MAX1668MEE+ ,Multichannel Remote/Local Temperature SensorsApplications PART TEMP RANGE PIN-PACKAGEMAX1668MEE -55°C to +125°C 16 QSOPDesktop and Notebook Cent ..
MAX1668MEE+T ,Multichannel Remote/Local Temperature SensorsELECTRICAL CHARACTERISTICS(V = +3.3V, STBY = V , configuration byte = X0XXXX00, T = 0°C to +125°C, ..
MAX1668MEE+T ,Multichannel Remote/Local Temperature SensorsFeaturesThe MAX1668/MAX1805/MAX1989 are precise multi-♦ Multichannelchannel digital thermometers th ..
MAX1668MEE-T ,Multichannel Remote/Local Temperature SensorsGeneral Description ________
MAX1669 ,Fan Controller and Remote Temperature Sensor with SMBus Serial InterfaceApplicationsMAX1669EEE -40°C to +85°C 16 QSOP®Pentium CPU CoolingDesktop ComputersPin Configuration ..
MAX1669EEE ,Fan Controller and Remote Temperature Sensor with SMBus Serial InterfaceApplicationsMAX1669EEE -40°C to +85°C 16 QSOP®Pentium CPU CoolingDesktop ComputersPin Configuration ..
MAX4426ESA ,Dual High-Speed 1.5A MOSFET Drivers
MAX4427CPA ,Dual High-Speed 1.5A MOSFET Drivers
MAX4427CSA ,Dual High-Speed 1.5A MOSFET Drivers
MAX4427CSA ,Dual High-Speed 1.5A MOSFET Drivers
MAX4427CSA ,Dual High-Speed 1.5A MOSFET Drivers
MAX4427CSA+ ,Dual High-Speed, 1.5A MOSFET Drivers
MAX1668MEE+-MAX1668MEE+T-MAX1668MEE-T-MAX1989MEE+-MAX1989MEE+T-MAX1989MUE+-MAX1989MUE+T
Multichannel Remote/Local Temperature Sensors
________________General DescriptionThe MAX1668/MAX1805/MAX1989 are precise multi-
channel digital thermometers that report the tempera-
ture of all remote sensors and their own packages. The
remote sensors are diode-connected transistors—typi-
cally low-cost, easily mounted 2N3904 NPN types—that
replace conventional thermistors or thermocouples.
Remote accuracy is ±3°C for multiple transistor manu-
facturers, with no calibration needed. The remote chan-
nels can also measure the die temperature of other ICs,
such as microprocessors, that contain an on-chip,
diode-connected transistor.
The 2-wire serial interface accepts standard system
management bus (SMBus™) write byte, read byte, send
byte, and receive byte commands to program the alarm
thresholds and to read temperature data. The data for-
mat is 7 bits plus sign, with each bit corresponding to
1°C, in two’s-complement format.
The MAX1668/MAX1805/MAX1989 are available in
small, 16-pin QSOP surface-mount packages. The
MAX1989 is also available in a 16-pin TSSOP.
________________________Applications
____________________________FeaturesMultichannel
4 Remote, 1 Local (MAX1668/MAX1989)
2 Remote, 1 Local (MAX1805)No Calibration RequiredSMBus 2-Wire Serial InterfaceProgrammable Under/Overtemperature AlarmsSupports SMBus Alert ResponseAccuracy
±2°C (+60°C to +100°C, Local)
±3°C (-40°C to +125°C, Local)
±3°C (+60°C to +100°C, Remote)3µA (typ) Standby Supply Current700µA (max) Supply CurrentSmall, 16-Pin QSOP/TSSOP Packages
MAX1668/MAX1805/MAX1989
Multichannel Remote/Localemperature SensorsSMBCLK
ADD0ADD1
VCCSTBY
GND
ALERT
SMBDATA
DXP1
DXP4
DXN4
INTERRUPT
TO μC
3V TO 5.5V
200Ω0.1μF
CLOCK
10kΩ EACH
DATA
DXN1
2200pF
2200pF
* DIODE-CONNECTED TRANSISTORMAX1668
MAX1805
MAX1989
Pin ConfigurationDXP1GND
STBY
SMBCLK
SMBDATA
ALERT
ADD0
ADD1
VCC
TOP VIEW
MAX1668
MAX1805
MAX1989
QSOP/TSSOPDXN1
DXP2
(N.C.) DXN3
DXN2
(N.C.) DXP3
(N.C.) DXP4
( ) ARE FOR MAX1805.
