MCP3421 - Microchip

MCP34216.2.5 TEMPERATURE MEASUREMENTFigure 6-10 shows an example of temperaturemeasurement using a thermocouple sensor and theMCP9800 silicon temperature sensor. The MCP9800 isa high accuracy temperature sensor that can detect thetemperature in the range of -55°C to 125°C with 1°Caccuracy.The type K thermocouple sensor senses thetemperature at the hot junction (T HJ ) with respect to thecold junction temperature (reference, T CJ ). Thetemperature difference between the hot and coldjunctions is represented by the voltage V1. This voltageis then converted to digital codes by the MCP3421.In the circuit, the MCP9800 is used for cold junctioncompensation. The MCU computes the difference ofthe hot and cold junction temperatures, which isproportional to the hot junction temperature (T HJ ).With Type K thermocouple, it can measuretemperature from 0°C to 1250°C degrees. The fullscale output range of the Type K thermocouple isabout 50 mV. This provides 40 µV/°C(= 50 mV/1250°C) of measurement resolution.Equation 6-2 shows the measurement budget for thermocouplesensor signal using the MCP3421 devicewith 18 bits and PGA = 8 settings. With this configuration,it can detect the input signal level as low asapproximately 2 µV. The internal PGA boosts the inputsignal level eight times. The 40 µV/°C input from thethermocouple is amplified internally to 320 µV/°Cbefore the conversion takes place. This results in20.48 LSB/°C output codes. This means there areabout 20 LSB output codes (or about 4.32 bits) per 1°Cof change in temperature.EQUATION 6-2: MEASUREMENT BUDGETFOR THERMOCOUPLESENSORDetectable Input Signal Level = 15.625μV/PGA= 1.953125μV for PGA = 8Input Signal Level after gain of 8:= ( 40μV/°C) • 8 = 320μV/°CNo. of LSB/°C =320μV/°C------------------------ = 20.48 Codes/°C15.625μVWhere:1 LSB = 15.625 µV with 18-bit configurationHot JunctionV DDIsothermal Block(T HJ )Heat Cold Junction (T CJ )MCP9800Thermocouple Sensor~ 40 µV°CV DDFIGURE 6-10:Measurement.Example of TemperatureEquation 6-3 shows an example of calculating theexpected number of output code with various PGA gainsettings for Type K thermocouple output.EQUATION 6-3:V1ExpectedNumber of Output Code =Where:1µFMCP3421SDA10 kΩ10 kΩ10 µFSCLTo MCU(MASTER)EXPECTED NUMBER OFOUTPUT CODE FORTYPE K THERMOCOUPLE⎛ ⎞50 mVlog ⎜2----------------------- ⎟⎜15.625μV⎟⎝-----------------------⎠PGA= 11.6 bits for PGA = 1= 12.6 bits for PGA = 2= 13.6 bits for PGA = 4= 14.6 bits for PGA = 81 LSB = 15.625 µV with 18-bit configuration.© 2009 Microchip Technology Inc. DS22003E-page 31