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I am trying to connect an analog temperature sensor LMT84 (http://www.ti.com/product/lmt84) to a micro controller (3.3VDC) using its ADC pin which is 1.4V tolerant (max). My Vcc is 3.3V, ADC max input is 1.4V and temperature IC minimum voltage is 1.5V. Two options I could think of is, either to place a voltage divider at the input of temperature IC source or at the output of temperature IC. Schematics shown below

Option 1 Schematic

schematic

simulate this circuit – Schematic created using CircuitLab

Option 2 Schematic

schematic

simulate this circuit

Can someone please help me with choosing the correct schematic (considering the pros and cons) and calculate the resistance values of R1 and R2 (internal input impedance of the Microcontroller ADC is high i.e 200K approx).

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    \$\begingroup\$ Have I misread the datasheet or is the output voltage in the range of 138 mV to 1299 mV, so inside your ADC range? \$\endgroup\$ – Arsenal Jul 23 '18 at 11:40
  • \$\begingroup\$ 200K is not all that high input impedance. You sure it's that low? \$\endgroup\$ – Spehro Pefhany Jul 23 '18 at 12:10
  • \$\begingroup\$ @Arsenal I was under the assumption that the output is proportional to the input (Vcc). \$\endgroup\$ – Zacson Jul 23 '18 at 18:27
  • \$\begingroup\$ @SpehroPefhany I did some measurement and it seems to 200K approx. \$\endgroup\$ – Zacson Jul 23 '18 at 18:27
  • \$\begingroup\$ So that means that for 10 bit measurements you should not have a source impedance more than around 200 ohms. \$\endgroup\$ – Spehro Pefhany Jul 23 '18 at 18:35
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It appears that your are confusing some part of the specifications for the LMT84. According to its datasheet on pages 9 and 10, the normal operating output for the LMT84 is 183mV to 1299mV which is within the 1.4V that the ADC supports. This chip will output this voltage as long as supply voltage is within the recommended 1.5V to 5.5V and the temperature is within its rated range of -50 to 150°C.

Your resistors as shown are not required and should be removed. However depending on the total capacitance load placed on the output of the LMT84, you may need a resistor on the output as explained in the datasheet on page 12.

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  • \$\begingroup\$ Thanks CoreyF. I misread the datasheet. The MCU uses switched sampling input on an ADC, hence a 0.1uF (100nF) capacitor is recommended. So I may need 1.5K resistance in series. \$\endgroup\$ – Zacson Jul 23 '18 at 18:37

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