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TMP61-Q1: Design guide

Part Number: TMP61-Q1
Other Parts Discussed in Thread: TMP61

Team, I'm designing on the Temp sensor Acquistion circuit. Been studying on different designs for a while. Mind is foggy now. Please guide. 

www.ti.com/.../sboa506.pdf

p/n: TMP6131QLPGQ1

Design Specs:

Operating Ambient (Max): +85ºC

Measurement Temperature: 0ºC to +125ºC or +25ºC to +125ºC

Supplies Available: +12V, +3.3V, (+1.5 V reference can be additionally added to design if needed)

ADC: ADS7138QRTERQ1

Note: +-1ºC error is acceptable. Also To Temp sensors, Will be attaching a wire of 30cm on both the terminals. Please guide. 

  • Hi Mohammed,

    The circuit design guide you shared will help when you use low-resolution ADCs (usually one that is self-integrated in an MCU). My first recommendation would be to make use of the thermistor design tool. Using this tool, you can determine the effective resistance of the TMP61-Q1 given your supply and using a 10k ohm bias resistor--with your conditions, you should use the 3.3 V supply since the Vmax across the thermistor is 5.5 V.

    Using your setup conditions and the thermistor effective resistance values, you can then determine the dynamic range of your signal accross temperature and calculate your resolution. This yields roughly about 0.2 C/bit at 125C. Depending on the noise floor of your ADC, you can make the decision of using the amplifier circuitry to make use of all of the resolution bits that you have available from your ADC. 

    I should also add that while the thermistor is rated for +-1.5% effective R tolerance accross temperature, you can implement the post-processing techniques described in this app note to achieve higher temperature accuracy. Keep in mind that adding signal processing circuitry (amplifier and R network) will introduce additional error into your system which can be determined from the non-linearity and GE of the opamp and the PPM and tolerance of the R network.

    Having 30 cm wires should be ok especially if you apply a 1-point calibration offset. This offset will get rid of any series resistance error from the cables as well as error from the TMP61-Q1 manufacturing variation. 

    Best regards,

    Simon Rojas

  • Thanks for the Response. 

    What all hardware is required, is it just simple Rbias resistor and TMP61 directly connected to ADC or A signal chain is required?

    if signal chain is required, please guide on the circuit design approach and what parameters to be taken care during design and what percentage precision components to be used? 

    Can you please guide in design steps. like step1, step2, step3 with actual tested circuit design or an application note.

    FYI, there is no local MCU, only ADC. 

  • Mohammed,

    In order to use TMP61 you need to bias it with either a voltage divider or a current source as seen below:

    In this configuration, you can calculate temperature by referring to the "4th order polynomial TMP vs Vdc" page found in the thermistor design tool.

    The thermistor can be directly connected to an ADC in that configuration. Doing any signal conditioning will depend on your requirements--in your case, if you are trying to hit +-1C with a 12 bit ADC, I would recommend implementing signal conditioning since your resolution is about 0.2C. All of the steps to design the amplifier are given in the document you highlighted (link here for reference). This includes component list and simulation results. You can then use the gain equation along with the thermistor design tool to calculate temperature.

    Best regards,

    Simon Rojas