Temperature Measurement Method - Analog Devices dBCool ADT7467 Manual

Table 5. 10-Bit A/D Output Code vs. V
V (+5 V ) V
CC IN
<0.0065 <0.0042
0.0065–0.0130 0.0042–0.0085
0.0130–0.0195 0.0085–0.0128
0.0195–0.0260 0.0128–0.0171
0.0260–0.0325 0.0171–0.0214
0.0325–0.0390 0.0214–0.0257
0.0390–0.0455 0.0257–0.0300
0.0455–0.0521 0.0300–0.0343
0.0521–0.0586 0.0343–0.0386
1.6675–1.6740 1.100–1.1042
3.330–3.3415 2.200–2.2042
5.0025–5.0090 3.300–3.3042
6.5983–6.6048 4.3527–4.3570
6.6048–6.6113 4.3570–4.3613
6.6113–6.6178 4.3613–4.3656
6.6178–6.6244 4.3656–4.3699
6.6244–6.6309 4.3699–4.3742
6.6309–6.6374 4.3742–4.3785
6.6374–6.4390 4.3785–4.3828
6.6439–6.6504 4.3828–4.3871
6.6504–6.6569 4.3871–4.3914
6.6569–6.6634 4.3914–4.3957
>6.6634 >4.3957

TEMPERATURE MEASUREMENT METHOD

A simple method of measuring temperature is to exploit the
negative temperature coefficient of a diode, measuring the base-
emitter voltage (V ) of a transistor, operated at constant
BE
current. Unfortunately, this technique requires calibration to
null out the effect of the absolute value of V
from device to device.
The technique used in the ADT7467 is to measure the change in
V when the device is operated at three different currents.
BE
Previous devices have used only two operating currents, but the
use of a third current allows automatic cancellation of
resistances in series with the external temperature sensor.
IN
Input Voltage
(3.3 V ) V
CC IN CCP
<0.00293
0.0293–0.0058
0.0058–0.0087
0.0087–0.0117
0.0117–0.0146
0.0146–0.0175
0.0175–0.0205
0.0205–0.0234
0.0234–0.0263
0.7500–0.7529
1.5000–1.5029
2.2500–2.2529
2.9677–2.9707
2.9707–2.9736
2.9736–2.9765
2.9765–2.9794
2.9794–2.9824
2.9824–2.9853
2.9853–2.9882
2.9882–2.9912
2.9912–2.9941
2.9941–2.9970
>2.9970
, which varies
BE
Decimal
0
1
2
3
4
5
6
7
8
•
•
•
256 (1/4-scale)
•
•
•
512 (1/2-scale)
•
•
•
768 (3/4 scale)
•
•
•
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
Figure 24 shows the input signal conditioning used to measure
the output of an external temperature sensor. This figure shows
the external sensor as a substrate transistor, but it could equally
be a discrete transistor. If a discrete transistor is used, the
collector is not grounded and should be linked to the base. 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. C1 can optionally be added as a noise filter
(recommended maximum value 1000 pF). However, a better
option in noisy environments is to add a filter, as described in
the Noise Filtering section.
Rev. 0| Page 15 of 80
ADT7467
A/D Output
Binary (10 Bits)
00000000 00
00000000 01
00000000 10
00000000 11
00000001 00
00000001 01
00000001 10
00000001 11
00000010 00
01000000 00
10000000 00
11000000 00
11111101 01
11111101 10
11111101 11
11111110 00
11111110 01
11111110 10
11111110 11
11111111 00
11111111 01
11111111 10
11111111 11