well as a discrete transistor.
If a discrete transistor is used, the collector is not grounded,
and 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.
If the sensor is used in a very noisy environment, a
capacitor value up to 1000 pF may be placed between the D+
and D inputs to filter the noise. However, additional
parasitic capacitance on the lines between D+, D, and the
thermal diode should also be considered. The total
capacitance should never be greater than 1000 pF.
To measure each DV
BE
, the sensor is switched between
operating currents of I, (N1 ?I), and (N2 ?I). The resulting
waveform is passed through a 65 kHz low-pass filter to
remove noise, then to a chopper-stabilized amplifier that
amplifies and rectifies the waveform. This produces a dc
voltage proportional to DV
BE
. These voltage measurements
determine the temperature of the thermal diode, while
automatically compensating for any series resistance on the
D+ and/or D lines. The temperature is stored in two
registers as a 13-bit word.