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I have a question for all high frequency and RF electrical engineers.

Thin film resistors have a vapor deposition of resistive material on one face of the component. Let's presume that the resistor provides feedback to amp with open-loop bandwidth of 800Mhz. Has anyone ever seen unexpected degradation of performance when the face of the resistor with the deposition ends up closer to any metal such as ground planes or adjacent traces on the PCB?

The proper answer I guess is to moat the trace and pads of any resistor for which parasitic shunt capacitance could be an issue, but I am curious if anyone has witnessed this in real life for thin film types before they moated any traces.

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  • \$\begingroup\$ Thank you for your time \$\endgroup\$
    – BHS
    Commented Sep 19, 2021 at 0:29

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In addition to other answers, ground plane capacitance and trace inductance make these types of feedback amplifiers more sensitive to modelling the physical reactance and loss tangent, before you commit to a design.

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This is why they started to make SMD parts much smaller to reduce coupling reactance and shorter to reduce inductance.

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This is a common issue in high frequency designs. To first order I estimate the parallel capacitance of the resistor to ground and model it in a simulator to see if it looks critical. In higher frequency, more sensitive circuits I use a full EM simulation.

This is also true for other components such as series blocking caps on RF signal lines.

There are enough factors (dielectric material, thickness, node impedance, various sensitivities, etc.) that it's hard to make a very general statement but it looks like you've got a good handle on the concept.

As you say, we commonly use ground cutouts to mitigate this. Best luck!

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The capacitance to ground of a 'film up' resistor is slightly different to that of 'film down'. However, not by much, compared to the resistor pads, and the traces beyond the pads.

It's well known that excess capacitance on the inverting input of a fast amplifier can cause instability. Manufacturers of such amplifiers usually provide guidance on relieving the ground plane under the entire non-inverting node - the IC pin pad, all the resistors connecting to it, and the traces between the IC and resistors. The effect of doing this will be far more significant than whether a resistor is mounted film up or down. Some high speed amplifiers use a non-standard pinout which places IN- next to OUT, to reduce such trace lengths. Using physically small resistors will also result in lower parasitic capacitance.

Precision work at microwave frequencies can benefit from knowing which way up your resistors are.

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  • \$\begingroup\$ Thank you for your time \$\endgroup\$
    – BHS
    Commented Sep 19, 2021 at 0:29

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