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Hello,

we have build a Design with ADS124S08 ADC for measuring 6 Themocouples.

Everything is working as expected when the Thermocouple shield is not connected.

The Shield is connected to the junction.

If we connect the Shield to GND the Measured Voltage (Diff) is wrong.

TC+ = +4mV

TC- = -4mV

So what is wrong in our Design?

Should we use a Bipolar Supply?

Greetings

• V_AN = 3V3

• Hi Eric,

If I understand correctly you are using a grounded thermocouple like what is shown in the middle of figure 3 in A Basic Guide to Thermocouple Measurements.  Is this a third wire? Or can you be more precise in explaining how the thermocouple is connected to ground?

What I believe is happening is the thermocouple is no longer a floating voltage relative to analog ground of the ADS124S08 when connected to ground.  Instead the thermocouple potential is now directly connected to ground.  If you have the PGA enabled and using any gain, the input is now outside of the input range of the ADC.  This can be verified by disabling and bypassing the PGA to see if the condition improves.

Best regards,

Bob B

• Dear Bob,

yes it's a grounded Thermocouple like the middle TC in figure 3.

The Shield is a third cable (shielded of the cable) and connected with protective earth.

Also our GND (ADC GND) is connected to Earth.

So, the TC Shield is connected to ADC GND.

My measurements with Oscilloscope against GND:

Without shield connected to GND -> TC+ and TC- are around 1,5V (+/-4mV when heated)

With shield connected to GND -> TC+ is +4mV and TC- is -4mV

I also read about the PGA input range and that we are outside of this.

Wen can try to disable the PGA and see if it help.

My question is, if it will solve the Problem when using +/- 2,5V for ADC Supply? (AVDD and AVSS)

• Hi Eric,

If you use bipolar analog supplies the input range will follow with the supply range.  This will allow measurement below ground even with PGA enabled.  If you make this change also consider other connections in your schematic that will need to change from ground to AVSS such as the pulldown resistors on the inputs.  Also REFCOM must connect to the same potential as AVSS.

Best regards,

Bob B

• Thank you Bob,

i changed the Schematic to bipolar supply. If you don't see any issues with the Schematic for grounded Themocouples, it will go to production.

Should AINCOM connected to -2V5 or GND?

• Hi Eric,

I don't see any issues.  AINCOM is the same as any other analog input.  See the section in the datasheet 10.1.5 regarding Unused Inputs and Outputs.

Best regards,

Bob B