A/D Conversion Sequence
If a start command is written (or generated automatically
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/MAX1989 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 do
not work at all. Also, ensure that the base resistance is
less than 100Ω. Tight specifications for forward-current
gain (+50 to +150, for example) indicate that the manu-
facturer 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/MAX1989s’ effective accuracy. The thermal
time constant of the 16-pin QSOP package is about
140s in still air. For the MAX1668/MAX1805/MAX1989
junction temperature to settle to within +1°C after a
sudden +100°C change requires about five time con-
stants or 12 minutes. The use of smaller packages for
remote sensors, such as SOT23s, improves the situa-
tion. 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
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 opera-
tion places constraints on high-frequency noise rejec-
tion; therefore, careful PC board layout and proper
external noise filtering are required for high-accuracy
remote measurements in electrically noisy environments.
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 the Typical
Operating Characteristics).
PC Board Layout
1) Place the MAX1668/MAX1805/MAX1989 as close as
practical to the remote diode. In a noisy environment,
such as a computer motherboard, this distance can
MAX1668/MAX1805/MAX1989
†
Multichannel Remote/Local
Temperature Sensors
8 _______________________________________________________________________________________
CMPT3904Central Semiconductor (USA)
MMBT3904Motorola (USA)
MMBT3904
SST3904Rohm Semiconductor (Japan)
KST3904-TFSamsung (Korea)
FMMT3904CT-NDZetex (England)
MANUFACTURER MODEL NO.
SMBT3904Siemens (Germany)
Table 1. Remote-Sensor Transistor
Manufacturers
Note: Transistors must be diode connected (base shorted to
collector).
National Semiconductor (USA)
