nRF52 SAADC power management, settling time

[Ladasky]

I am working on an application for the nRF52 Preview board, using SDK 0.9.2. I need to acquire about five analog samples from a single channel at roughly 100 Hz, then wait. Sometimes I will not need the ADC for several minutes. Leaving the ADC powered on when I am not using it would unnecessarily drain the battery. I have two questions.

When I turn on the ADC, how long should I wait before I can be confident that I can acquire an unbiased measurement? I do not see this information in the electrical specification for the chip (section 35 of the nRF52832 data sheet). Obviously this settling time should depend on the resolution of the measurement. I am interested in 12-bit resolution.

Are there specific functions which switch the ADC hardware on and off? For now, I'm developing a busy-wait version of my application. I suppose that when I switch to using the App Timer, the whole SoC will sleep between interrupts. But in the mean time, I want to selectively cycle the power on the SAADC. I want to test the waking-up procedure, and verify that I can get a reproducible measurement.

[Øyvind Karlsen]

Hi,

The settling time for the nRF52 SAADC is less than 3 µs, which is the minimum acquisition time you can set. This means that you do not have to worry about waiting for the circuit to settle.

For your second question you could design functions that will turn the ADC on and off, is this what you are asking?

Best regards,

Øyvind

[Ladasky]

Thanks for your reply, Øyvind. A 3 µs settling time is quite fast for my purposes. And since you say that the power-up procedure is guaranteed to last 3 µs, I can take a sample right away. That's good.

Yes, I am wondering whether I can turn the ADC off with a function call -- for example, nrf_drv_saadc_uninit(). I want to save power if I can.