[FAQ] ADS7128: Using the RMS module

The procedure for using the RMS module is outlined in the steps below:

1. Select the channel for the RMS computation using the RMS_CHID field in the RMS_CFG register.
2. Define the time over which the RMS is to be computed by configuring the RMS_SAMPLES field.
3. Start the RMS computation by setting RMS_EN to 1 in the GENERAL_CFG register.
4. The device starts computing the RMS result when the sample size defined by RMS_SAMPLES is converted on the analog input selected for RMS computation. An additional 40 samples must be converted to complete the RMS computation.
5. To monitor for when the RMS computation completes, poll the RMS_DONE bit in the SYSTEM_STATUS register. The ALERT pin can also be used for requesting an interrupt by configuring the ALERT_RMS bit in the ALERT_MAP register.
6. Read the RMS result, clear RMS_DONE bit and disable RMS module.

Shown in the table below is an example of an RMS computation and what is written to each register according to the steps outlined above, using analog input Channel 0, 4096 samples, No DC subtraction, in Autonomous Conversion mode.

Please reference data sheet to see what each individual bit in each register is configuring.

A similar flow chart may be used for different configurations. Different number of samples, input channels, sampling frequency, subtracting DC value, or any other available configurations can be achieved by setting the appropriate bits on the registers.

Manual mode follows a similar set of steps. The difference is you need to read the data each time to trigger a new conversion start.  As per datasheet section 8.4.2 "A new conversion is initiated on the ninth falling edge of SCL (ACK bit) when the last byte of output data is read." so you would need to read out the data for the number of RMS samples requested (1024, 4096, 16384, or 65536 in the configuration) plus an additional 40 conversions before the RMS_DONE flag will be set. In autonomous mode the ADC will be making conversions even if the master device is not reading them, and the RMS result will be available on the RMS_LSB and RMS_MSB registers once the necessary number of conversions have been made.

What we found to be a common missing step when using the RMS module is that to reinitiate another RMS calculation, the RMS_EN bit should be cleared (set to 0) and then set back to '1' for the next calculation in the GENERAL_CFG register.

Best Regards,

Angel