ADM1024
http://onsemi.com
13
(R2 has no effect as the input voltage at the device pin is
2.5 V when VIN = plus full scale).
R1
R3 +
Vf
s)
* 2.5
2.5
(eq. 4)
Offset voltages other than 2.5 V can be used, but the
calculation becomes more complicated.
Temperature Measurement System
Local Temperature Measurement
The ADM1024 contains an on-chip band gap temperature
sensor, whose output is digitized by the on-chip ADC. The
temperature data is stored in the Temperature Value Register
(address 27h) and the LSB from Bits 6 and 7 of the
Temperature Configuration Register (address 4Bh). As both
positive and negative temperatures can be measured, the
temperature data is stored in twos complement format, as
shown in Table
8. Theoretically, the temperature sensor and
ADC can measure temperatures from 128C to +127C
with a resolution of 1C, although temperatures below
40C and above +125C are outside the operating
temperature range of the device.
External Temperature Measurement
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 setting 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 VBE 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 VBE when the device is operated at two different
currents.
This is given by:
(eq. 5)
DVbe + KT q ln(N)
where:
K is Boltzmann’s constant.
q is the charge on the carrier.
T is the absolute temperature in Kelvins.
N is the ratio of the two currents.
Figure
17 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.
I
D+
LPF
N y I
IBIAS
VDD
BIAS
DIODE
D–
LOWPASS
FILTER
fC = 65kHz
VOUT+
VOUT–
TO
ADC
REMOTE
SENSING
TRANSISTOR
Figure 17. 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.
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
Layout Considerations section for more information on C1.
To measure
DVBE, the sensor is switched between
operating currents of I and N I. The resulting waveform is
passed through a 65 kHz lowpass filter to remove noise,
then to a chopperstabilized amplifier that performs the
functions of amplification and rectification of the waveform
to produce a dc voltage proportional to
DVBE. 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