MAX6694
5-Channel Precision Temperature Monitor
with Beta Compensation
can be violated. The forward voltage at the highest
expected temperature must be greater than 0.25V at
10µA, and at the lowest expected temperature, the for-
ward voltage must be less than 0.95V at 100µA. Large
power transistors must not be used. Also, ensure that
the base resistance is less than 100Ω. Tight specifica-
tions for forward current gain (50 < ß < 150, for exam-
ple) indicate that the manufacturer has good process
controls and that the devices have consistent V
BE
char-
acteristics. Manufacturers of discrete transistors do not
normally specify or guarantee ideality factor. This is
normally not a problem since good-quality discrete
transistors tend to have ideality factors that fall within a
relatively narrow range. We have observed variations in
remote temperature readings of less than ±2°C with a
variety of discrete transistors. Still, it is good design
practice to verify good consistency of temperature
readings with several discrete transistors from any
manufacturer under consideration.
Unused Diode Channels
If one or more of the remote diode channels is not
needed, disconnect the DXP and DXN inputs for that
channel, or connect the DXP input to V
CC
. The status
register indicates a diode "fault" for this channel and the
channel is ignored during the temperature-measure-
ment sequence. It is also good practice to mask any
unused channels immediately upon power-up by set-
ting the appropriate bits in the Configuration 2 and
Configuration 3 registers. This will prevent unused
channels from causing ALERT or OVERT to assert.
Thermal Mass and Self-Heating
When sensing local temperature, the MAX6694 mea-
sures the temperature of the PCB to which it is sol-
dered. The leads provide a good thermal path between
the PCB traces and the die. As with all IC temperature
sensors, thermal conductivity between the die and the
ambient air is poor by comparison, making air tempera-
ture measurements impractical. Because the thermal
mass of the PCB is far greater than that of the
MAX6694, the device follows temperature changes on
the PCB with little or no perceivable delay. When mea-
suring the temperature of a CPU or other IC with an on-
chip sense junction, thermal mass has virtually no
effect; the measured temperature of the junction tracks
the actual temperature within a conversion cycle.
When measuring temperature with discrete remote
transistors, the best thermal response times are
obtained with transistors in small packages (i.e., SOT23
or SC70). 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.
ADC Noise Filtering
The integrating ADC has good noise rejection for low-
frequency signals, such as power-supply hum. In envi-
ronments with significant high-frequency EMI, connect
an external 100pF capacitor between DXP_ and DXN_.
Larger capacitor values can be used for added filter-
ing, but do not exceed 100pF because it can introduce
errors due to the rise time of the switched current
source. High-frequency noise reduction is needed for
high-accuracy remote measurements. Noise can be
reduced with careful PCB layout as discussed in the
PCB Layout
section.
Slave Address
The slave address for the MAX6694 is shown in Table 11.
PCB Layout
Follow these guidelines to reduce the measurement
error when measuring remote temperature:
1) Place the MAX6694 as close as is practical to the
remote diode. In noisy environments, such as a com-
puter motherboard, this distance can be 4in to 8in
(typ). This length can be increased if the worst noise
sources are avoided. Noise sources include CRTs,
clock generators, memory buses, and PCI buses.
2) Do not route the DXP-DXN lines next to the deflec-
tion coils of a CRT. Also, do not route the traces
across fast digital signals, which can easily intro-
duce +30°C error, even with good filtering.
