INA226: INA226 - ADC Resolution Spec Req

Hello Rohith,

Welcome to the E2E Amplifiers forum.

1. I do not know what this ADC reference voltage is. It is set internally in the device and cannot be adjusted. It is most likely Vs/2, but knowing the reference voltage is not critical to use of INA226 or most of our digital current/power monitors. The INA226 will always sense the differential shunt voltage (Vsense) regardless of what the common-mode voltage (VCM). Of course the device only operates within spec for a VCM range of 0V to 36V.

2. The ADC offset error is simply the input offsets we specify for the shunt and bus voltage measurements (see below a picture from the datasheet). So the ADC offset when measuring shunt voltages is maximum ±10µV (or 4 LSB) and offset when measuring bus voltages is maximum ±7.5mV (or 6 LSB).

3. If you wish to ensure the ADC (thus INA226) works to its best performance then you will want to follow the simple Layout guidelines example in datasheet section 10. As for all amplifiers using a decoupling bypass capacitor of 0.1µF will ensure a stable power rail internally in the device. The capacitors should be placed as close as possible to the VS pin.

Hope this helps.
Peter Iliya

Hey Rohith,

When thinking about error, you need to consider it in terms of shunt voltage (Vshunt) and input offset voltage (Vos). Additionally, you need to understand what the minimum and maximum currents you want to sense. You will most always have the greatest error at the minimum sense current because offset error will begin to dominate. Error% = 100*Vos/Vshunt

1. What is the minimum current you want to sense and at what error?
2. What is the largest current you need to sense?
3. What is maximum allowable shunt voltage your system can tolerate?

The questions above all relate to each other and it's hard to propose a solution without knowing all of these bounding factors.
If you want better accuracy at lower current, then you could choose a larger shunt resistor. Although, this will increase your shunt power dissipation at higher currents and you need to ensure that Vshunt <= 81.92mV when ILOAD= max current because this is maximum voltage INA226 can measure.

The INA226 and INA233 are the most accurate digital current shunt monitors with 10uV max input offset, so your only option is to increase shunt resistance or employ a calibration procedure where you make a measurement of a known current, record the measurement, and then subtract this offset from all your other measurements.

For the EVM, the 50 pin connector (J1) is what the SM-USB-DIG physically connects to and this interface card is included in the INA226EVM Kit. You can view the evm kit contents in section 1.1 of EVM User's Guide.

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Why do you want to filter the input shunt voltage? Is your sense current noisy? The datasheet only recommends this filter is the current is noisy, current contains harmonic transients at 500kHz or harmonics of this, or there are very fast voltage transients at the input that could damage internal ESD cells. Even if the current is noisy, you can increase the conversion time or also the number of averages the INA226 measures. These values are programmed by user into INA226. See section 7.4.2.

If you must need the input filter, then the value of the input resistors (R10 and R11) do not matter as long as they are less than 10-Ohms. We do not have input impedance for ADC, but it will not load your VBUS line. The input bias current (IB) is limited to 10uA per input pin. You are dealing with amps of current so it will not load down your bus.

The input impedance of the VBUS pin is 830 kOhm as shown in datasheet Electrical Characteristics table. Thus, the resistor divider (R125, R13i, and R14i) will actually load down this 830 kOhm internal impedance. So ideally the VBUS node should equal 400V*(4.75/(300+4.75)) = 6.2346V, but VBUS will actually equal 400V*(Rx/(300+Rx)) = 6.1996V, where Rx = 830||4.75.

Hope this helps.
Peter