´

Power Register

Current Register

I C

2

Interface

Voltage Register

GND

BUS

ADC

V

I

A0

ALERT

A1

SDA

SCL

C

0.1 F

BYPASS

m

Load

Alert Register

V

(Supply Voltage)

S

Power Supply

(0 V to 28 V)

R

10

FILTER

W

£

R

10

FILTER

W

£

C

0.1 F to 1 F

Ceramic

Capacitor

FILTER

m

m

IN-

IN+

INA231

www.ti.com

SBOS644 – FEBRUARY 2013

Filtering and Input Considerations

Measuring current is often a noisy task, and such noise can be difficult to define. The INA231 offers several

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

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

measurements to allow added flexibility in configuring the monitoring of the power-supply bus.

The internal ADC is based on a delta-sigma (

ΔΣ) front-end with a 500-kHz (±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 signals are at 1 MHz and higher; therefore, manage them by

incorporating filtering at the input of the INA231. The high frequency enables the use of low-value series resistors

on the filter with negligible effects on measurement accuracy. In general, filtering the INA231 input is only

necessary if there are transients at exact harmonics of the 500-kHz (±30%) sampling rate (greater than 1 MHz).

Filter using the lowest possible series resistance (typically 10

Ω or less) and a ceramic capacitor. Recommended

values for this capacitor are 0.1

μF to 1.0 μF. Figure 22 shows the INA231 with an additional filter added at the

input.

Overload conditions are another consideration for the INA231 inputs. The INA231 inputs are specified to tolerate

30 V across the inputs. A large differential scenario might be a short to ground on the load side of the shunt. This

type of event can result in full power-supply voltage across the shunt (as long as the power supply or energy

storage capacitors support it). Keep in mind that removing a short to ground can result in inductive kickbacks that

could exceed the 30-V differential and common-mode rating of the INA231. 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 electrolytics on one or both sides of the shunt, an input

overstress condition may 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 with no large electrolytics present. This problem

occurs because an excessive dV/dt can activate the ESD protection in the INA231 in systems where large

currents are available. Testing has demonstrated that the addition of 10-

Ω resistors in series with each input of

the INA231 sufficiently protect the inputs against this dV/dt failure up to the 30-V rating of the INA231. Selecting

these resistors in the range noted has minimal effect on accuracy.

Figure 22. INA231 with Input Filtering

Copyright © 2013, Texas Instruments Incorporated

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