Other Parts Discussed in Thread: INA129
Hello,
I have built the application circuit from Figure 6. in the INA128/INA129 datasheet entitled "Thermocouple Amplifier with RTD Cold-Junction Compensation". I am using a type-T thermocouple and therefore used R1=R2=102k. The RTD is a platinum 100 ohm thermistor, which adheres to the RTD polynomial and is Class A (lower error).
I am simply testing to see if I can record a reasonable temperature right now by having my thermocouple in ambient air. My gain resistor is 165 Ohms, for a gain of G=304.03. Taking this into account, at room temperature the thermocouple output voltage should be roughly 300 to 400 uV. The output from the INA128 however is roughly 2.7V. Dividing by the gain, the thermocouple voltage is thus implied to be 8mV, which is terribly high.
A few notes about my setup:
1) I am acquiring the data using a USB-DAQ from National Instruments, with LabView. The acquisition is being performed by the DAQ as a "Differential" setup, and it is designated as such in LabView.
2) The ground for the entire circuit is the AI Ground for the DAQ (NI-USB 6211) and this is NOT earth ground. I had read that I should not tie this ground to earth ground for a differential measurement.
3) The load resistor on the INA128 is 10kohm as per the datasheet.
4) To ensure stability at the thermocouple inputs, I have added a 10uF resistor between the leads.
5) My V+ is 12V and my V- is -12 V.
6) The entire arrangment is on a two-sided PCB.
In LabView, I take continuous samples over a given time interval and then divide by that number of samples to obtain and average Vo. This Vo is then divided by the gain, and multiplied by 1 000 000 to convert it from Volts to microvolts. This new number in microvolts is then put through the appropriate Type-T polynomial to convert voltage to temperature (this has been checked repeatedly to make sure the math and polynomial values are correct).
Any advice as to why the output voltage is not representative of my input voltage would be greatly appreciated. I am new to this forum, so if I have not included enough information or if anything was unclear I apologize. Please ask and I will provide more information if necessary.
Thank you,
Mike
