ADM1024
Register (address 4Bh). As both positive and negative tempera-
tures can be measured, the temperature data is stored in twos
complement format, as shown in Table IV. Theoretically, the
temperature sensor and ADC can measure temperatures from
–128°C to +127°C with a resolution of 1°C, although tempera-
tures below –40°C and above +125°C are outside the operating
temperature range of the device.
External 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. As the sensor is operating in a noisy environment, C1 is
provided as a noise filter. See the section on layout considerations
for more information on C1.
To measure
∆V
BE
, the sensor is switched between operating
currents of I and N
×
I. The resulting waveform is passed through
a 65 kHz low-pass filter to remove noise, thence to a chopper-
stabilized amplifier that performs the functions of amplification
and rectification of the waveform to produce a dc voltage pro-
portional to
∆V
BE
. This voltage is measured by the ADC to give
a temperature output in 8-bit twos complement format. To
further reduce the effects of noise. Digital filtering is performed
by averaging the results of 16 measurement cycles. An external
temperature measurement takes nominally 9.6 ms.
The results of external temperature measurements are stored in
8-bit, twos-complement format, as illustrated in Table IV.
Table IV. Temperature Data Format
The ADM1024 can measure the temperature of two external
diode sensors or diode-connected transistors, connected to Pins
13 and 14 or 17 and 18.
Pins 13 and 14 are a dedicated temperature input channel. Pins
17 and 18 can be configured to measure a diode sensor by set-
ting Bit 2 of the Channel Mode Register to 1.
The forward voltage of a diode or diode-connected transistor,
operated at a constant current, exhibits a negative temperature
coefficient of about –2 mV/°C. Unfortunately, the absolute value of
V
BE
, varies from device to device, and individual calibration is
required to null this out, so the technique is unsuitable for mass-
production.
The technique used in the ADM1024 is to measure the change
in V
BE
when the device is operated at two different currents.
This is given by:
∆V
BE
=
KT/q
×
ln(N)
where:
K
is Boltzmann’s constant
q
is charge on the carrier
T
is absolute temperature in Kelvins
N
is ratio of the two currents.
Figure 14 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, provided for temperature
monitoring on some microprocessors, but it could equally well
be a discrete transistor.
V
DD
I
N
I
I
BIAS
LOW-PASS
FILTER
f
C
= 65kHz
D+
REMOTE
SENSING
TRANSISTOR
V
OUT+
TO
ADC
D–
BIAS
DIODE
V
OUT–
Temperature
–128°C
–125°C
–100°C
–75°C
–50°C
–25°C
0°C
+0.5°C
+10°C
+25°C
+50°C
+75°C
+100°C
+125°C
+127°C
LAYOUT CONSIDERATIONS
Digital Output
1000 0000
1000 0011
1001 1100
1011 0101
1100 1110
1110 0111
0000 0000
0000 0000
0000 1010
0001 1001
0011 0010
0100 1011
0110 0100
0111 1101
0111 1111
Digital boards can be electrically noisy environments, and care
must be taken to protect the analog inputs from noise, particularly
when measuring the very small voltages from a remote diode
sensor. The following precautions should be taken:
1. Place the ADM1024 as close as possible to the remote sensing
diode. Provided that the worst noise sources such as clock
generators, data/address buses and CRTs are avoided, this
distance can be 4 to 8 inches.
2. Route the D+ and D– tracks close together, in parallel, with
grounded guard tracks on each side. Provide a ground plane
under the tracks if possible.
3. Use wide tracks to minimize inductance and reduce noise
pickup. Ten mil track minimum width and spacing is
recommended.
Figure 14. Signal Conditioning for External Diode
Temperature Sensors
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.
–12–
REV. 0
AD [ ANALOG DEVICES ]
