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.
[Paul Allen’s] SigZig data logger is a good case in point. It has an input Z of about 330K ohms. Not as high as a modern scope or digital volt meter, but still fairly high compared to what you usually want to measure. The SigZig has a very low noise figure, so the guys at SigZig decided to see if they could exploit the 330K input Z to measure very small currents.
Using some basic math modelling they were able to dectect 115 picoamp changes. The plot on the right shows the difference between a 1 gigaohm resistor and a 1.1 gigaohm resistor loading a 1.25V source. You can see there is a little noise, but the signal is clearly discernible. Obviously, without the low noise floor, it would be difficult to pull the signal out of the noise at this low level. But as we’ve mentioned before, the SigZig is very quiet. If you need a brush up on impedance, we’ve got you covered there too.
via [Dangerous Prototypes]