TMP35/TMP36/TMP37

Rev. F | Page 17 of 20

the temperature sensor is reached. Potentiometer P1 is required

to calibrate the absolute accuracy of the AD654. The table in

Figure 34 illustrates the circuit element values for each of the

three sensors. The nominal offset voltage required for 0 Hz

output from the TMP35 is 50 mV; for the TMP36 and TMP37,

the offset voltage required is 100 mV. For the circuit values

shown, the output frequency transfer characteristic of the

circuit was set at 50 Hz/°C in all cases. At the receiving end, a

frequency-to-voltage converter (FVC) can be used to convert

the frequency back to a dc voltage for further processing. One

such FVC is the AD650.

For complete information about the AD650 and the AD654,

consult the individual data sheets for those devices.

DRIVING LONG CABLES OR HEAVY CAPACITIVE

LOADS

Although the TMP3x family of temperature sensors can drive

capacitive loads up to 10,000 pF without oscillation, output

voltage transient response times can be improved by using a

small resistor in series with the output of the temperature

sensor, as shown in Figure 35. As an added benefit, this resistor

forms a low-pass filter with the cable capacitance, which helps

to reduce bandwidth noise. Because the temperature sensor is

likely to be used in environments where the ambient noise level

can be very high, this resistor helps to prevent rectification by

the devices of the high frequency noise. The combination of this

resistor and the supply bypass capacitor offers the best protection.

TMP3x

0.1µF

GND

750Ω

LONG CABLE OR

HEAVY CAPACITIVE

LOADS

Figure 35. Driving Long Cables or Heavy Capacitive Loads

COMMENTARY ON LONG-TERM STABILITY

The concept of long-term stability has been used for many years

to describe the amount of parameter shift that occurs during

the lifetime of an IC. This is a concept that has been typically

applied to both voltage references and monolithic temperature

sensors. Unfortunately, integrated circuits cannot be evaluated

at room temperature (25°C) for 10 years or more to determine

this shift. As a result, manufacturers very typically perform

accelerated lifetime testing of integrated circuits by operating

ICs at elevated temperatures (between 125°C and 150°C) over a

shorter period of time (typically, between 500 and 1000 hours).

As a result of this operation, the lifetime of an integrated circuit

is significantly accelerated due to the increase in rates of reaction

within the semiconductor material.