Using digital temperature-sensing methods in a noisy environment is difficult, if not downright impossible. Noise is easily coupled into the temperature-sensing circuit and can result in a large temperature error. In fact, a very noisy environment can swamp the temperature measurement completely, rendering any result useless.
To make matters worse, adding an adequate filter in front of the digital temperature sensor for such applications has been impossible up until now. That's because the capacitance and resistance associated with a filter interfere with the measurement technique used, causing offsets that lead to errors in the temperature result. The advent of series-resistance cancellation techniques embedded in the latest digital sensors means this is no longer the case.
For example, adding a simple R-C-R filter between the D+ and D− inputs of the ADT7461 reduces or eliminates the effect of noise on the temperature measurement (see the figure). The remote sensor is a diode-connected standard pnp transistor. Its emitter is connected to the D+ pin on the ADT7461, and its base and collector connect to the D− input. The filter consists of two resistors with values of 100 Ω and a 1-nF capacitor.
Place the filter as near as possible to the D+ and D− inputs, and connect it as shown. This filter has a cutoff frequency of 1.6 MHz. Without a filter in place, temperature measurement can be in error by 80°C or more! With a filter, it's possible to drop error to below 1°, making this circuit configuration ideal for noisy environments.
Other values of resistance and capacitance can be used to build a filter with the required cutoff frequency. The value of the capacitance is limited to a maximum of 2.2 nF, because any higher value affects the temperature measurement. Likewise, the resistance value is limited to a total of 3 kΩ for the D+ and D− paths combined.
Normally, any resistance in the path between the remote sensor and a standard digital temperature sensor affects the accuracy of the temperature measurement. Typically, there's a 0.5°C offset in the measured temperature value per ohm of parasitic resistance in series with the sensor.
However, the ADT7461 automatically cancels out the effect of up to 3 kΩ of series resistance. This attribute allows a filter to be built between the ADT7461 and the remote sensor. The filter in the figure shows 100 Ω of resistance on both the D+ and D− paths to the external sensor. No user calibration is required. Also, any resistance associated with the pc-board tracks or other connectors will be cancelled, allowing the remote temperature sensors to be placed some distance from the ADT7461.
Figure 2 shows how effective the filter is in a very noisy environment. The noise in this instance was a 100-mV square wave, applied with the same phase to both the D+ and D− paths. Without a filter in place, the ADT7461 temperature measurement is in error by up to 50°C. This level of noise in the system makes use of a temperature-monitor IC without a filter impossible. By employing a filter, the error is reduced to less than 1°, below the specified ±1°C accuracy of the device. Similarly, the filter can reduce differential noise, where the noise on the D+ and D− paths isn't in phase.