SBOA596 May   2024 ADS124S08 , INA326 , MSPM0G3507 , OPA387 , PGA900 , REF200

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Temperature Measure
  5. 2Key Challenge to Design a Temperature Monitor
  6. 3Proposed Designs for Temperature Monitor Based NTC or RTD
    1. 3.1 Voltage Excitation
    2. 3.2 Current Excitation
    3. 3.3 Ratio Measure
    4. 3.4 Integrated Designs
  7. 4Summary
  8. 5References

3.1 Voltage Excitation

Traditionally, bridge circuit was used with RTD and NTC temperature sensor for remove the bias voltage or common voltage and just sampling difference voltage by an instrumentation amplifier. The function between output of bridge and sensor resistor varies is non-linearity. Bridge amplifier can solve this issue. The bridge amplifier as dash block shown in Figure 3-1. Assume the reference voltage is VR and output of amplifier is Vo, then it is easy to receive Equation 1.

Discrete RTD Signal Conditions CircuitFigure 3-1 Discrete RTD Signal Conditions Circuit
Equation 1. Vo=-VR2R×ΔR

From Equation 1, the designer can see that the output voltage is linear with resistor varies. Assume the reference is 4.096V and resistor of RTD is 1000Ω under 0℃, the output can be approximately 7.9872mV for 1℃ temperature rise under approximately 2mA excite current. in actually design, the excite current can need less than 1mA for avoid self-heating and then the output voltage can small than 7.9872mV for 1℃ temperature rises. For get better resolution and precise, there need a post-amplifier to amplify the signals from bridge amplifier. Note, from Equation 1, the output voltage is inverting with varied resistor. So, first stage amplifier potential needs dual-power supply in case function abnormal.

Since the signals is quite small, for improved system precision, the key requirements for bridge amplifier is low offset and low offset drift. OPA387 is a very low offset amplifier and can meet this application. Maximum 2uV offset voltage can work to improve system precision. The second amplifier was used to inverter output of bridge amplifier to non-inverter and amplify the signals again. The third amplifier is optional in case the need to amplify further in actual design. Also, good idea if customer inserts the filter between amplifier circuits for reduced system noise and improve SNR of system.

Performance of reference is another factor which can potentially impact system precision according Equation 1. For getting best precision, REF54410 was proposed for exciting the sensor. The 0.02% precision and 0.8ppm/C drift can help to improve system precision. REF54410 is one parts of REF54 series reference with 4.096V output voltage. Customer can select other parts with different output voltage to meet specific requirements.