SLYS021A January 2021 – May 2022 INA228
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 range of 1x to 1024x is implemented and can be selected by the AVG bits in ADC_CONFIG register. The sample conversion time and the averaging are a part of the integrated digital filter described in Section 184.108.40.206.
The INA228 can measure the shunt voltage, bus voltage, and die temperature, or a combination of any based on the selected MODE bits setting in the ADC_CONFIG register. This permits selecting modes to convert only the shunt voltage or bus voltage to further allow the user to configure the monitoring function to fit the specific application requirements. When no averaging is selected, once an ADC conversion is completed, the converted values are independently updated in their corresponding registers where they can be read through the digital interface at the time of conversion end. The conversion time for shunt voltage, bus voltage, and temperature inputs are set independently from 50 µs to 4.12ms depending on the values programmed in the ADC_CONFIG register. Enabled measurement inputs are converted sequentially so the total time to convert all inputs depends on the conversion time for each input and the number of inputs enabled. When averaging is used, the intermediate values are subsequently stored in an averaging accumulator, and the conversion sequence repeats until the number of averages is reached. After all of the averaging has been completed, the final values are updated in the corresponding registers that can then be read. These values remain in the data output registers until they are replaced by the next fully completed conversion results. In this case, reading the data output registers does not affect a conversion in progress.
The ADC has two conversion modes—continuous and triggered—set by the MODE bits in ADC_CONFIG register. In continuous-conversion mode, the ADC will continuously convert the input measurements and update the output registers as described above in an indefinite loop. In triggered-conversion mode, the ADC will convert the input measurements as described above, after which the ADC will go into shutdown mode until another single-shot trigger is generated by writing to the MODE bits. Writing the MODE bits will interrupt and restart triggered or continuous conversions that are in progress. Although the device can be read at any time, and the data from the last conversion remains available, the Conversion Ready flag (CNVRF bit in DIAG_ALRT register) is provided to help coordinate triggered conversions. This bit is set after all conversions and averaging is completed.
The Conversion Ready flag (CNVRF) clears under these conditions:
While the INA228 device is used in either one of the conversion modes, a dedicated digital engine is calculating the current, power, charge and energy values in the background as described in Section 7.3.2. In triggered mode, the accumulation registers (ENERGY and CHARGE) are invalid, as the device does not keep track of elapsed time. For applications that need critical measurements in regards to accumulation of time for energy and charge measurements, the device must be configured to use continuous conversion mode, as the accumulated results are continuously updated and can provide true system representation of charge and energy consumption in a system. All of the calculations are performed in the background and do not contribute to conversion time.
For applications that must synchronize with other components in the system, the INA228 conversion can be delayed by programming the CONVDLY bits in CONFIG register in the range between 0 (no delay) and 510 ms. The resolution in programming the conversion delay is 2 ms. The conversion delay is set to 0 by default. Conversion delay can assist in measurement synchronization when multiple external devices are used for voltage or current monitoring purposes. In applications where an time aligned voltage and current measurements are needed, two devices can be used with the current measurement delayed such that the external voltage and current measurements will occur at approximately the same time. Keep in mind that even though the internal time base for the ADC is precise, synchronization will be lost over time due to internal and external time base mismatch.