Hi,

I am wondering if routing a signal to an ADC input over one or several vias is an issue concerning precision of the measurement results. I always tried to avoid this the closer I come to the input the more, but I can't give a real reason why.

What is a vias equivalent circuit? Is there any influence on the signal at all?

My circuit deals with low frequency signals and I am using an ADuC847 with 24 bit ADC

Thanks for any answers.

Martin

Hi, Martin.

Vias can be thought of as a lumped circuit consisting of inductance and capacitance at low frequencies. at high frequencies, its model is too complex but still consists of networks of capacitance and inductance.

At low frequencies, a via will behave capacitively or has low impedance. Vias, as much as possible, should be avoided along the input's path to minimize small voltage drop (Note: a trace alone has equivalent impedance which is dependent to its length, thickness, material used, etc.). This will somehow degrade the signal a little when it reaches the ADC. If this voltage drop is equivalent to 1 LSB of the ADC, then you will be reading a wrong data.

The best approach to prevent this is to rout the input signal as close as possible to the input pins of the ADC and avoid the use of via. If via is really necessary, its impedance can be reduced by having a smaller pad, reducing its cylinder radius and decreasing its ground plane radius. You can contact your PCB manufacturer for the limits of these specifications.

Hope this helps.

Regards,

Johnny