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MAX6682MUA-MAX6682MUA+-MAX6682MUA+T
Thermistor-to-Digital Converter
General DescriptionThe MAX6682 converts an external thermistor’s temper-
ature-dependent resistance directly into digital form.
The thermistor and an external fixed resistor form a volt-
age-divider that is driven by the MAX6682’s internal
voltage reference. The MAX6682 measures the voltage
across the external resistor and produces a 10-bit +
sign output code dependent on that voltage.
The MAX6682 does not linearize the highly nonlinear
transfer function of a typical negative temperature coef-
ficient (NTC) thermistor, but it does provide linear out-
put data over limited temperature ranges when used
with an external resistor of the correct value. Over theto +50°C temperature range, the MAX6682 produces
output data that is scaled to 8LSBs/°C (for 0.125°C res-
olution), provided that the correct thermistor and exter-
nal resistor values are used. Other temperature ranges
can be easily accommodated, but do not necessarily
yield data scaled to an even number of LSBs per
degree.
The 3-wire SPI™-compatible interface can be readily
connected to a variety of microcontrollers.
The MAX6682 is a read-only device, simplifying use in
systems where only temperature data is required.
Power-management circuitry reduces the average ther-
mistor current, minimizing self-heating. Between con-
versions, supply current is reduced to 21µA (typ). The
internal voltage reference is shut down between mea-
surements.
The MAX6682 is available in a small, 8-pin µMAX pack-
age and is specified over the -55°C to +125°C temper-
ature range.
ApplicationsHVAC
Medical Devices
Battery Packs/Chargers
Home Appliances
FeaturesConverts Thermistor Temperature to Digital DataLow Average Thermistor Current Minimizes Self-
Heating ErrorsLow Supply Current, 21µA (typ) Including 10kΩ
Thermistor CurrentInternal Voltage Reference Isolates Thermistor
from Power-Supply Noise10-Bit ResolutionAccommodates Any Thermistor Temperature
RangeOutput Data Scaled for Direct Temperature
Readings from 0°C to +50°CSimple SPI-Compatible InterfaceSmall, 8-Pin µMAX Package
MAX6682
Thermistor-to-Digital Converter
Ordering Information19-2219; Rev 0; 2/02
PARTTEMP RANGEPIN-PACKAGEMAX6682MUA-55°C to +125°C8 µMAX
SPI is a trademark of Motorola, Inc.
Pin Configuration appears at end of data sheet.3.3V
0.1μF
MC68HCXX
I/O
SCLK
MISO
SCLK
VCC
GND
MAX6682REXT
THERMISTOR
ypical Operating Circuit
MAX6682
Thermistor-to-Digital Converter
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(VCC= 3V to 5.5V, TA= -55°C to +125°C, unless otherwise noted. Typical values are specified at VCC= 3.3V and TA= +25°C.) (Note 1)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
Supply Voltage (VCCto GND) .................................-0.3V to +6V
SO, SCK, CS, R-, R+ to GND ....................-0.3V to (VCC+ 0.3V)
R+ Current........................................................................±20mA
R- Current...........................................................................±1mA
SCK, CS, SO Current .........................................-1mA to +50mA
ESD Protection (Human Body Model).............................±2000V
Continuous Power Dissipation (TA= +70°C)
8-Pin µMAX (derate 4.1mW/°C above +70°C)............328mW
Operating Temperature Range
(TMINto TMAX)...............................................-55°C to +125°C
Storage Temperature Range.............................-65°C to +150°C
Junction Temperature .....................................................+150°C
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
Supply VoltageVCC3.05.5V
ADC Total Unadjusted ErrorTUEDOUT = 768.935 x (VREXT/VR+) - 134.0923;
VIN > 0.1VREF-3+3LSB
ADC Conversion TimetCONV6480ms
R- Input ImpedanceZIN1MΩ
R- Leakage Current150nA
Conversion Rate0.5Hz
Reference Voltage OutputVREFILOAD = 1mA1.101.221.40V
Reference Load Regulation0 < ILOAD < 2mA00.1%/mA
Reference Supply Regulation0.7mV/V
Conversion Supply CurrentICDuring conversion, no load220300µA
Average Supply CurrentIA0.5 conversions/s, no load1729µA
Standby CurrentISCS low, SCK inactive37µA
Idle CurrentIIDCS high, analog circuits off1017µA
SERIAL INTERFACE
Input Low VoltageVIL0.2 x
VCCV
Input High VoltageVIH0.8 x
VCCV
Input Leakage CurrentILEAKVIN = GND or VCC1µA
Output High VoltageVOHISOURCE = 1.6mAVCC -
0.4V
Output Low VoltageVOLISINK = 1.6mA0.4V
MAX6682
Thermistor-to-Digital Converter
TIMING CHARACTERISTICS
(VCC= 3V to 5.5V, TA= TMINto TMAX, unless otherwise noted. Typical values are specified at VCC= 3.3V and TA= +25°C.) (Note 2)
Note 1:All specifications are 100% tested at TA= +25°C. Specification limits over temperature are guaranteed by design,
not production tested.
