SBASAY5 June 2024 ADS8681W
PRODUCTION DATA
The ADS868xW features an input alarm and an AVDD alarm. For the input alarm, the low and high threshold values are user programmable and an input outside the specified range activates the alarm. The input alarm also incorporates hysteresis, which is also user programmable. This section focuses on the application of the user programmable input alarm thresholds and hysteresis using a negative-temperature-coefficient (NTC) thermistor for temperature sensing.
When temperature rises, the NTC thermistor resistance decreases. When temperature falls, the NTC thermistor resistance increases. Figure 8-3 shows a diagram the NTC thermistor placed in a voltage divider circuit with a sensing resistor. As indicated in this figure, Vout increases as temperature increases and decreases as temperature decreases. In a temperature-sensitive application, monitor if the temperature is too high or low. Program and adjust the input alarm thresholds to alert whether a system has gotten too hot in operation or is running abnormally cool.
When the temperature fluctuates near the alarm thresholds, the temperature potentially goes beyond the programmed limits and falls back into the thresholds multiple times in succession. To prevent false triggering of the alarm resulting from noise or interference, apply and adjust the hysteresis that is applied to the signal.
Determine if the temperature is near the alarm high threshold. Ambient noise potentially causes the voltage measured to exceed this threshold momentarily. However, the actual temperature does not exceed the predetermined limit, and therefore causes a false alarm. If the temperature exceeds the predetermined limit and the voltage momentarily exceeds the alarm high threshold, no alarm is issued when an alarm is needed. This condition is caused by the ambient noise triggering the voltage measured to fall below the alarm high threshold. By applying and properly adjusting the amount of hysteresis, these situations are prevented. Furthermore, the noise immunity of the input alarm feature is improved to more accurately represent the temperature conditions of the system. Figure 8-4 depicts the alarm functionality when hysteresis is applied.