MAX6682
NTC thermistors are often described by the resistance
at +25°C. Therefore, a 10kΩ thermistor has a resistance
of 10kΩ at +25°C. When choosing a thermistor, ensure
that the thermistor’s minimum resistance (which occurs
at the maximum expected operating temperature) in
series with R
EXT
does not cause the voltage reference
output current to exceed about 1mA. Some standard
10kΩ thermistors with similar characteristics are listed
in Table 2. When used with one of these thermistors
and the recommended series resistor, the MAX6682
provides output data scaled in °C over the +10°C to
+40°C temperature range.
Choosing the External Resistor
Choose R
EXT
to minimize nonlinearity errors from the
thermistor:
1) Decide on the temperature range of interest (for
example 0°C to +70°C).
2) Find the thermistor values at the limits of the tem-
perature range. R
MIN
is the minimum thermistor
value (at the maximum temperature) and R
MAX
is
the maximum thermistor value (at the minimum tem-
perature). Also find R
MID
, the thermistor resistance
in the middle of the temperature range (+35°C for
the 0°C to +70°C range).
3) Find R
EXT
using the equation below:
Table 3 shows nominal output data for several tempera-
tures when R
EXT
has been chosen according to the
equation above for a temperature range of 0°C to
+70°C. The output data is not conveniently scaled to
the actual temperature over this range, but the linearity
is better than 2.4°C over the 0°C to +70°C range
(Figure 2). The temperature weighting over this range is
0.14925°C/LSB.
Serial Interface
The Typical Application Circuit shows the MAX6682
interfaced with a microcontroller. In this example, the
MAX6682 processes the reading from R
EXT
and trans-
mits the data through an SPI-compatible interface.
Force CS low and apply a clock signal at SCK to read
the results at SO. Forcing CS low immediately stops
any conversion in process. Initiate a new conversion by
forcing CS high.
Force CS low to output the first bit on the SO pin. A
complete read requires 11 clock cycles. Read the 11
output bits on the rising edge of the clock, if the first bit
D10 is the sign bit. Bits D10–D0 contain the converted
temperature in the order of MSB to LSB.
After the 11th clock cycle, SO goes to a high-imped-
ance state. SO remains high impedance until CS is
pulsed high and brought back low. Figure 4 is the SO
output.
Power-Supply Considerations
The MAX6682 accuracy is relatively unaffected by
power-supply coupled noise. In most applications,
bypass V
CC
to GND by placing a 0.1µF ceramic
bypass capacitor close to the supply pin of the
devices.
Thermal Considerations
Self-heating degrades the temperature measurement
accuracy of thermistors. The amount of self-heating
depends on the power dissipated in the thermistor and
the dissipation constant of the thermistor. Dissipation
constants depend on the thermistor’s package and can
vary considerably.
A typical thermistor might have a dissipation constant
equal to 1mW/°C. For every mW the thermistor dissi-
pates, its temperature rises by 1°C. For example, con-
vs. TEMPERATURE
