INA228 datasheet(13/50 Pages) TI

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7.3.2 Internal Measurement and Calculation Engine

The current and charge are calculated after a shunt voltage measurement, while the power and energy are

calculated after a bus voltage measurement. Power and energy are calculated based on the previous current

calculation and the latest bus voltage measurement. If the value loaded into the SHUNT_CAL register is zero,

the power, energy and charge values will be reported as zero.

The current, voltage, and temperature values are immediate results when the number of averages is set to one

as shown in Figure 7-2. However, when averaging is used, each ADC measurement is an intermediate result

which is stored in the corresponding averaging registers. Following every ADC sample, the newly-calculated

values for current, voltage, and temperature are appended to their corresponding averaging registers until the

set number of averages is achieved. After all of the samples have been measured the average current and

voltage is determined, the power is calculated and the results are loaded to the corresponding output registers

where they can then be read.

The energy and charge values are accumulated for each conversion cycle. Therefore the INA228-Q1 averaging

function is not applied to these.

Calculations for power, charge and energy are performed in the background and do not add to the overall

conversion time.

Q1 =

i1 x t1

Q.. =

i.. x t..

Q.. =

i.. x t..

Q.. =

i.. x t..

Q.. =

i.. x t..

E.. =

p.. x t..

E.. =

p.. x t..

E.. =

p.. x t..

E.. =

p.. x t..

E1 =

p1 x t2

Power register

Energy register

Charge register

ADC,

Temperature, Current,

Voltage

Figure 7-2. Power, Energy and Charge Calculation Scheme

7.3.3 Low Bias Current

The INA228-Q1 features very low input bias current which provides several benefits. The low input bias current

of the INA228-Q1 reduces the current consumed by the device in both active and shutdown state. Another

benefit of low bias current is that it allows the use of input filters to reject high-frequency noise before the signal

is converted to digital data. In traditional digital current-sense amplifiers, the addition of input filters comes at

the cost of reduced accuracy. However, as a result of the low bias current, the reduction in accuracy due to

input filters is minimized. An additional benefit of low bias current is the ability to use a larger shunt resistor to

accurately sense smaller currents. Use of a larger value for the shunt resistor allows the device to accurately

monitor currents in the sub-mA range.

The bias current in the INA228-Q1 is the smallest when the sensed current is zero. As the current starts to

increase, the differential voltage drop across the shunt resistor increases which results in an increase in the bias

current as shown in Input Bias Current vs. Differential Input Voltage.

7.3.4 High-Precision Delta-Sigma ADC

The integrated ADC is a high-performance, low-offset, low-drift, delta-sigma ADC designed to support

bidirectional current flow at the shunt voltage measurement channel. The measured inputs are selected through

the high-voltage input multiplexer to the ADC inputs as shown in Figure 7-1. The ADC architecture enables

lower drift measurement across temperature and consistent offset measurements across the common-mode

voltage, temperature, and power supply variations. A low-offset ADC is preferred in current sensing applications

to provide a near 0-V offset voltage that maximizes the useful dynamic range of the system.

ADC conversion time for each input can be set independently by the VSHCT, VBUSCT, and VTCT bits in register

ADC_CONFIG, respectively, in the range of 50 µs to 4.12 ms. Furthermore, a sample averaging function in the

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INA228-Q1

SLYS022A – JUNE 2020 – REVISED JUNE 2021

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