MAX1668_00 Datasheet, PDF (8/16 Pages) Maxim Integrated Products – Multichannel Remote/Local Temperature Sensor

Multichannel Remote/Local
Temperature Sensor
A/D Conversion Sequence
If a Start command is written (or generated automatical-
ly in the free-running autoconvert mode), all channels
are converted, and the results of all measurements are
available after the end of conversion. A BUSY status bit
in the status byte shows that the device is actually per-
forming a new conversion; however, even if the ADC is
busy, the results of the previous conversion are always
available.
Remote-Diode Selection
Temperature accuracy depends on having a good-qual-
ity, diode-connected small-signal transistor. Accuracy
has been experimentally verified for all of the devices
listed in Table 1. The MAX1668/MAX1805 can also
directly measure the die temperature of CPUs and other
ICs having on-board temperature-sensing diodes.
The transistor must be a small-signal type, either NPN
or PNP, with a relatively high forward voltage; other-
wise, the A/D input voltage range can be violated. The
forward voltage must be greater than 0.25V at 10µA;
check to ensure this is true at the highest expected
temperature. The forward voltage must be less than
0.95V at 100µA; check to ensure this is true at the low-
est expected temperature. Large power transistors
don’t work at all. Also, ensure that the base resistance
is less than 100Ω. Tight specifications for forward-cur-
rent gain (+50 to +150, for example) indicate that the
manufacturer has good process controls and that the
devices have consistent VBE characteristics.
For heat-sink mounting, the 500-32BT02-000 thermal
sensor from Fenwal Electronics is a good choice. This
device consists of a diode-connected transistor, an
aluminum plate with screw hole, and twisted-pair cable
(Fenwal Inc., Milford, MA, 508-478-6000).
Thermal Mass and Self-Heating
Thermal mass can seriously degrade the MAX1668/
MAX1805’s effective accuracy. The thermal time con-
stant of the QSOP-16 package is about 140s in still air.
For the MAX1668/MAX1805 junction temperature to set-
tle to within +1°C after a sudden +100°C change
requires about five time constants or 12 minutes. The
use of smaller packages for remote sensors, such as
SOT23s, improves the situation. Take care to account
for thermal gradients between the heat source and the
sensor, and ensure that stray air currents across the
sensor package do not interfere with measurement
accuracy.
Self-heating does not significantly affect measurement
accuracy. Remote-sensor self-heating due to the diode
current source is negligible. For the local diode, the
Table 1. Remote-Sensor Transistor
Manufacturers
MANUFACTURER
MODEL NUMBER
Central Semiconductor (USA)
Motorola (USA)
National Semiconductor (USA)
Rohm Semiconductor (Japan)
Samsung (Korea)
Siemens (Germany)
Zetex (England)
CMPT3904
MMBT3904
MMBT3904
SST3904
KST3904-TF
SMBT3904
FMMT3904CT-ND
Note: Transistors must be diode connected (base shorted to
collector).
worst-case error occurs when sinking maximum current
at the ALERT output. For example, with ALERT sinking
1mA, the typical power dissipation is V_CC x 400µA plus
0.4V x 1mA. Package theta J-A is about 150°C/W, so
with V_CC = 5V and no copper PC board heat sinking,
the resulting temperature rise is:
dT = 2.4mW x 150°C/W = 0.36°C
Even with these contrived circumstances, it is difficult
to introduce significant self-heating errors.
ADC Noise Filtering
The ADC is an integrating type with inherently good
noise rejection, especially of low-frequency signals
such as 60Hz/120Hz power-supply hum. Micropower
operation places constraints on high-frequency noise
rejection; therefore, careful PC board layout and proper
external noise filtering are required for high-accuracy
remote measurements in electrically noisy environ-
ments.
High-frequency EMI is best filtered at DXP_ and DXN_
with an external 2200pF capacitor. This value can be
increased to about 3300pF (max), including cable
capacitance. Higher capacitance than 3300pF intro-
duces errors due to the rise time of the switched cur-
rent source.
Nearly all noise sources tested cause additional error
measurements, typically by +1°C to +10°C, depending
on the frequency and amplitude (see Typical Operating
Characteristics).
PC Board Layout
1) Place the MAX1668/MAX1805 as close as practical
to the remote diode. In a noisy environment, such
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