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How can I connect an LMT70 temperature sensor (4Pins) to Arduino Mega board for atmospheric temperature measurement. LM35 sensor has got only 3 pins and connections are known. But LMT70 high precision sensor got 4 pins. How to connect it to arduino mega board Please help. Thank you.

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Every electronics component has a document that describes it. It is called a "datasheet".

This is the page for the LMT70: ti.com LMT70
The red text is for the datasheet as pdf document, or else you can use the tab "Online datasheet", and the datasheet can also be found under the tab "Technical documents".

According to the datasheet it works with 5V, you can use it with the Arduino Mega (which is a 5V Arduino board). Connect the analog output of the sensor to an analog input of the Arduino Mega.

If you don't use the T_ON, then the datasheets says: "Tie this pin to VDD if not used". That means you can connect T_ON to 5V.

Do you have the chip (the sensor) only, or do you have a module with the sensor on it ?

Since the sensor has an anlog output, the accuracy depends not only on the sensor but also on the Arduino and the Arduino voltage reference. If you need a highly accurate temperature measurement, then you better buy a digital temperature sensor. For example the Si7051.

I suggest to use the 2.56V Internal Analog Voltage Reference.
The sensor has 1375.2mV output at -55°C and 302.8mV at 150°C.

const float aref = 2.56;  // adjust it to the actual AREF voltage
int adc_value = analogRead(A0);
float voltage = float(adc_value) * aref / 1024.0;

float t1 = voltage - 302.8;  // get rid of voltage offset
float t2 = t1 * (150.0 + 55.0) / (1375.2 - 302.8); // match range
float temperature = 150 - t2;  // negate and offset.

When you measure the AREF pin with a good multimeter and fill in the correct voltage in the calculation in the sketch, then you can get 1°C accuracy for the temperature. Perhaps you can get near 0.5°C accuracy, but no more. The sensor is more accurate than the Arduino Mega.

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In situations like this, you always start with reading the device datasheet, understand how it works, how to communicate with the device, and then write your code to implement that communications protocol.

That strategy always works, regardless of the specifics of a given device.

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