ADM1021A
Rev. 7 | Page 9 of 19 | www.onsemi.com
The technique used in the ADM1021A is to measure the change
in V
BE
when the device is operated at two different currents.
This is given by
()
qKT
BE
ln/ ×=
where:
K is Boltzmann’s constant.
q is the charge on the electron (1.6 × 10
–19
coulombs).
T is the absolute temperature, in kelvins.
N is the 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 tem-
perature monitoring on some microprocessors, but it could be
a discrete transistor. If a discrete transistor is used, the collector
will not be grounded and should be linked to the base. To
prevent ground noise 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. If the sensor is operating in a noisy environment,
one can optionally be added as a noise filter. Its value is typically
2200 pF, but it should be no more than 3000 pF. See the Layout
Considerations section for more information on C1.
To measure ΔV
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, and then to
a chopper-stabilized amplifier that performs the functions of
amplification and rectification of the waveform to produce
a dc voltage proportional to ΔV
BE
. This voltage is measured by
the ADC to give a temperature output in 8-bit, twos comple-
ment format. To reduce the effects of noise further, digital
filtering is performed by averaging the results of 16 measure-
ment cycles.
Signal conditioning and measurement of the internal
temperature sensor is performed in a similar manner.
DIFFERENCES BETWEEN THE ADM1021 AND THE
ADM1021A
Although the ADM1021A is pin-for-pin compatible with the
ADM1021, there are some differences between the two devices.
Below is a summary of these differences and reasons for the
changes.
1.
The ADM1021A forces a larger current through the remote
temperature sensing diode, typically 205 μA vs. 90 μA for
the ADM1021. The primary reason for this is to improve
the noise immunity of the part.
2.
As a result of the greater remote sensor source current, the
operating current of the ADM1021A is higher than that of
the ADM1021, typically 205 μA vs. 160 μA.
3.
The temperature measurement range of the ADM1021A is
0°C to 127°C, compared with −128°C to +127°C for the
ADM1021. As a result, the ADM1021 should be used if
negative temperature measurement is required.
4.
The power-on reset values of the remote and local
temperature values are −128°C in the ADM1021A as
compared to 0°C in the ADM1021. As the part is powered
up converting (except when the part is in standby mode,
that is, Pin 15 is pulled low), the part measures the actual
values of remote and local temperature and writes these to
the registers.
5.
The four MSBs of the revision register can be used to
identify the part. The ADM1021 revision register reads
0x0x, and the ADM1021A reads 0x3x.
6.
The power-on default value of the address pointer register
is undefined in the ADM1021A and is equal to 0x00 in
the ADM1021. As a result, a value must be written to the
address pointer register before a read is performed in the
ADM1021A. The ADM1021 is capable of reading back
local temperature without writing to the address pointer
register, as it defaulted to the local temperature
measurement register at power-up.
7.
Setting the mask bit (Bit 7 Config Reg) on the
ADM1021A masks current and future ALERTs. On the
ADM1021, the mask bit, masks only ALERTs. Any current
ALERT has to be cleared using an ARA.
TEMPERATURE DATA FORMAT
One LSB of the ADC corresponds to 1°C so the ADC can
theoretically measure from −128°C to +127°C, although the
device does not measure temperatures below 0 C; therefore, the
actual range is 0°C to 127°C. The temperature data format is
shown in Table 4.
The results of the local and remote temperature measurements
are stored in the local and remote temperature value registers
and are compared with limits programmed into the local and
remote high and low limit registers.
Table 4. Temperature Data Format
Temperature Digital Output
0°C 0 000 0000
1°C 0 000 0001
10°C 0 000 1010
25°C 0 001 1001
50°C 0 011 0010
75°C 0 100 1011
100°C 0 110 0100
125°C 0 111 1101
127°C 0 111 1111