(N.C.) DXN4
ypical Operating Circuit19-1766; Rev 2; 5/03
PART
MAX1668MEE-55°C to +125°C
TEMP RANGEPIN-PACKAGE16 QSOP
_______________Ordering InformationSMBus is a trademark of Intel Corp.†Pg
MAX1805MEE-55°C to +125°C16 QSOP
Desktop and Notebook
Computers
LAN Servers
Industrial Controls
Central-Office Telecom
Equipment
Test and Measurement
Multichip Modules
MAX1989MEE-55°C to +125°C16 QSOP
MAX1989MUE-55°C to +125°C16 TSSOP
MAX1668/MAX1805/MAX1989
Multichannel Remote/Local emperature Sensors
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS(VCC= +3.3V, STBY= VCC, configuration byte = X0XXXX00, TA
= 0°C to +125°C, unless otherwise noted.) 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.
VCCto GND..............................................................-0.3V to +6V
DXP_, ADD_, STBY to GND........................-0.3V to (VCC+ 0.3V)
DXN_ to GND........................................................-0.3V to +0.8V
SMBCLK, SMBDATA, ALERTto GND......................-0.3V to +6V
SMBDATA, ALERTCurrent.................................-1mA to +50mA
DXN_ Current......................................................................±1mA
Continuous Power Dissipation (TA= +70°C)
QSOP (derate 8.30mW/°C above +70°C)....................667mW
TSSOP (derate 9.40mW/°C above +70°C)..................755mW
Operating Temperature Range.........................-55°C to +125°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +150°C
Lead Temperature (soldering, 10s).................................+300°C
DXP_ forced to 1.5VRemote-Diode Source Current
Low level (POR state)
Configuration byte =
X0XXXX10, high level
Configuration byte =
X0XXXX01, high level
High level (POR state)
DXN_ Source Voltage0.7V
Hardware or software standby,
SMBCLK at 10kHz
SMBus static
TA = 0°C to +85°C
TA = +60°C to +100°C
Average measured over 4s; logic inputs forced
VCCor GND
Temperature Error, Local Diode
(Notes 1, 2)-3.5+3.5°C-2.5+2.5Including long-term drift
Temperature Error, Remote Diode
(Notes 2, 3)-5+5°C-3+3
TR = -55°C to +125°C
TR = +60°C to +100°C
PARAMETERMINTYPMAXUNITSUndervoltage Lockout Hysteresis50mV
Undervoltage Lockout Threshold2.602.82.95V
Supply Voltage Range3.05.5V
Initial Temperature Error,
Local Diode (Note 2)-3+3°C
Power-On Reset (POR) Threshold1.31.82.3V
POR Threshold Hysteresis50mV
Standby Supply Current
Temperature Resolution (Note 1)8Bits+2
Average Operating Supply Current400700µA
Conversion Time260320380ms100130
Address Pin Bias Current160µA
CONDITIONSVCCinput, disables A/D conversion, rising edge
TA = 0°C to +125°C
VCC, falling edge
From stop bit to conversion complete (all channels)
Logic inputs
forced to VCC
or GND
ADD0, ADD1; momentary upon power-on reset
Monotonicity guaranteed
TA = +60°C to +100°C
ADC AND POWER SUPPLY
MAX1668/MAX1805/MAX1989
Multichannel Remote/Local emperature Sensors
