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7.4.3 Filtering and Input Considerations

Measuring current is often noisy, and such noise can be difficult to define. The INA3221 offers several

filtering options by allowing conversion times and the number of averages to be selected independently in

the Configuration register. The conversion times can be set independently for the shunt- and bus-voltage

measurements as well, for added flexibility in configuring power-supply bus monitoring.

The internal ADC is based on a delta-sigma (ΔΣ) front-end with a 500kHz (±30%) typical sampling rate. This

architecture has good inherent noise rejection; however, transients that occur at or very close to the sampling-

rate harmonics can cause problems. These transient signals are at 1MHz and higher; therefore, the signals are

managed by incorporating filtering at the INA3221 input. High-frequency signals allow for the use of low-value

series resistors on the filter, with negligible effects on measurement accuracy. In general, filtering the INA3221

input is only necessary if there are transients at exact harmonics of the 500kHz (±30%) sampling rate that

are greater than 1MHz. Filter using the lowest-possible series resistance (typically 10Ω or less) and a ceramic

capacitor. Recommended capacitor values are 0.1μF to 1.0μF. Figure 7-8 shows the INA3221 with an additional

filter added at the input.

Load 2

Power Supply

(0V to 26V)

Ch 1

Ch 2

VIN 2

VIN+2

Ch 3

ADC

RFILTER

≤

CFILTER: 0.1- F to 1- F

Ceramic Capacitor

RFILTER

≤

CFILTER

Figure 7-8. INA3221 With Input Filtering

The INA3221 inputs are specified to tolerate 26V across the inputs. However, overload conditions are another

consideration for the INA3221 inputs. For example, a large differential-input scenario can be a short to ground

on the load side of the shunt. This type of event results in the full power-supply voltage applied across the

shunt, if supported by the power supply or energy-storage capacitors. Keep in mind that removing a short to

ground can result in inductive kickbacks that can exceed the 26V differential and common-mode rating of the

INA3221. Inductive kickback voltages are best controlled by zener-type transient-absorbing devices (commonly

called transzorbs) combined with sufficient energy-storage capacitance.

In applications that do not have large energy-storage electrolytic capacitors on one or both sides of the shunt, an

input overstress condition can result from an excessive dV/dt of the voltage applied to the input. A hard physical

short is the most likely cause of this event, particularly in applications without large electrolytic capacitors

present. This problem occurs because an excessive dV/dt can activate the INA3221 ESD protection in systems

where large currents are available. Testing has demonstrated that the addition of 10Ω resistors in series with

each INA3221 input sufficiently protects the inputs against this dV/dt failure up to the 26V device rating.

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INA3221

SBOS576C – MAY 2012 – REVISED SEPTEMBER 2025

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