Temperature Measurement System; Remote Temperature Measurement - Analog Devices ADM1063 Manual

TEMPERATURE MEASUREMENT SYSTEM

The ADM1063 contains an on-chip, band gap temperature
sensor, whose output is digitized by the on-chip, 12-bit ADC.
Theoretically, the temperature sensor and ADC can measure
temperatures from −128°C to +127°C with a resolution of
0.125°C. Because this exceeds the operating temperature range
of the device, local temperature measurements outside this
range are not possible. Temperature measurements from
−128°C to +127°C are possible using a remote sensor. The
output code is in offset binary format, with −128°C given by
Code 0x400, 0°C given by Code 0x800, and +127°C given by
Code 0xC00.
As with the other analog inputs to the ADC, a limit register is
provided for each of the temperature input channels. Therefore,
a temperature limit can be set such that if it is exceeded, a warning
is generated and available as an input to the sequencing engine.
This enables users to control their sequence or monitor functions
based on an overtemperature or undertemperature event.

REMOTE TEMPERATURE MEASUREMENT

The ADM1063 can measure the temperature of two remote
diode sensors or diode-connected transistors connected to the
DxN and DxP pins.
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 varies from device to device, and individual calibration
be
is required to null this, making the technique unsuitable for
mass production. The technique used in the ADM1063 is to
measure the change in V when the device is operated at two
be
different currents.
This is given by
∆ V = kT / q × ln ( N )
be
where:
k is Boltzmann's constant.
q is charge on the carrier.
T is absolute temperature in Kelvin.
N is ratio of the two currents.
Figure 34 shows the input signal conditioning used to measure the
output of a remote temperature sensor. This figure shows the
external sensor as a substrate transistor provided for temperature
monitoring on some microprocessors, but it could equally be a
discrete transistor such as a 2N3904 or 2N3906.
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 DxN input and the emitter is connected to the
DxP input. If an NPN transistor is used, the emitter is connected
to the DxN input and the base is connected to the DxP input.
Figure 35 and Figure 36 show how to connect the ADM1063
to an NPN or PNP transistor for 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
DxN input.
To measure ∆V
currents of I and N × I. The resulting waveform is passed
through a 65 kHz low-pass filter to remove noise and through
a chopper-stabilized amplifier that amplifies and rectifies the
waveform to produce a dc voltage proportional to ∆V
voltage is measured by the ADC to produce a temperature
output in 12-bit offset binary. To further reduce the effects of
noise, digital filtering is performed by averaging the results of
16 measurement cycles. A remote temperature measurement
takes nominally 600 ms. The results of remote temperature
measurements are stored in 12-bit, offset binary format, as
shown in Table 10. This provides temperature readings
with a resolution of 0.125°C.
Table 10. Temperature Data Format
Temperature
−128 °C
−125 °C
−100 °C
−75 °C
−50 °C
−25 °C
−10 °C
0 °C
+10.25 °C
+25.5 °C
+50.75 °C
+75 °C
+100 °C
+125 °C
+128 °C
Rev. 0 | Page 23 of 36
, the sensor is switched between operating
be
Digital Output (Hex) Digital Output (Bin)
400 010000000000
418 010000011000
4E0 010011100000
5A8 010110101000
600 011000000000
670 011001110000
7B0 011110110000
800 100000000000
852 100001010010
8CC 100011001100
996 100110010110
A58 101001011000
B48 101101001000
BE8 101111101000
C00 110000000000
ADM1063
. This
be