# Impedance

#### mlkcampion

Joined Nov 16, 2006
59
Hey Forum
I have a device that hasa a very high output impedance (less then 2000 MOhms). I want to interface it to a PLC. My understanding of a high output impedance is that it is a very poor current source?? Therefore i would need to connect it to a low input impedance??? How would i decide that the PLC has a low enough input impedance (1Mohm)?? If it wasn 't what op-amp would give me a high enough input impedance??
How do i calculate approxiamtely or intutively what sort of op-amp or op-amp configuration i would need (eg voltage follower)??
How
Thanks
Michael

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

You will need to buffer the signal through an op amp with a higher input resistance. It can just be set up as a voltage follower. You can also add gain to make the signal larger, if desired.

One op amp that will do the trick is the OPA134PA. The PA makes it come in the DIP8 package. The signal source is 2 X 10^10 - the OPA134 has an input resistance of 5 X 10^13. They're available through either TI or Digi-Key for about \$2.45 ea.

#### mlkcampion

Joined Nov 16, 2006
59
Thanks for the help!!
When you say the signal source is 2 X 10^10 ???

Also i came across a site which was talking about impedance and started stating that such and such was 100kv i.e. giving the impedance in Volts instead of ohms, why is this???

Thanks

#### beenthere

Joined Apr 20, 2004
15,819
Hi,

That reads as "two times ten to the tenth power".

Impedance is a measure of resistance, but with a frequency-dependant aspect to it. With op amps, the input impedance figure is more properly a resistance. I imagine the 100KV was meaning something more like insulation resistance, meaning that it would withstand that much voltage without failing. Sounds like a lot, though.

#### rootboy

Joined Jan 4, 2007
13
What device?, what model PLC?