MAX6698
7-Channel Precision Remote-Diode, Thermistor,
and Local Temperature Monitor
16 ______________________________________________________________________________________
VREF supplies a reference voltage (1V nominal) to bias
the thermistor/REXT_ voltage-divider. The voltage
across REXT is measured by the MAX6698’s ADC,
resulting in a voltage that is directly proportional to tem-
perature. The data in the thermistor registers gives the
voltage across REXT as a fraction of the reference volt-
age (1LSB = 0.5% of VREF).
Because thermistors have nonlinear temperature-resis-
tance functions, and because different thermistors have
different functions, it is important to understand the
relationship between temperature, REXT, and the volt-
age across REXT for a given thermistor. Table 13
shows temperature vs. the thermistor channel data for a
Betatherm 10k3A1 thermistor and REXT=1600Ω.
Thermal Mass and Self-Heating
When sensing local temperature, the MAX6698 mea-
sures the temperature of the printed-circuit board
(PCB) to which it is soldered. 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 compar-
ison, making air temperature measurements impracti-
cal. Because the thermal mass of the PCB is far greater
than that of the MAX6698, the device follows tempera-
ture changes on the PCB with little or no perceivable
delay. When measuring 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 conver-
sion cycle.
When measuring temperature with discrete remote tran-
sistors, 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 2200pF capacitor between DXP_ and
DXN_. Larger capacitor values can be used for added
filtering, but do not exceed 3300pF 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 dis-
cussed in the PCB Layout section.
PCB Layout
Follow these guidelines to reduce the measurement
error when measuring remote temperature:
1) Place the MAX6698 as close as is practical to the
remote diode. In noisy environments, such as a
computer 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.