ELECTRICAL CHARACTERISTICS (continued)(VCC= +3.3V, STBY= VCC, configuration byte = X0XXXX00, TA
= 0°C to +125°C, unless otherwise noted.) STBY, SMBCLK, SMBDATA; VCC= 3V to 5.5V
tHIGH, 90% to 90% points
tLOW, 10% to 10% points
(Note 4)
SMBCLK, SMBDATA
Logic inputs forced to VCCor GND
ALERTforced to 5.5V
STBY, SMBCLK, SMBDATA; VCC= 3V to 5.5V
ALERT,SMBDATA forced to 0.4V
CONDITIONS4SMBCLK Clock High Time4.7SMBCLK Clock Low Time
kHzDC100SMBus Clock Frequency5SMBus Input Capacitance-1+1Logic Input Current1ALERTOutput High Leakage
Current2.2Logic Input High Voltage0.8Logic Input Low Voltage6Logic Output Low Sink Current
UNITSMINTYPMAXPARAMETERtSU:DAT, 10% or 90% of SMBDATA to 10% of SMBCLK
tSU:STO, 90% of SMBCLK to 10% of SMBDATA
tHD:STA, 10% of SMBDATA to 90% of SMBCLK
tSU:STA, 90% to 90% points250SMBus Data Valid to SMBCLK
Rising-Edge Time4SMBus Stop-Condition Setup Time4SMBus Start-Condition Hold Time250SMBus Repeated Start-Condition
Setup Time4.7SMBus Start-Condition Setup TimeSMBus Data-Hold Time
Master clocking in dataµs1SMBCLK Falling Edge to SMBus
Data-Valid Time
SMBus INTERFACE
ELECTRICAL CHARACTERISTICS (VCC= +5V, STBY= VCC, configuration byte = X0XXXX00, TA
= -55°C to +125°C, unless otherwise noted.) (Note 6)
CONDITIONSMonotonicity guaranteed= +60°C to +100°C
Bits8Temperature Resolution+2= +60°C to +100°C= -55°C to +125°C°C-3+3
Initial Temperature Error,
Local Diode (Note 2)4.55.5Supply-Voltage Range
From stop bit to conversion complete (both channels)ms260380Conversion Time+3= -55°C to +125°C°C
UNITSMINTYPMAX+5
PARAMETERTemperature Error, Remote Diode
(Notes 2, 3)
ADC AND POWER SUPPLYtHD:DAT, slave receive (Note 5)0
MAX1668/1805 toc03
FREQUENCY (MHz)
TEMPERATURE ERROR (
TEMPERATURE ERROR
vs. SUPPLY NOISE FREQUENCY100mVP-P
WITH VCC 0.1μF CAPACITOR REMOVED
2200pF BETWEEN DXN_ AND DXP_
250mVP-P
TEMPERATURE ERROR
vs. PC BOARD RESISTANCE
MAX1668/1805 toc01
LEAKAGE RESISTANCE (MΩ)
TEMPERATURE ERROR (
°C)
PATH = DXP_ TO GND
PATH = DXP_ TO VCC (5V)
TEMPERATURE ERROR
vs. TEMPERATURE
MAX1668/1805 toc02
TEMPERATURE (°C)
TEMPERATURE ERROR (NPN (CMPT3904)
PNP (CMPT3906)
INTERNAL
ypical Operating Characteristics(Typical Operating Circuit, VCC= +5V, STBY= VCC, configuration byte = X0XXXX00, TA = +25°C, unless otherwise noted.)
MAX1668/MAX1805/MAX1989
Multichannel Remote/Local emperature Sensors
ELECTRICAL CHARACTERISTICS (continued)(VCC= +5V, STBY= VCC, configuration byte = X0XXXX00, TA
= -55°C to +125°C, unless otherwise noted.) (Note 6)
Note 1:Guaranteed by design, but not production tested.
Note 2:Quantization error is not included in specifications for temperature accuracy. For example, if the MAX1668/MAX1805/
MAX1989 device temperature is exactly +66.7°C, the ADC may report +66°C, +67°C, or +68°C (due to the quantization
error plus the +0.5°C offset used for rounding up) and still be within the guaranteed ±1°C error limits for the +60°C to
+100°C temperature range. See Table 2.
