Thermocouple measurement interface

Hi S1640 ,

I am currently looking into this. I will get back to you soon. Thank you for your patience.

Regards,

JC

Hi S1640 ,

Firstly, thermocouples are floating sensors. They require biasing to set the sensor voltage DC operating point.

In your connections:

1. Thermocouple + side: Connect to LTC2487 via LPF / Thermocouple - side: Connect to GND, then connect to LTC2487 via LPF

1. • In this configuration, the negative thermocouple lead is referenced to ground. This is a valid connection. Since the sensor is biased to ground, the LTC2487 can measure the full differential thermocouple voltage within its allowable +/-FS differential input range.

 2. Thermocouple + side: Connect to LTC2487 via LPF / Thermocouple - side: Connect directly to LPF without connecting to GND

1. • This configuration is incorrect. With no bias, the thermocouple is electrically floating. This can cause unstable behavior, increased noise susceptibility, and inaccurate temperature readings.

 3. Thermocouple + side: Connect to LTC2487 via LPF / Thermocouple - side: Connect Insert a resistor and connect to GND, then connect to LPF

1. • This configuration is also valid. Adding a high-value resistor such as 1Mohm from the negative thermocouple lead to ground provides a proper DC bias. The small leakage currents of the LTC2487 produce voltage drop across this resistor, while still supplying a stable bias point for the thermocouple.

Would you be open to considering our newer Sigma--Delta ADCs? These devices provide higher integration and flexible digital filter options, enabling user to optimize performance by balancing speed and noise performance. They also support simultaneous 50/60Hz rejection and offer simplified sensor interface for thermocouples and thermistors.

The AD7124-4 and AD7124-8 are 24-bit ADCs with integrated PGA (gain up to 128) and a built-in VBIAS generator that simplifies thermocouple biasing. The  AD7124-8 supports up to 8 differential or 15 pseudo-differential inputs, while the AD7124-4 supports 4 differential or 7 pseudo-differential inputs, making them suitable for multi-channel, multiplexed sensor systems. The built-in diagnostics enhances system robustness and reliability. We also have available reference design for thermocouple and thermistor measurement using the AD7124 that you may use as reference.

Meanwhile, the AD4129-8 is an ultra-low power 16-bit ADC that includes a PGA (gain up to 128), a VBIAS generator, and an integrates FIFO. It supports 8 differential or 16 pseudo-differential inputs and also offers diagnostics feature.

Regards,

JC

Hi JCCillion-san,

Thank you for your advice.
My worries have been resolved. I will inquire with the customer about the new product.
However, the AD7124 may be too high spec for the customer.
If I receive any inquiries from customers, would appreciate your support again.

Thank you !

Hi S1640 ,

Additional note for connection 1, ensure that neither thermocouple lead is referenced to earth elsewhere in the system.

Regards,

JC

Hi JCCillion-san

Understood. I will take care of it. I will also inform our customers to be extra careful.
Thank you so much for your support.