Software specifications based on MAX6675 hardware specifications are as follows.
【 H-CMP Architecture 】
The H-CMP Architecture of this product is shown in the figure below.
Temperature Conversion
The MAX6675 includes signal-conditioning hardware to convert the thermocouple’s signal into a voltage compatible with the input channels of the ADC. The T+and Tinputs connect to internal circuitry that reduces the introduction of noise errors from the thermocouple wires.
Before converting the thermoelectric voltages into equivalent temperature values, it is necessary to compensate for the difference between the thermocouple cold-junction side (MAX6675 ambient temperature) and a 0°C virtual reference. For a type-K thermocouple, the
voltage changes by 41μV/°C, which approximates the thermocouple characteristic with the following linear equation:
VOUT = (41μV / °C) x (TR – TAMB)
Where:
VOUT is the thermocouple output voltage (μV).
TR is the temperature of the remote thermocouple junction(°C).
TAMB is the ambient temperature (°C).
Cold-Junction Compensation
The function of the thermocouple is to sense a difference in temperature between two ends of the thermocouple wires. The thermocouple’s hot junction can be read from 0°C to +1023.75°C. The cold end (ambient temperature of the board on which the MAX6675 is mounted) can only range from -20°C to +85°C.
While the temperature at the cold end fluctuates, the MAX6675 continues to accurately sense the temperature difference at the opposite end.
The MAX6675 senses and corrects for the changes in the ambient temperature with cold-junction compensation.The device converts the ambient temperature reading into a voltage using a temperature-sensing diode. To make the actual thermocouple temperature measurement, the MAX6675 measures the voltage from the thermocouple’s output and from the sensing diode. The device’s internal circuitry passes the diode’s voltage (sensing ambient temperature) and thermocouple voltage (sensing remote temperature minus ambient temperature) to the conversion function stored in the ADC to calculate the thermocouple’s hot-junction temperature.
Optimal performance from the MAX6675 is achieved when the thermocouple cold junction and the MAX6675 are at the same temperature. Avoid placing heat-generating devices or components near the MAX6675 because this may produce cold-junction-related errors.
Digitization
The ADC adds the cold-junction diode measurement
with the amplified thermocouple voltage and reads out
the 12-bit result onto the SO pin. A sequence of all
zeros means the thermocouple reading is 0°C. A
sequence of all ones means the thermocouple reading
is +1023.75°C.
Serial Interface
The Typical Application Circuit shows the MAX6675 interfaced with a microcontroller. In this example, the MAX6675 processes the reading from the thermocou-ple and transmits the data through a serial interface. Force CS low and apply a clock signal at SCK to read the results at SO. Forcing CS low immediately stops any conversion process. Initiate a new conversion
process by forcing CS high.
Force CS low to output the first bit on the SO pin. A complete serial interface read requires 16 clock cycles. Read the 16 output bits on the falling edge of the clock. The first bit, D15, is a dummy sign bit and is always zero. Bits D14–D3 contain the converted temperature in the order of MSB to LSB. Bit D2 is normally low and goes high when the thermocouple input is open. D1 is low to provide a device ID for the MAX6675 and bit D0 is three-state.
Figure 1a is the serial interface protocol and Figure 1b shows the serial interface timing. Figure 2 is the SO output.
Open Thermocouple
Bit D2 is normally low and goes high if the thermocou-ple input is open. In order to allow the operation of the open thermocouple detector, T- must be grounded.Make the ground connection as close to the GND pin as possible.
Noise Considerations
The accuracy of the MAX6675 is susceptible to power- supply coupled noise. The effects of power-supply noise can be minimized by placing a 0.1μF ceramic bypass capacitor close to the supply pin of the device.
Thermal Considerations
Self-heating degrades the temperature measurement accuracy of the MAX6675 in some applications. The magnitude of the temperature errors depends on the thermal conductivity of the MAX6675 package, the mounting technique, and the effects of airflow. Use a large ground plane to improve the temperature mea-surement accuracy of the MAX6675.
The accuracy of a thermocouple system can also be improved by following these precautions:
●Use the largest wire possible that does not shunt heat away from the measurement area.
●If small wire is required, use it only in the region of the measurement and use extension wire for the region with no temperature gradient.
●Avoid mechanical stress and vibration, which could strain the wires.
●When using long thermocouple wires, use a twisted-pair extension wire.
●Avoid steep temperature gradients.
●Try to use the thermocouple wire well within its temperature rating.
●Use the proper sheathing material in hostile environments to protect the thermocouple wire.
●Use extension wire only at low temperatures and only in regions of small gradients.
●Keep an event log and a continuous record of thermocouple resistance.
Reducing Effects of Pick-Up Noise
The input amplifier (A1) is a low-noise amplifier designed to enable high-precision input sensing. Keep the thermocouple and connecting wires away from electrical noise sources.
