## Changing the temperature coefficient of linear thermistors. | |

Linear thermistors ( RTDs or Resistance Temperature Detectors ) are used for temperature measurement or temperature compensation in analog circuits.

RTDs are based on a platinum or nickel-alloy film, giving them a positive temperature coefficient ( TC ) in the range of 150 ppm/°C to 7000 ppm/°C.

For temperature measurement circuits, the actual TC is not important - the circuit is designed for the RTD in use.

In temperature compensation applications, the TC is normally the most important parameter. RTD temperature coefficients comes in discrete values ( except the one you need ).

Putting a fixed resistor in series with a RTD will reduce the TC of a RTD to almost any value you want while maintaining its linearity.

As an example a logarithmic amplifier based on the voltage/current ratio of a P-N junction has a gain TC of 3333ppm/°C. This can be compensated by a RTD with a TC of 3333ppm/°C.

All calculations here assumes that the RTD temperature/resistance function is linear ( it is not, but very close ).

The resistance of a RTD is:

where:

Tr is the reference temperature, typically 25 °C or 0 °C.

T is current temperature.

TH_{Tr} is the RTD resistance at Tr.

TH_{TCR} is the RTD temperature coefficient in ppm/°C.

TH_{T} is the RTD resistance at T.

The resistance of a RTD in series with a resistor is:

where:

R is the series resistor.

RTH_{T} is the series combination resistance at T.

or:

where:

RTH_{Tr} is the series combination resistance at Tr.

RTH_{TCR} is the series combination temperature coefficient in ppm/°C.

RTH_{T} is the series combination resistance at T.

To find R:

Spread-sheet with TC calculations.

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