This FAQ is meant to explain why the INA3221 and INA219 devices are marked as high side only and in which situations they can be useful in other configurations.
The problem:
The datasheet shows the readable absolute maximum range (VCM_max) as -0.3V to 26V, which would indicate the device can read into and below ground. This generates confusion as to why this part is marketed as an high side device only.
The solution:
The INA3221 has an ADC that can measure up to 26V on each input channel. This is accomplished by measuring the voltage across the VIN+ side of the channel. When the part is configured as a high side application, this pin is directly connected to the Vs rail and therefore cuts the requirement for an extra pin on the device to measure the bus supply. The datasheet clearly specifies that although the common range is +/-26V, using a differential measurement, the voltage at each of these pins must not go below -300mV. This describes the -26V as a short circuit to ground in which, depending on the current capability of the power supply and the electrolytic storage capacitors, a fault could result in the full power supply voltage applied across the shunt resistor.
The valid configurations for a differential measurement are: [(VIN+)-(VIN-)]=(26-0) = 26V or [(VIN-)-(VIN+)]=(0-26) = -26V
Partially supported configurations and what to look for before recommending this part:
The INA3221 can read both positive and negative values with a range of +/-163.5mV and unlike the analog parts it doesn’t require any offset voltage in order to do so. The only consideration is that the voltage at each pin never goes below -300mV. It should be expressed that if a fault occurs, in which the voltage generated across the shunt resistor goes lower than -300mV the customer is required to add clamping devices or device failure will occur. Internally the device has an ADC that measures the bus voltage. In order to save a pin TI has decided to internally connect it to the VIN+ pin, which on a high side measurement, is directly connected to VBUS. Low side measurements are still possible, however, since this pin now sits close to ground potential the device will only be able to operate as a current sense amplifier. Alerts can still be configured if a given current limit threshold is reached, but power and voltage alerts will no longer work.
There is no application to measure negative currents with these devices, however many computer systems have the requirement to measure small negative voltages such as -1.2 or -1.8V and I was approached by a customer inquiring if our part was able to do it. Negative voltages require differential measurement. Under this configuration one of the leads on the sense resistor is connected to ground and follows to the load, completing the circuit to the negative rail. In a differential configuration, either input can be connected to either lead of the sense resistor depending on the designer needs. Connecting VIN+ to GND and VIN- to the load side of the sense resistor will return positive values for the current flowing to the load, while reversing the connections will return negative values. In both configurations the pins are driven below ground with Vsense = Rsense*Isense and therefore considerations must be taken to not exceed 300mV below ground. Since the part has an active range of 163.5mV, this configuration supports an overload of about 180%. Finally, since both inputs are closed to GND, bus voltage measurement will not work and again the part will be good only as a current sense amplifier.
The part itself can’t measure the voltage on systems using either a low side or a negative rail configuration, as discussed above. And for single input parts such as the INA226 this is final. On the INA3221 we have multiple input channels and each ADC can be used to conveniently measure a range of input voltages within the allowable +/-163.5mV range. If the customer understands the limitations associated with the bias current of 10uA per input (20uA total) it is possible to use additional channels on the part as an ADC to measure such voltages. In fact, it is possible to use the parts to measure AC signals using this very same configuration. In order to do so we require a voltage divider in which the lower resistor is minimally affected by the 20uA bias current. Ideally we would use a standard voltage divider buffered by an OP-AMP, but in situations where the output voltage is low and stable, such as -5V it is possible to use a voltage divider of low impedance, such that the total current across the bus is 90% higher than the bias currents. So 200uA or higher, would prove a good compromise between power dissipation and accuracy. For systems of high or variable voltage, such as AC waveforms, the OPAMP is mandatory as a decrease in the voltage will result in a lower proportion of the current in the resistive divider and therefore error would increase as the voltage decreases.
Both positive and negative currents are fully supported providing that the device is used in a high side sense configuration with the voltage to each pin not exceeding the allowable -0.3 to 26V or -26 to 26V differential. Other measurements such as AC, low side or negative rails are supported, but require additional protection against fault conditions. Depending on the current capability of the power supply and the electrolytic storage capacitors, a fault could result in the full power supply voltage applied across the shunt resistor. Under the measurement of positive currents the device pins can go all the way up to 26, with a total swing of 26.3V, representing an overload of 161 times the nominal current. Excluding inductive kickback voltages, that can be limited by employing TVS diodes and, where applicable, suitable DC Link capacitors with low ESR, this should cover most failure situations typically involved. However, in AC applications or those on which the current can go negative, the device must be clamped to 300mV below ground, effectively limiting the overload capability of the device to 1.83x the nominal negative current value and therefore requiring additional protection methods to avoid part failure, whenever applicable.
Note: All the tests were carried out with an INA3221 but should apply to similar devices.
