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A poor ground connection, for example, from a board connected to the supply with a thin cable, will have high ground impedance, which could generate glitches during power-up. In addition, in some electromagnetic environments (EME), the parasitic gate capacitance of the metal-oxide semiconductor (MOS) transistor can charge, causing it to malfunction until the capacitance is discharged. This could cause a failure in the POR initialisation.

Drift and tolerance need to be considered as well. In some cases, discrete components such as capacitors have high tolerances—up to 40 per cent—and high drift versus temperature, voltage and time. In addition, the threshold voltages have a negative temperature coefficient. For example, VT1 could vary from 0.8V at room temperature to 0.9V at –40°C and 0.7V at +105°C.

Importance of voltage accuracy and transitional behaviour

Now you know some common issues that could cause system problems when powering a board, and the basic rules to guarantee correct board initialisation. The supply is often overlooked, but both its final voltage accuracy and its transitional behaviour are important.

Click here to read about the difference between power-off and power-down modes of an IC.

Reference
Insight into digiPOT Specifications and Architecture Enhances AC Performance – Analog Dialogue, Volume 45, Number 3.


The author is an applications engineer in the Linear and Precision Technology Group at Analog Devices Inc in Valencia, Spain.

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