5 Low-power systems: Power down or power off?

Low-power DAQ systems often conserve energy by using different power-down methods. Some ADCs offer a power-down mode that helps reduce system power consumption by putting the device in a low-power state when it is not in use. The ADC data sheet then specifies the current consumption in this mode. Another popular power-saving technique is to simply turn off the power supply when the ADC is not in use and turn the power supply back on when needed. This method should result in no power consumption while the system is off.

The latter method is subject to the transient currents discussed in this article, however, because any capacitors must recharge every time the supply cycles. You can estimate how much current the system consumes when the supply is turned off by using the standard equations for charge (Q) and current (I), and then compare this value to the ADC data-sheet value in power-down mode.

For example, the ADS1261 data sheet recommends 10- and 0.1-µF decoupling capacitors in parallel from AVDD to AVSS. The data sheet also specifies that AVDD must be 5 V. Equation 2 and Equation 3 calculate that the average current is 50.5 µA if the power supply cycles once per second:

where, C = 10.1 µF (10 µF + 0.1 µF), V = 5 V and t = 1 s.

Recall from the green highlighted section in Table 1 that the ADS1261 power-down current in power-down mode is only 8 μA (maximum). Comparing both options reveals that using the ADC power-down mode conserves >6 times more power relative to turning off the supplies. Therefore, it is important to consider the effect that transient currents can have on overall power consumption. Choosing to put the ADC in a power-down state can often be the more energy-efficient solution.