Electronics > Beginners
Does impedance value of a voltage rail change at different points ?
muthukural001:
Hello All,
Today, my colleague asked me to fill impeance value of the voltage rails.He told me to take impedance value of the same rail at different point.For example, a 5V usb input is going to three DC-DC converters for 3.3, 1.2, 1.8. He told like, take impedance value of 5V input rail at USB connector, 3.3V DC-DC converter's input side,1.8V DC-DC converter's input side and 1.2V DC-DC converter's input side...
Is he right?...My assumption is all the values are same...Please correct me if I am wrong...
Thanks,
Muthu
AndyC_772:
Yes, it does vary, because the power plane has inductance and resistance. If it were a perfect conductor, there would be no need for ICs to have decoupling capacitors physically located next to power pins.
It's incredibly difficult to measure accurately, though, especially at the kinds of frequencies where this inductance has a significant effect. High end CAD software (eg. Cadence Allegro) can simulate the PCB layout and derive the impedance of a supply at different places on the PCB.
To do it in a meaningful way, you'd need a network analyser and some specialist training in how to probe the board correctly. I'm guessing you don't have these.
What equipment do you have?
muthukural001:
Hi Andy,
Yes, we do not have network analyzer. We will measure with Multimeter...
Ok, Generally, how much ohms difference they will have? milli Ohms (OR) Ohms ?.
Thanks,
Muthu
AndyC_772:
It's highly unlikely that a multimeter will tell you anything useful whatsoever.
If you need to know the impedance of a supply, this implies that you need to know for a good reason.
If the board were being used to supply very large currents to a resistive load like a heater, then it might make sense to measure the dc resistance (not the impedance) of the tracks on the board.
However, I'm guessing that this isn't the case, since you're using dc/dc converters to produce low voltage rails which are typical of low power digital logic. It's much more likely that, if you need to know the impedance at all, you need to know it across a range of frequencies that a multimeter simply cannot measure.
Ask what this information is needed for, and how it will be used. It may well be that you don't have the ability, or the need, to make a measurement which is correct and meaningful for the purpose.
radiolistener:
--- Quote from: muthukural001 on May 21, 2019, 07:08:03 am ---Ok, Generally, how much ohms difference they will have? milli Ohms (OR) Ohms ?.
--- End quote ---
it may vary in range from milliohms to gigaohms, it depends on frequency, power supply and wires.
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