Note 3:A remote diode is any diode-connected transistor from Table 1. TRis the junction temperature of the remote diode. See the
Remote-Diode Selectionsection for remote-diode forward-voltage requirements.
Note 4:The SMBus logic block is a static design that works with clock frequencies down to DC. While slow operation is possible, it
violates the 10kHz minimum clock frequency and SMBus specifications, and can monopolize the bus.
Note 5:Note that a transition must internally provide at least a hold time in order to bridge the undefined region (300ns max) of
SMBCLK’s falling edge tHD:DAT.
Note 6:Specifications from -55°C to +125°C are guaranteed by design, not production tested.
CONDITIONSUNITSMINTYPMAXPARAMETERSTBY, SMBCLK, SMBDATA; VCC= 4.5V to 5.5VLogic Input High VoltageV2.4
ALERTforced to 5.5VµA1ALERTOutput High Leakage
Current
Logic inputs forced to VCCor GNDµA-2+2Logic Input Current
ALERT,SMBDATA forced to 0.4VmA6Logic Output Low Sink Current
STBY, SMBCLK, SMBDATA; VCC= 4.5V to 5.5VV0.8Logic Input Low Voltage
SMBus INTERFACE
STANDBY SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX1668/1805 toc07
SUPPLY VOLTAGE (V)
SUPPLY CURRENT (
STBY = GND
ADD0 = ADD1 = HIGH-Z
ADD0 = ADD1 = GND
RESPONSE TO THERMAL SHOCK
MAX1668/1805 toc08
TIME (s)
TEMPERATURE (
16 QSOP IMMERSED IN
+115°C FLUORINERT BATH
MAX1668/MAX1805/MAX1989
Multichannel Remote/Local emperature Sensors TEMPERATURE ERROR
vs. COMMON-MODE NOISE FREQUENCY
MAX1668/1805 toc04
FREQUENCY (MHz)
TEMPERATURE ERROR (100
SQUARE-WAVE AC-COUPLED INTO DXN
2200pF BETWEEN DXN_ AND DXP_
100mVP-P
50mVP-P
ypical Operating Characteristics (continued)(Typical Operating Circuit, VCC= +5V, STBY= VCC, configuration byte = X0XXXX00, TA = +25°C, unless otherwise noted.)
TEMPERATURE ERROR
vs. DXP_ TO DXN_ CAPACITANCEMAX16681805 toc05
DXP_ TO DXN_ CAPACITANCE (nF)
TEMPERATURE ERROR (
STANDBY SUPPLY CURRENT
vs. CLOCK FREQUENCY
MAX1668/1805 toc06
SMBCLK FREQUENCY (kHz)
SUPPLY CURRENT (101001000
STBY = GND
VCC = 5V
VCC = 3.3V
MAX1668/MAX1805/MAX1989
Multichannel Remote/Local emperature Sensors
_______________Detailed DescriptionThe MAX1668/MAX1805/MAX1989 are temperature
sensors designed to work in conjunction with an exter-
nal microcontroller (µC) or other intelligence in thermo-
static, process-control, or monitoring applications. The
µC is typically a power-management or keyboard con-
troller, generating SMBus serial commands by “bit-
banging” general-purpose input-output (GPIO) pins or
through a dedicated SMBus interface block.
These devices are essentially 8-bit serial analog-to-digi-
tal converters (ADCs) with sophisticated front ends.
However, the MAX1668/MAX1805/MAX1989 also contain
a switched current source, a multiplexer, an ADC, an
SMBus interface, and associated control logic (Figure 1).
In the MAX1668 and MAX1989, temperature data from
the ADC is loaded into five data registers, where it is
automatically compared with data previously stored in
10 over/undertemperature alarm registers. In the
MAX1805, temperature data from the ADC is loaded into
three data registers, where it is automatically compared
with data previously stored in six over/undertemperature
alarm registers.
ADC and Multiplexer The ADC is an averaging type that integrates over a
64ms period (each channel, typical), with excellent
noise rejection.
