You probably know that to transfer the most energy between a source and a load their impedance needs to match. That’s why a ham radio transmitter needs a 50 ohm antenna (at least, usually). The transmitter is 50 ohms and you want a match. Some test equipment matches impedance, but for multimeters, oscilloscopes and a lot of other gear, the instrument just presents a very large impedance. As long as it is much larger than the measured circuit’s impedance, the effect will be small.
With today’s MOSFET instrumentation amplifiers, it isn’t uncommon to see very high input impedances. However, you sometimes run into something that has a low input Z and that can cause issues if you don’t account for them. On the other hand, where some people see issues, others see opportunities.
A couple of years ago he worked on a standalone chemical sensor and had a few extra boards sitting around after the project was done. As any resourceful hacker will do, he reached for them as the closest and easiest solution when needing to log data as a quick test. It wasn’t for quite some time that he went back to try out commercially available loggers and found a problem in doing so.
