EVAL-ADT7470EB Analog Devices Inc, EVAL-ADT7470EB Datasheet - Page 24

• Manufacturer Part Number: EVAL-ADT7470EB
• Description: BOARD EVALUATION FOR ADT7470
• Manufacturer: Analog Devices Inc

Specifications of EVAL-ADT7470EB

• Sensor Type: Temperature
• Interface: SMBus (2-Wire/I²C)
• Voltage - Supply: 3 V ~ 5.5 V
• Embedded: No
• Utilized Ic / Part: ADT7470
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

ADT7470
Calculating Fan Speed
Assuming that the measured number of tach pulses per rotation
corresponds to the number of pulses counted as set in register
0x43, fan speed is calculated by
where Fan Tach Reading is the 16-bit fan tachometer reading.
For example:
What is Fan 1 speed in RPM?
Tach 1 High Byte (Reg 0x2B) = 0x17
Tach 1 Low Byte (Reg 0x2A) = 0xFF
Fan 1 tach reading = 0x17FF = 6143 decimal
RPM = (f × 60)/Fan 1 tach reading
RPM = (90000 × 60)/6143
Fan Speed = 879 RPM
Fan Speed (RPM) = (90,000 × 60)/Fan Tach Reading
Rev. C | Page 24 of 40
Fan Pulses per Revolution
Different fan models can output either 1, 2, 3, or 4 tach pulses
per revolution. The number of tach pulses per rotation for each
fan should be programmed into the fan pulses per revolution
register (Register 0x43). If an incorrect value is programmed,
then the fan speed cannot be determined using the equation in
the Calculating Fan Speed section.
Alternatively, if the number of tach pulses per rotation is not
know, this register can be used in determining the number
of pulses/revolution output by a given fan. By plotting fan
speed measurements at maximum speed with different pulses/
revolution settings, the smoothest graph with the lowest ripple
determines the correct pulses/revolution value.