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My PCB has high-precision SMD resistors (0.01%, 25ppm/C) - some pre-installed, some I added. When I measured their values in-circuit with a multimeter directly at the pins, some of them showed incoherent values compared to theoretical ones. When I removed one resistor and tested it separately, it was fine. I also checked the manufacturer-installed resistors, and the same issue occurred. Since the manufacturer uses automated soldering, I doubt it's a soldering problem.

Is there something wrong or am I missing out something? If so what could be causing this, and how can I diagnose and resolve it?

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Is there something wrong or am I missing out something ?

When the resistor is in-circuit and, you try and measure it, not only are you measuring the resistor but all the other components it connects to.

how can I diagnose and resolve it?

Nothing to resolve. Test the circuit properly as it is intended to work and don't hope to measure precise values for any component when they are soldered (or otherwise connected) into a circuit.

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You generally cannot measure values in circuit, it's confounded by the parallel paths of other components.

For some components you will be lucky, they may have a capacitor in series which isolates them from other components at DC, and then you get the correct reading.

It's even more of a problem when the parallel components are non-linear things like diodes or transistors. Then you may get different readings of the same component depending on which DMM you use, or which way round you measure them.

There is a technique to increase the number of components you can measure in circuit, called guarding.

To measure a component between nodes A and B, you first have to identify all the other paths between A and B, and then find another node on each of those paths. These are the guard nodes, and become our reference terminal. Drive node A with a voltage source with respect to the guard nodes. Measure the current out of node B using a transresistance/virtual ground amplifier that forces B to the guard node voltage.

A little thought will show that this isolates the target component. The current flowing from node A to guard is irrelevant. There is no voltage between guard and node B, so no current flows through those paths. This works with AC for capacitors/inductors, as well as at DC for resistors.

It may not allow you to measure every component, as the guard nodes may not exist or be easy to find, or to connect to.

Precision will be hard to achieve. I would not like make measurements to 0.01% using this method, as input offsets in the amplifiers used will cause errors through the parallel paths, exacerbated if their resistance is much lower than the device under test. If they are orders of magnitude lower, then the technique becomes pretty much unusable.

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  • \$\begingroup\$ Thank you for your great response. Is this true even when measurements are taken directly at the pins of the resistors? I understand that for other types of components, it can be more challenging... \$\endgroup\$ Commented Oct 7, 2023 at 10:28
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    \$\begingroup\$ @flower.fish even when the measurements are mad at the resistor pins, I'm not sure how you think measuring at the pins defeats the problems of the parallel paths. \$\endgroup\$
    – Neil_UK
    Commented Oct 7, 2023 at 10:33

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