ADS1118 - Bizarre temperature data readings

Hi there,

I’m working on a project with a K-Type thermocouple using ADS1118 as an ADC and data converter. I’ve spent three days struggling read correct data from the chip.

My setup is as follows:

I have one thermocouple using one channel of the ADS1118 schematic Fig. 1

Fig. 1

I have a 10KΩ pull-up resistor on DIN and pull-up enabled on DOUT, SCLK at 4Mhz and timings as Table 1:

 Table 1.

When I program ADS1118 to read and convert data from the thermocouple I’ve readings around 0 degrees Celsius at environment temperature around 20-22 degrees Celsius.

The temperature is going up and down changing only between 0xFFFF and 0x00XX. Depending on how the PGA is programmed the data register changes in very limited boundaries. See Fig. 2, 3, 4, 5

N/A

Fig. 2

Fig. 3

Fig. 4

Fig. 5

When the ADS is programmed to read and convert data from the internal sensor it gives temperature readings around a 100 degrees Celsius and above. Fig. 6, 7, 8, 9

N/A

Fig. 6

Fig. 7

Fig. 8

Fig. 9

I have tried many different setups without any succes. I tried wiring another K-Type thermocouple on the other ADS1118 channel without the filter – it measuring the same.

I red carefully every post here in the support forum. I tried to work with CS always low and I see that the ADS1118 DRDY asserts low every 7.86629 mS – the data reading is the same. Fig. 10

Fig. 10

I forgot to mention that I'm using Labfacility K-Type thermocouple with Miniature Plug & Socket Connector. I've measured that thermocouple and the results are matching  with the K-Type Thermocouple Reference Table. I have more than one thermocouple and I've tried with all of them. 

What I’m doing wrong?

Best regards,

Zhivko Zhelev