ADS1256: Power-supply sequencing

Hi Roy,

Welcome to the TI E2E Forums!

There is no required sequence for powering the analog and digital supplies on the ADS1256; however the device will not operate properly until both supplies have been provided.

After you've powered the device and the supply voltages have settled, you will need to wait at least 8192 clock cycles (about 1.1 ms with the nominal fclk) before trying to communicate with the device. I usually also recommend issuing a RESET command or toggling of the /RESET pin after powering up to help ensure that the device has powered up into a known state.

Another thing to be cautious of is the potential to damage the device when it is not powered. The absolute maximum ratings for the device also hold true when the device is not powered, so generally you would not want to apply any analog input signals or digital control signals to the device until the power supplies have been provided. If you cannot meet those conditions in your system then you may need to consider adding some protection componets (often series resistors can suffice) to ensure you limit any input currents to the device while it is powered off, as it will not take much voltage to forward bias the internal ESD diodes in this state.

Best regards,
Chris

Hi Chris,

Thanks for your comprehensive response.  You are saying that no digital signals should be supplied to the device unless the digital supply is present, and no analog signals should be supplied unless the analog supply is present.  Combined with no requirement for power-supply sequencing, your answer implies, I think, that one could supply digital signals to the device as long as the digital supply is present, even when no analog supply is present, and vice versa.  I ask for this clarification, because, in my application, the digital and analog supply voltages come from completely different sources.  In general, the supplies will be turned on and off more or less concurrently, but one could imagine a case, due to a malfunction or whatever, when only one supply is present.  It is the case that no analog signals will be supplied to the IC when the analog supply is absent, and vice versa, so my application will not violate the conditions you specified in your answer.

Roy

Hi Roy,

Roy Mallory said:

You are saying that no digital signals should be supplied to the device unless the digital supply is present, and no analog signals should be supplied unless the analog supply is present.  Combined with no requirement for power-supply sequencing, your answer implies, I think, that one could supply digital signals to the device as long as the digital supply is present, even when no analog supply is present, and vice versa.

That is correct!

Generally, you can run the ADC's digital supply off of the same supply used by your MCU and, by default, ensure that digital signals are only ever present while the ADC's digital supply is powered.

In the case of the analog signals, you may need to consider what types of fault conditions may occur...are the analog inputs exposed to the outside world, if so you may need to consider additional ESD protection and perhaps consider the possibility of end-user error (should they be responsible for wiring the sensor to the ADC).

Best regards,
Chris