Note 2:Guaranteed by design.
PARAMETERSYMBOLCONDITIONSMINTYPMAXUNITS
SERIAL INTERFACE TIMING (Figures 5 and 6)
Serial Clock FrequencyfSCL5MHz
SCK Pulse High WidthtCH50ns
SCK Pulse Low WidthtCL50ns
CS Fall to SCK RisetCSS35ns
CS Fall to Output Data ValidtDVCL = 10pF35ns
SCK Fall to Output Data ValidtDOCL = 10pF35ns
CS Rise to Output High-ZtTRCL = 10pF25ns
SCK Fall to Output High-ZtHIZCL = 10pF35ns
CS Pulse WidthtCSW75ns
Typical Operating Characteristics
(VCC= 5V, thermistor = 10k nominal, REXT= 7680Ω, TA = +25°C, unless otherwise noted.)
TEMPERATURE ERROR
vs. POWER-SUPPLY NOISE FREQUENCYMAX6682 toc01
FREQUENCY (MHz)
TEMPERATURE ERROR (
VIN = SQUARE WAVE
APPLIED TO VCC WITH
NO VCC BYPASS CAPACITOR
VIN = 250mVP-P
100100k10k1M10M
AVERAGE SUPPLY CURRENT
vs. CLOCK FREQUENCYMAX6682 toc02
SCK FREQUENCY (Hz)
SUPPLY CURRENT (
SCK IS DRIVEN
RAIL-TO-RAIL®
AVERAGE SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX6682 toc03
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (
Rail-to-Rail is a registered trademark of Nippon Motorola, Ltd.
MAX6682
Detailed DescriptionThe MAX6682 is a sophisticated interface circuit that
energizes a low-cost thermistor and converts its tem-
perature-dependent resistance to 10-bit digital data.
The MAX6682 powers the thermistor only when a mea-
surement is being made; the power dissipated in the
thermistor is minimized. This virtually eliminates self-
heating, a major component of thermistor error. The
simple serial interface is compatible with common
microcontrollers.
Temperature ConversionThe MAX6682 converts the voltage drop across the
resistor REXTto a digital output using an internal 10-bit
ADC. By measuring the voltage across REXT, the output
code is directly related to temperature when using an
NTC thermistor.
Although the relationship between a thermistor’s resis-
tance and its temperature is very nonlinear, the voltage
across REXTis reasonably linear over a limited temper-
ature range, provided that REXTis chosen properly. For
example, over a +10°C to +40°C range, the relationship
between the voltage across REXTand temperature is
linear to within approximately 0.2°C. Wider temperature
ranges result in larger errors.
The digital output is available as a 10-bit + sign word.
The relationship between the 11-bit digital word and the
voltage across REXT(normalized to VR+) is given by:
where VREXT/VR+is the voltage across REXTnormal-
ized to the value of VR+.
Table 1 shows the relationship between the voltage
across REXTand the MAX6682’s digital output code. It
also shows the temperature that would produce the list-
ed value of VREXTwhen a standard thermistor is used
in conjunction with REXT= 7680Ω. The MAX6682 pro-
duces output codes scaled to the actual temperature
when used with the standard thermistor and REXT=
7680Ωover the +10°C to +40°C temperature range.
Under these conditions, the nominal accuracy is about
0.2°C between +10°and +40°C, and about 1.5°C from
0°C to +50°C. In Table 1, the 3LSBs of the output code
represent fractional temperatures. The LSB has a value
of 0.125°C.
All table entries assume no errors in the values of REXT
or the thermistor resistance. Table 1 also assumes the
use of one of the following standard thermistors:
Betatherm 10K3A1, Dale 1M1002, or Thermometrics
C100Y103J. These thermistors have a nominal resis-
tance of 10kΩat +25°C and very similar temperature-
to-resistance functions. They give the results shown in
Table 1.
Different temperature ranges can be accommodated as
well using different values of REXT(see Choosing the
External Resistor). The MAX6682 works with thermistors
other than the ones listed above, but the transfer func-
tions vary somewhat.
Applications Information
Thermistors and Thermistor SelectionNTC thermistors are resistive temperature sensors
whose resistance decreases with increasing tempera-
ture. They are available in a wide variety of packages
that are useful in difficult applications such as measure-
ment of air or liquid temperature. Some can operate
over temperature ranges beyond that of most ICs. The
relationship between temperature and resistance in an
OUT
REXT=×+01743878010404
Thermistor-to-Digital Converter
PINNAMEFUNCTIONI.C.Internally Connected. Connect to GND or leave unconnected.R+Reference Voltage Output. External resistor positive input.