The multiplexer automatically steers bias currents
through the remote and local diodes, measures their
forward voltages, and computes their temperatures.
Each channel is automatically converted once the con-
version process has started. If any one of the channels
is not used, the device still performs measurements on
these channels, and the user can ignore the results of
the unused channel. If any remote-diode channel is
unused, connect DXP_ to DXN_ rather than leaving the
pins open.
The DXN_ input is biased at 0.65V above ground by an
internal diode to set up the A/D inputs for a differential
measurement. The worst-case DXP_ to DXN_ differential
input voltage range is 0.25V to 0.95V.
Excess resistance in series with the remote diode caus-
es about +0.5°C error per ohm. Likewise, 200µV of offset
voltage forced on DXP_ to DXN_causes about 1°C error.
MAX1668/
MAX1989
FUNCTION1, 3, 5, 7DXP_
Combined Current Source and A/D Positive Input for Remote-Diode Channel. Do not
leave DXP floating; connect DXP to DXN if no remote diode is used. Place a 2200pF
capacitor between DXP and DXN for noise filtering.
PINALERTSMBus Alert (Interrupt) Output, Open DrainADD0SMBus Slave Address Select PinADD1
SMBus Address Select Pin (Table 8). ADD0 and ADD1 are sampled upon power-up.
Excess capacitance (>50pF) at the address pins when floating can cause address-
recognition problems.STBYHardware Standby Input. Temperature and comparison threshold data are retained in
standby mode. Low = standby mode, high = operate mode.SMBCLKSMBus Serial-Clock InputSMBDATASMBus Serial-Data Input/Output, Open Drain
1, 3
Pin Description
NAMEMAX18052, 4, 6, 8DXN_Combined Current Sink and A/D Negative Input. DXN is normally biased to a diode volt-
age above ground. 2, 4VCCSupply Voltage Input, 3V to 5.5V. Bypass to GND with a 0.1µF capacitor. A 200Ωseries
resistor is recommended but not required for additional noise filtering.9GNDGround16N.C.No Connection. Not internally connected. Can be used for PC board trace routing.5–8
MAX1668/MAX1805/MAX1989
Multichannel Remote/Local emperature Sensors Figure 1. MAX1668/MAX1805/MAX1989 Functional Diagram
DXP4
DXP3DXP2DXP1
DXN4DXN3
DXN2
DXN1
LOCAL
CURRENTSOURCES
MUX
DIODEFAULT
ADC
CONTROL
LOGIC
SMBus
ADDRESSDECODER
STBY ADD ADD1
SMBDATASMBCLK
ALERT
DIGITAL COMPARATORS
ALERT RESPONSE
CONFIGURATION BYTE
ADDRESS REGISTER
REGISTER
STATUS BYTE REGISTERS
1 AND 2
COMMAND BYTE REGISTER
TEMPERATURE DATA REGISTERS
HIGH LIMITS REGISTERSLOW LIMITS REGISTERS
ALERT MASK
REGISTER
NOTE: DOTTED LINES ARE FOR MAX1668 AND MAX1989.
A/D Conversion SequenceIf 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 SelectionTemperature 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-HeatingThermal 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 ALERToutput. For example, with ALERTsinking
1mA, the typical power dissipation is VCCx 400µA plus
0.4V x 1mA. Package theta J-A is about 150°C/W, so
with VCC= 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 FilteringThe 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 LayoutPlace 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 emperature SensorsCMPT3904Central Semiconductor (USA)
MMBT3904Motorola (USA)
MMBT3904
SST3904Rohm Semiconductor (Japan)
KST3904-TFSamsung (Korea)
FMMT3904CT-NDZetex (England)
MANUFACTURERMODEL NO.SMBT3904Siemens (Germany)
Table 1. Remote-Sensor Transistor
Manufacturers
Note:Transistors must be diode connected (base shorted to
collector).
