Impedance Vs Resistance

[balaseven]

impedance vs resistance

Hi all,

I need answer for the folllowing questions relevant to printed circuit board technology.

What is the exact difference between resistance and impedance?

If impedance mean for the restriction of AC current, why we have to consider it for DC.?

In PCB technology,we have to maintain impedance for each traces....For example,we have to maintain 50 ohm impedance for single ended nets.and 100 ohm differential impedance for differential pairs....In most of our board,we are using DC power supply.In that case,why we have to consider impedance(50ohm,100ohm)......we should consider resistance...isn't it?

Thanks
bala

 

[antarveena1982]

difference between impedance and resistance

Hi Bala

Actually the is thing is like that there some part of frequency is aways present when said DC we think it is of 0 frequency.But we get pulsating DC there is some part of frequency is present?

So we consider impedance rather than resistance.

Hope this will clear to you.

regards
Antarveena

 

[zirtapoz]

difference between resistance and impedance

if your design works pure dc, thats ok Forget about impedance.

 

[RF-OM]

impedance and resistance

The difference between resistance and impedance is that impedance have reactive part beside resistive, which means that there is some phase shift between voltage and current. For AC systems reactive part is responsible for reflecting part of the received energy back to the source. This the major difference from DC when only ohmic or heat losses are present. But DC may be in the form of very fast rising and falling pulse or pulses. Actually such a pulse consists from DC and many AC signals. In this case there are a lot of additional losses.

When you design PCB you need to carefully consider what kind of signal will go through this particular trace. If it is pure DC you need to consider heat loss and possible coupling to other traces or elements. But if your trace will care any kind of AC signal you need to consider the signal properties and accommodate trace properties to match them. Usually RF signals goes through 50 Ohm lines and differential signals use 100 Ohm. Differential lines are more complicated, but they are more protected from noise and parasitic EM coupling. Any kind of traces or lines have resistance and according losses that must be accounted. However, AC impedance has much higher effect on board interconnections and often is major loss contributor. 50 Ohm was chosen primarily from feasibility and usability considerations. It is widely used value, but when it is necessary designer may chose other characteristic impedance for the trace. There is also important to divide or decouple DC power supply lines and AC interconnections, not only well visible forward path lines, but also practically invisible return path lines. This is not easy story, but if you want to be a good designer you need to learn it. The PCB design often underestimated, but it is probably not easier than all the circuits on the board. High frequency RF and high speed designs are considered as black magic (you may find the books with such words in the title). So, good PCB designer is often a magician.

Best regards,
RF-OM

 

[LvW]

maintain impedance

Well, I try a rather short explanation:

Impedance is simply the ratio of voltage-to-current for a two-pole device (e.g. part or input resp. output of a four-pole) .
1.) If there is no phase shift between both, it is an pure ohmic impedance - and that´s called resistance
2.) If there is a phase shift, there is some sort of capacitice or inductive (or both) influence. That means the ratio has a real as well as an imaginary part and their combination is called impedance.

 

[balaseven]

resistance vs impedance

Thank you all....

 

[dmd4raj]

resistance vs. impedance

All linear voltage regulators working under finite bandwidth,inductance(Impedance(collection of Res,inductance,cap.,)) increases with frequency.So if the board operates @>100MHZ then the inductance in power line increases.Due to this there is a problem with grounding and due to high inductance power ll be fluctuate.So we have to provide the decoupling capacitors near to the power pin of Both Analog and Digital ICs.


Regards
RAJAN.K