Measuring current is often noisy and such noise can be difficult to define. The INA4235 offers several options for filtering by allowing the conversion times and number of averages to be selected independently in the Configuration register (0h). The conversion times can be set independently for the shunt voltage and bus voltage measurements to allow added flexibility when configuring the monitoring of the power-supply bus.

The internal ADC is based on a delta-sigma (ΔΣ) front-end with a 500kHz (±0.5% max) 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 1MHz and higher and can be managed by incorporating filtering at the device input. The high frequency enables the use of low-value series resistors on the filter with negligible effects on measurement accuracy. In general, filtering the device input is only necessary if there are transients at exact harmonics of the 500kHz (±0.5% max) sampling rate (greater than 1MHz). Filter using the lowest possible series resistance (typically 100Ω or less) and a ceramic capacitor. Recommended values for this capacitor are between 0.1µF and 1µF. Figure 8-1 illustrates the device with a filter added at the input.

Figure 8-1 Input Filtering

Overload conditions are another consideration for the device inputs. The device inputs are specified to tolerate 26V across the inputs. A large differential scenario can be a short to ground on the load side of the shunt. This type of event can result in the full bus power-supply voltage across the shunt (as long the power supply or energy storage capacitors can support this voltage). Removing a short to ground can result in inductive kickbacks that can exceed the 26V differential and 48V common-mode rating of the device. Inductive kickback voltages are best controlled by Zener-type, transient-absorbing devices (commonly called transzorbs) combined with sufficient energy storage capacitance. The Current Shunt Monitor with Transient Robustness Reference Design describes a high-side, current-shunt monitor used to measure the voltage developed across a current-sensing resistor and how to better protect the current-sense device from transient overvoltage conditions.

In applications that do not have large energy storage electrolytics 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, and the excessive dV/dt can activate the ESD protection in systems with large currents. Testing demonstrates that the addition of 10Ω resistors in series with each input of the device sufficiently protects against dV/dt failures up to the 48V rating of the device. Selecting these resistors in the range noted has minimal effect on accuracy.