National Semiconductor (USA)
be 4in to 8in (typ) or more as long as the worst noise
sources (such as CRTs, clock generators, memory
buses, and ISA/PCI buses) are avoided.Do not route the DXP_ to DXN_ lines next to the
deflection coils of a CRT. Also, do not route the
traces across a fast memory bus, which can easily
introduce +30°C error, even with good filtering.
Otherwise, most noise sources are fairly benign.Route the DXP_ and DXN_ traces in parallel and in
close proximity to each other, away from any high-
voltage traces such as +12VDC. Leakage currents
from PC board contamination must be dealt with
carefully, since a 20MΩleakage path from DXP_ to
ground causes about +1°C error.Connect guard traces to GND on either side of the
DXP_ to DXN_ traces (Figure 2). With guard traces
in place, routing near high-voltage traces is no
longer an issue. Route through as few vias and crossunders as possi-
ble to minimize copper/solder thermocouple effects. When introducing a thermocouple, make sure that
both the DXP_ and the DXN_ paths have matching
thermocouples. In general, PC board-induced ther-
mocouples are not a serious problem. A copper-sol-
der thermocouple exhibits 3µV/°C, and it takes
about 200µV of voltage error at DXP_ to DXN_ to
cause a +1°C measurement error. So, most para-
sitic thermocouple errors are swamped out. Use wide traces. Narrow ones are more inductive
and tend to pick up radiated noise. The 10mil
widths and spacings recommended in Figure 2 are
not absolutely necessary (as they offer only a minor
improvement in leakage and noise), but try to use
them where practical.Copper cannot be used as an EMI shield, and only
ferrous materials such as steel work well. Placing a
copper ground plane between the DXP_ to DXN_
traces and traces carrying high-frequency noise sig-
nals does not help reduce EMI.
PC Board Layout ChecklistPlace the MAX1668/MAX1805/MAX1989as close as
possible to the remote diodes.Keep traces away from high voltages (+12V bus).Keep traces away from fast data buses and CRTs.Use recommended trace widths and spacings.Place a ground plane under the traces.Use guard traces flanking DXP_ and DXN_ and con-
necting to GND.Place the noise filter and the 0.1µF VCCbypass
capacitors close to the MAX1668/MAX1805/
MAX1989.Add a 200Ωresistor in series with VCCfor best noise
filtering (see the Typical Operating Circuit).
Twisted-Pair and Shielded Cables For remote-sensor distances longer than 8in, or in partic-
ularly noisy environments, a twisted pair is recommend-
ed. Its practical length is 6ft to 12ft (typ) before noise
becomes a problem, as tested in a noisy electronics lab-
oratory. For longer distances, the best solution is a
shielded twisted pair like that used for audio micro-
phones. For example, Belden #8451 works well for dis-
tances up to 100ft in a noisy environment. Connect the
twisted pair to DXP_ and DXN_ and the shield to GND,
and leave the shield’s remote end unterminated.
Excess capacitance at DX_ _ limits practical remote-sen-
sor distances (see the Typical Operating Characteristics).
For very long cable runs, the cable’s parasitic capaci-
tance often provides noise filtering, so the 2200pF capac-
itor can often be removed or reduced in value.
Cable resistance also affects remote-sensor accuracy;series resistance introduces about +0.5°C error.
Low-Power Standby ModeStandby mode disables the ADC and reduces the sup-
ply-current drain to less than 12µA. Enter standby
mode by forcing the STBYpin low or through the
RUN/STOP bit in the configuration byte register.
Hardware and software standby modes behave almost
identically: all data is retained in memory, and the SMB
interface is alive and listening for reads and writes.
Activate hardware standby mode by forcing the STBY
pin low. In a notebook computer, this line can be con-
nected to the system SUSTAT# suspend-state signal.
The STBYpin low state overrides any software conversion
command. If a hardware or software standby command
is received while a conversion is in progress, the conver-
MAX1668/MAX1805/MAX1989
Multichannel Remote/Local emperature Sensors MINIMUM
10mils
10mils
10mils
10mils
GND
GND
DXN_
DXP_
Figure 2. Recommended DXP_/DXN_ PC Traces
