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Thermocouple 16 bit & differential low voltage
Several ACE include interfaces that are individually configurable in vBuilder for either providing:
- thermocouple interfaces
- or as analog inputs for differential analog signals
Input characteristics:
- Input impedance is about 2.2 MΩ.
- Absolute maximum input voltage for each analog input pin is between 3.3 VDC and GND.
- The differential voltage is the difference between the two input pins, but both input pins must remain within the absolute voltage limits 3.3 to GND.
Thermocouple inputs
- Can be configured for J, K, T or N type thermocouples
- Connect each thermocouple to one of the four wire pairs shown in the diagram on the right
- The negative wire should be connected to the negative input (indicated by a ‘-’ sign on the diagram)
- The positive wire should be connected to the positive input (indicated by a ‘+’ sign on the diagram)
Low level differential voltage signals
Each differential input can also be used to measure low level differential voltage signals.
The following ranges are available :
- +/- 0.256 V
- +/- 0.512 V
- +/- 1.024 V
- +/- 2.048 V
Setup in vBuilder
The vBuilder Setup window (Setup Hardware) for the thermocouple / differential analog configuration
The vBuilder thermocouple cailbration
See How to Wire and Calibrate a Thermocouple?
In operation, the signal is converted to a 16 bits floating point value. That value is either temperature, in degrees Celcius, or volts.
Questions / Answers
- Does ACE PLC use the internal temperature sensor (Cold Junction) for compensation?
- The thermocouple chip has an internal temperature sensor that senses its temperature.
- ACE PLC read that out of the chip and use that to calculate cold junction compensation
- Do you use lookup table or polynomial for calculation of temperature out of voltage?
- ACE PLC uses a polynomial calculation
- What value should be visible when no thermocouple is connected? Will this value be stable or fluctuate?
- Without a thermocouple connected, the voltage input is pulled to around 3.3V, which yields a ridiculously large number.
- What is the cutoff frequency of ACE PLC RC filter. What is the performance of this filter for the 1kHz PWM signal?
- For the differential signal, around 240Hz.
- What is the data rate (SPS) and Full Scale Range (FSR) used when measuring a K-type thermocouple?
- The ACE thermocouple chip has an SPI interface.
- Thermocouple measurements messages take place between program logic scans.
- ACE PLC limit the reads to once every 16 ms.
- There are 2 thermocouples on a chip – so once every 32 ms.
- There is also periodic (once every 24 reads), when we read the chip temperature.
- So, we read each individual channel about 30 times per second.
- Does ACE PLC use “continuous” conversion or “single shot” mode for measuring a thermocouple?
- “Single shot”
ADS-1118 thermocouple chip specifications
