REV. A
ADM1027
15
If a discrete transistor is used, the collector will not be 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. Figure 14 shows
how to connect the ADM1027 to an NPN or PNP transistor for
temperature measurement. To prevent ground noise from inter-
fering 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 DV
be
, the sensor is switched between operating cur-
rents 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 recti-
fication of the waveform to produce a dc voltage proportional to
DV
be
. This voltage is measured by the 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 measure-
ment takes nominally 25.5 ms. The results of remote temperature
measurements are stored in 10-bit, twos complement format,
as illustrated in Table III. 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
o
C.
Table III. Temperature Data Format*
Temperature
Digital Output (10-Bit)
128rC
1000 0000 00
125rC
1000 0011 00
100rC
1001 1100 00
75rC
1011 0101 00
50rC
1100 1110 00
25rC
1110 0111 00
10
o
C
1111 0110 00
0rC
0000 0000 00
+10.25rC
0000 1010 01
+25.5rC
0001 1001 10
+50.75rC
0011 0010 11
+75rC
0100 1011 00
+100rC
0110 0100 00
+125rC
0111 1101 00
+127rC
0111 1111 00
*Bold denotes 2 LSBs of measurement in Extended Resolution
Register 2 (Reg. 0x77) with 0.25
o
C resolution.
ADM1027
2N3904
NPN
D+
D
Figure 14a. Measuring Temperature Using an
NPN Transistor
2N3906
PNP
ADM1027
D+
D
Figure 14b. Measuring Temperature Using a
PNP Transistor
NULLING OUT TEMPERATURE ERRORS
As CPUs run faster, it is getting more difficult to avoid high
frequency clocks when routing the D/D+ traces around a system
board. Even when recommended layout guidelines are followed,
there may still be temperature errors attributed to noise being
coupled onto the D+/D lines. High frequency noise generally
has the effect of giving temperature measurements that are too
high by a constant amount. The ADM1027 has temperature
offset registers at addresses 0x70, 0x71, and 0x72 for the Remote 1,
Local, and Remote 2 temperature channels. By doing a one-time
calibration of the system, you can determine the offset caused
by system board noise and null it out using the offset registers.
The offset registers automatically add a twos complement 8-bit
reading to every temperature measurement. The LSB adds a 1rC
offset to the temperature reading so the 8-bit register effectively
allows temperature offsets of up to 127rC with a resolution of
1rC. This ensures that the readings in the temperature measure-
ment registers are as accurate as possible.
TEMPERATURE OFFSET REGISTERS
Reg. 0x70 Remote 1 Temperature Offset = 0x00 (0rC default)
Reg. 0x71 Local Temperature Offset = 0x00 (0rC default)
Reg. 0x72 Remote 2 Temperature Offset = 0x00 (0rC default)
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