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ADS1120: Fill Level Sensor TMR + RTD PT1000 measurement

Part Number: ADS1120
Other Parts Discussed in Thread: ADS1220, , OPA391, TLV9001, ADS114S06, ADS124S06

Hello Team, 

can you provide a solution for customer request I have received. The customer want to measure the TMR resistor chain and the RTD with the same ADC. Can you provide the best fitting ADC for this and a schematic how to connect the TMR Sensor to the ADC? The ADC should be connect to an MCU like MSPM0 via SPI or I2C. The sample rate can be very low. <100 SPS.

Briefly summarized:

- TMR with open-drain output
- Switch between resistors and change it like a rheostat / potentiometer
- For operation 1.8-5Volt and approx. 1.5µA are needed.
- Problem: Open drain switches to GND - thus rheostat is connected to GND on one side
-> influence BIAS resistance at load by current consumption TMR

TMR are available with simple "switching function" and also with COS/SIN output or analog output.

Here a simple switching function is used: TMR1340 or comparable model of a competitor.


4-20mA Output for TMR Liquid-Level Sensors.pdf

  • Hi Jan,

    As per the solution introduced by the document, you need a circuit/op-amp to convert the resistance to a voltage output, then the voltage signal can be sampled by the ADC. 

    Regarding to ADC selection, what's the RTD configuration the customer wanted? for example, 2-wire, 3-wire and so on. This requirement determines how many input channels of the ADC the customer needs.



  • Hi Dale, 

    here some additional information: 

    We want to measure with the ADC to a variable (level) resistance and a thermal resistance (RTD).

    Primary is 2 wire. Here it is to be noted that the level is NOT a RTD but and a rheostat. So a variable resistance in the range of 800Ohm to approx. 2500Ohm with which the height of the measuring current has no influence on measurand (thermal measuring error by self-heating).

    If it is possible it would be good to keep the option open to 3 wires (for the additional RTD).

    as I read it here
    *A Basic Guide to RTD Measurements (Rev. A) ( on page 13, top two paragraphs;

    I see, you can omit the rbias if necessary. Then each of the Ain0-2 voltages relates directly to AVSS. So Ain2 to AVSS is always =0.
    I would switch off the PGA if necessary.
    With a LOW level in the tank I would have a resistance of 800Ohm. The resistance cannot drop any further in the normal state.
    For the Rtd the min value is -20°C. With PT1000 this is 921,6Ohm



  • Hi Jan,

    Thanks for providing more information. ADS1120 or ADS1220 is the best ADC to meet the customer's requirements (ADS1220 is 4-ch, 24-bit ADC):

    1. 3 input pins can be can be configured for RTD ratiometric measurement, for example, AIN0 for IDAC output, AIN1 and AIN2 for RTD measurement as a differential input, AIN3 for TMR measurement as a single-ended input, REFP0 and REFN0 for external reference input with a reference resistor.
    2. An amplifier should be used converted the resistance of TMR to a linear voltage output, for example,  OPA391 or TLV9001 depending on if low power consumption is needed or system calibration can be done.
    3.  When the single-ended signal from the amplifier output (TMR sensor) is measured on AIN3, the negative ADC input should be internally connected to AVDD by the internal switch within the multiplexer, this can be done by setting MUX[3:0] in Configuration Register 0. Note that the gain is limited to 1,2 and 4 because the internal PGA is bypassed under this situation.
    4. Either 2-wire RTD with low or high-side reference can be supported.
    5. If 3-wire RTD measurement is expected, more input channels are required, a single ADC with multiple input channels (ADS114S06/ADS124S06) will be a good solution.

    Let me know if you have any questions.

    Best regards,