External system noise can significantly affect the ability of a comparator to accurately measure and detect whether input signals exceed the reference threshold levels and reliably indicate overrange conditions. The most obvious effect that external noise has on the operation of a comparator is to cause a false-alert condition. If a comparator detects a large noise transient coupled into the signal, the device can easily interpret this transient as an overrange condition.

External filtering helps reduce the amount of noise that reaches the comparator, and thus reduce the likelihood of a false alert from occurring. The tradeoff to adding this noise filter is that the alert response time is increased because of the input signal being filtered along with the noise. Figure 8-1 shows the implementation of an input filter for the device.

Figure 8-1 Input Filter

Limiting the input resistance this filter is important because this resistance can have a significant affect on the input signal that reaches the device input pins because of the device input bias currents. A typical system implementation involves placing the current-sensing resistor very near the device so that the traces are very short and the trace impedance is very small. This layout helps reduce the ability of coupling additional noise into the measurement. Under these conditions, the characteristics of the input bias currents have minimal affect on device performance.

As illustrated in Figure 8-2, the input bias currents increase in opposite directions when the differential input voltage increases. This increase results from a device design that allows common-mode input voltages to far exceed the device supply voltage range. With input filter resistors now placed in series with these unequal input bias currents, there are unequal voltage drops developed across these input resistors. The difference between these two voltage drops appears as an added signal that, in this case, subtracts from the voltage developed across the current-sensing resistor, thus reducing the signal that reaches the device input pins. Smaller-value input resistors reduce this effect of signal attenuation to allow for a more accurate measurement.

Figure 8-2 Input Bias Current vs. Differential Input Voltage

For example, with a differential voltage of 10 mV developed across a current-sensing resistor and using 20-Ω resistors, the differential signal that actually reaches the device is 9.85 mV. A measurement error of 1.5% is created as a result of these external input filter resistors. Use 10-Ω input filter resistors instead of the 20-Ω resistors to reduce this added error from 1.5% down to 0.75%.