ADS1018: Temperature sensing mode

Vrushali,

The ADS1018 conversion for temperature is relatively straightforward as explained on page 14 of the datasheet.

To use the device you would start a conversion with the device in temperature sensor mode (likely using a single-shot conversion). Then wait for the conversion to complete and then read back the conversion register.

The conversion register is 16 bits. To convert the temperature sensor data, you truncate the last four bits, using the first 12 bits for the temperature conversion. You could convert that conversion value to a number (with negative numbers represented in two's complement notation) and multiply that number by the LSB, which is 0.125C.

Joseph Wu

Vrushali,

The internal temperature sensor is integrated into the multiplexer of the ADC and requires a conversion cycle to output the data. However, even though the temperature sensor is part of the multiplexer, the measurement shouldn't be affected by the other connections of the multiplexer. It would not change the formula on page 14 of the datasheet. The internal temperature sensor is specified to ±1°C from 0°C to 70°C and ±2°C from –40°C to +125°C.

I would note that the internal temperature sensor measures the temperature of the ADS1018 on the die of the device. Because the device is soldered onto the board, the temperature will reflect board temperature at that site on the board, not the air temperature next to the device. If there are other devices on the board nearby, and they have a large power dissipation, their temperature may be measured as well.

Joseph Wu

Vrushali,

The answer to this depends on the proximity of the ADS1018 to the IGBT device. Again, the ADS1018 measures the board temperature where the ADS1018 is placed and if that is close to the IGBT, then it may be accurate. There should be good thermal connectivity between the ADS1018 and the IGBT. The internal temperature sensor uses the die temperature, so there is no specific lead or pin that is used for the temperature measurement, and all of the lead connections may act to thermally conduct heat to the device.

As an example, the ADS1118 (the 16-bit version of the ADS1018) is used in a thermocouple demonstration. For that application, the device is placed between the thermocouple contacts so that it can best measure the temperature there. For thermocouple applications, this measurement is critical, and it looks like this on the board:

However, a thermocouple application won't have the same requirements as a power transistor temperature measurement. The thermocouple measurement itself doesn't have any heat associated with it and the temperature is likely near ambient as long as there isn't much circuitry near the device.

For your application, I'm not sure how accurate you need to measure the temperature and how close it is to the device. In most power device applications, like this, I've seen a temperature sensor place between two power devices so that the temperature is closer to that seen in each of the devices.

Joseph Wu