VS

GND

OUT

ALERT

IN+

IN-

Load

Supply

(0 V to 36 V)

2.7 V to 5.5 V

RPULL-UP

10 k

RLIMIT

RESET

+

INA301

CBYPASS

0.1 PF

LIMIT

CFILTER

RFILTER

”10

19

INA301

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SBOS713A – SEPTEMBER 2015 – REVISED FEBRUARY 2016

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7.4 Device Functional Modes

7.4.1 Input Filtering

External system noise can significantly affect the ability of a comparator to accurately measure and detect

whether input signals exceed the reference threshold levels, thus reliably indicating an overrange condition. The

most obvious effect that external noise can have 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 can help reduce the amount of noise that reaches the comparator and 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 as well as the noise. Figure 39 shows the implementation of an input

filter for the device.

Figure 39. Input Filter

Limiting the amount of input resistance used in this filter is important because this resistance can have a

significant affect on the input signal that reaches the device input pins resulting from the device input bias

currents. A typical system implementation involves placing the current-sensing resistor very near the device so

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 40, the input bias currents increase in opposite directions when the differential input

voltage increases. This increase results from the design of the device 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 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.