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About ADS1220EVM

user4967788
Prodigy 70 points
Other Parts Discussed in Thread: ADS1220, ADCPRO, ADS1220EVM

When I was learning to use ADS1220, I tested with ADS1220evm, and my goal was to use 2-wire to measure the temperature of thermistor. The configuration is: R30 welding 0 ohm resistance, R3 welding 3.241K ohm resistance. In addition, I used the jumper cap to connect A3- and REF-, as well as A3+ and REF +, with a resistance of 110-ohm on both ends of the RTD. Wiring method as shown in picture, I am in the process of wiring series a current meter, if used to measure the flow of electric current is 500 microamps, but in the process of running ADCPro, I did not measure the current. I don't know what's going on here. Could you please give me some answers? Looking forward to your reply. thank your.

  • 0
    TI__Guru** 113765 points

    Hi user4967788,

    I would suggest you follow your routing and compare to the schematic in the ADS1220EVM user's guide.  You will find a couple of issues.  One is you are attempting to measure current where no current exists due to your connection.  You have AIN3 output (current source) connected directly to the reference resistor.  No current will flow through the RTD because it is shorted by the jumper.  Your current should flow from AIN3 which connects to the EXC- instead of EXC+.

    You should also change to reference to the other external reference input, and you will need to connect your RTD differently if you actually want to measure the RTD as the connection you now have does not connect to the analog inputs.

    As the ADS1220EVM provides for many different sensor types and combinations I would suggest printing out the schematic and using colored pencils or highlighters and trace out on the schematic the path and then connect your wiring appropriately.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 70 points
    Hi Bob B
    I really found the problem, according to what you have said, and I also read the data, but now it is a new problem, I set the gain of 8, when I am through sampling, read to the AD, I discovered that the AD value change is bigger, the error is relatively large, the calculated value has a great influence on me. The temperature range is also larger. What should I do? Looking forward to your reply. thank you
  • 0 in reply to user4967788
    TI__Guru** 113765 points

    Hi user4967788,

    When applying gain, the code value will increase because the full-scale range decreases.  When analyzing the data make sure you use the proper LSB size for your calculation.  The time domain plot makes this correction for you if you have entered the correct reference voltage into the reference tab of ADCPro, but when reviewing raw codes you need to manually make this calculation.

    When using the 3.24k bias resistor for the reference and the same current is used to excite both the reference resistor and the RTD, the current term drops out of the equation as is discussed in the RTD apps section of the ADS1220 datasheet as the measurement becomes ratiometric.  In other words the result code is the ratio of the RTD to the reference resistor.  Equation 40 on page 56 of the datasheet shows the result code is a ratio of the RTD divided by the reference resistor times the gain.  When applying gain, the result code will be much larger.

    Also consider that any noise that is picked up on the wiring from EMI/RFI sources will also be gained.  As the input filter resistors are 0  ohm, there is no RC filtering to filter out the noise or preventing aliasing so changing the filter resistors from 0 ohm to a larger value will help.  I would use a value in the range of 1 to 2k to see if it helps with the noise.

    Best regards,

    Bob B