3R-External Resistor Negative Input. Connect R- to the junction of the external resistor and the
thermistor.GNDGround. Ground connection for MAX6682 and ground return for external thermistor.CSChip Select. Drive CS low to enable the serial interface.SOSerial Data OutputSCKSerial Clock Input
8VCCPositive Supply. Bypass VCC to GND with a 0.1µF capacitor.
Pin Description
NTC thermistor is very nonlinear and can be described
by the following approximation:
1 / T = A + BlnR + C(lnR)3
where T is absolute temperature, R is the thermistor’s
resistance, and A, B, and C are coefficients that vary
with manufacturer and material characteristics. The
general shape of the curve is shown in Figure 1.
The highly nonlinear relationship between temperature
and resistance in an NTC thermistor makes it somewhat
more difficult to use than a digital-output temperature
sensor IC, for example. However, by connecting the
thermistor in series with a properly chosen resistor and
using the MAX6682 to measure the voltage across the
resistor, a reasonably linear transfer function can be
obtained over a limited temperature range. Errors
decrease for smaller temperature ranges.
Figures 2 and 3 show typical thermistor nonlinearity
curves for a standard thermistor in conjunction with
series resistors chosen to optimize linearity over two
different temperature ranges: +10°C to +40°C and 0°C
to +70°C.
MAX6682
Thermistor-to-Digital ConverterTHERMISTOR NONLINEARITY
vs. TEMPERATURE
TEMPERATURE (°C)
LINEARITY ERROR (°C)
Figure 2. Thermistor Nonlinearity vs. Temperature for a Standard
Thermistor from 0°C to +70°C
*Assumes VR+= 1.220V.
THERMISTOR
TEMPERATURE (°C)
VREXT (mV) WITH STANDARD
THERMISTOR AND REXT =
7680Ω*
DECIMAL VALUE OF DOUT
(1LSB = 0.125°C)DOUT+60.000921.6+55.875001 1011 1111
+50.000830.6+48.625001 1000 0101
+40.000720.5+40.000001 0100 0000
+30.000595.4+30.125000 1111 0001
+25.000530.1+25.000000 1100 1000
+20.000464.4+19.875000 1001 1111
+10.000339.7+10.000000 0101 0000232.3+1.500000 0000 1100
-0.725225.5+1.000000 0000 1000
-2.000213.60.125000 0000 0001
-5.000187.4-2.000111 1111 0000
Table 1. Temperature vs. Digital Output for Standard Thermistor with REXT= 7680ΩTHERMISTOR RESISTANCE
vs. TEMPERATURE
TEMPERATURE (°C)
THERMISTOR RESISTANCE (k
Figure 1. Thermistor Resistance vs. Temperature
MAX6682NTC thermistors are often described by the resistance
at +25°C. Therefore, a 10kΩthermistor has a resistance
of 10kΩat +25°C. When choosing a thermistor, ensure
that the thermistor’s minimum resistance (which occurs
at the maximum expected operating temperature) in
series with REXTdoes not cause the voltage reference
output current to exceed about 1mA. Some standard
10kΩthermistors 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 ResistorChoose REXTto minimize nonlinearity errors from the
thermistor:Decide on the temperature range of interest (for
example 0°C to +70°C).Find the thermistor values at the limits of the tem-
perature range. RMINis the minimum thermistor
value (at the maximum temperature) and RMAXis
the maximum thermistor value (at the minimum tem-
perature). Also find RMID, the thermistor resistance
in the middle of the temperature range (+35°C for
the 0°C to +70°C range). Find REXTusing the equation below:
Table 3 shows nominal output data for several tempera-
tures when REXThas 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 InterfaceThe Typical Application Circuitshows the MAX6682
interfaced with a microcontroller. In this example, the
MAX6682 processes the reading from REXTand trans-
mits the data through an SPI-compatible interface.
Force CSlow and apply a clock signal at SCK to read
the results at SO. Forcing CSlow immediately stops
any conversion in process. Initiate a new conversion by
forcing CShigh.
Force CSlow 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 CSis
pulsed high and brought back low. Figure 4 is the SO
output.
Power-Supply ConsiderationsThe MAX6682 accuracy is relatively unaffected by
power-supply coupled noise. In most applications,
bypass VCCto GND by placing a 0.1µF ceramic
bypass capacitor close to the supply pin of the
devices.
Thermal ConsiderationsSelf-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-RRRRRREXTMIDMINMAXMINMAX
MINMAXMID=+()−
Thermistor-to-Digital ConverterTHERMISTOR NONLINEARITY
vs. TEMPERATURE
TEMPERATURE (°C)
LINEARITY ERROR (
°C)
Figure 3. Thermistor Nonlinearity vs. Temperature for a Standard
Thermistor from +10°C to +40°C
Figure 4. SO Output
10-BIT TEMPERATURE READINGBit109876543210
MSB
(Sign)
LSB
