Hello,
I would like to understand this behavior: in measuring the resistance of R7 in the attached circuit, I observed the value reads differently depending on which "polarity" I place the probes. The "polarities" are marked as Config 1 and Config 2, where the '+' symbol indicates the input of the DMM and the '-' indicates the common/black input of the DMM. Furthermore, the resistance measurement changes over time, either increasing or decreasing, depending on the polarity.
+3.3V generated by a boost converter with an integrated bypass (schematic in 'dmm-resistance-boost-ckt.png'. When the boost is not used, +3V3 is fed by a CR2032 battery. In my measurement, no batteries are present.
The resistor is spec'd as +/- 5% and both readings fall within that tolerance. I observed this behavior with both 121GW and Rigol DM3058E (although there is a bit of difference in exact behavior) and I have no concern that either DMM is out of calibration (they both read a potentiometer sitting around just fine).
I also ruled out that I was applying pressure inconsistently on R27 (it is SMT 0402) by soldering some wires and using some clamps instead of sharp probes.
My best postulation for these observations is, assuming the DMMs are active as constant current sources while performing resistance measurements, that:
1. The measurement increasing or decreasing is caused by the capacitors charging or discharging
2. There is some internal path from Pin 9 (TP1) to GND or VCC within the MSP430 or another IC
Any help understanding this would be very much appreciated. My hope is to take away some 'best practices' for doing these types of measurements.
P.S.
1. I wasn't sure if this should go in the T&E section or Beginners - the example is somewhat specific but the understanding how to use the resistance measurement function properly seems like an important beginner skill
2. The reason I am measuring R7 is that I observed its value, surprisingly (and unsurprisingly not mentioned in documentation), influences the internal oscillator(DCO) and MCLK. In a small production run of this device, there was about a 25% failure rate where an output signal's timing failed specifications, and at this time I'm not convinced it's R7 causing the MCLK deviation.