The integrated ADC is a high-performance, low-offset, low-drift, delta-sigma ADC designed to support bidirectional current measurement. The measured inputs are selected through the high-voltage input multiplexer to the ADC inputs (see Figure 6-6). The ADC architecture enables lower drift measurement across temperature and consistent offset measurements across 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.

The INA740x measures the die temperature, current, and bus voltage. An internal temperature measurement is made before each current measurement. Temperature compensation is then applied to the current measurement to achieve low drift performance. The MODE bits setting in the ADC_CONFIG register permit selecting modes to convert only the current or bus voltage to further allow the user to configure the monitoring function to fit the specific application requirements. After an ADC conversion is complete, the converted values independently update in their corresponding registers where they can be read through the digital interface at the time of conversion end if no averaging is selected.

The conversion times for shunt voltage, bus voltage, and temperature inputs are set independently from 50 µs to 4.12 ms depending on the values programmed in the ADC_CONFIG register. The value for current is calculated after both the temperature and shunt voltage measurements are made. The total time to get the current measurement is the sum of the conversion times for these two parameters. Enabled measurement inputs are converted sequentially, which means 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 is complete, 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 the 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. The values from the device can be read at any time because data from the last conversion remains available until the next conversion is complete. The Conversion Ready flag is set after all conversions and averaging are complete.

The Conversion Ready flag (CNVRF) clears under these conditions:

• Writing to the ADC_CONFIG register (except for selecting shutdown mode); or
• Reading the DIAG_ALRT Register

While the INA740x 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 Internal Measurement and Calculation Engine. In triggered mode, the accumulation registers (ENERGY and CHARGE) are invalid, as the device does not keep track of elapsed time. For applications that require 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 INA740x conversion can be delayed by programming the CONVDLY bits in the CONFIG register in the range between 0 ms (no delay) and 510 ms. The resolution in programming the conversion delay is 2 ms. The conversion delay is set to 0 ms by default. Conversion delay can assist in measurement synchronization when multiple external devices are used for voltage or current monitoring purposes. In applications where 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.