Resistance of test leads

[djcarpentier]

Hi all,

First post here. I have a set up with a 100 ohm 0.1% resistor and after calibration i go to make a measurement of my test lead resistance. I consistently get a flat line value of 3.2 ohms. This doesn't make much sense. I really want to use REW for calculating T/S parameters. Does anyone have any insight on this?

 

[John Mulcahy]

Which REW version are you running? Did you run impedance calibration?

 

[djcarpentier]

REW 5.19. Yes i run the impedance calibration and it seems to come out ok.

 

[John Mulcahy]

What is the full setup - interface being used, connections etc.

 

[djcarpentier]

I followed the impedance design page for a standard measurement system. Just running it off my PC's onboard audio ins and outs. I do have an audio interface, but haven't tried to use it yet. I have very good electrical knowledge, so i'm thinking this has something to do with my onboard sound card. My test leads are also on the longer side as i need space to do any real measurements. I've attached some photos of my test rig. I guess my nest step would be to try my audio interface. It has fully balanced ins and outs though, so i was trying to avoid it. I also don't have the greatest work are (living room).

Thanks very much for having a look John, much appreciated.

 

[John Mulcahy]

The long leads are the ones to and from the onboard audio then. Even in that case 3 ohms seems high, that's around 10 m of 30 AWG. Could try a version with short leads to see how much difference that makes. If you measure a 100 ohm resistor do you get a 103 ohm result?

 

[djcarpentier]

The longer one is to the sound card output. Short one is input. Then there are the two speaker leads that are fairly long ~ 1m. R sense measures at 99.7 ohms. This is about right as it is supposed to be a 100 ohm 0.1% resistor. Going to hook up my Focusrite Forte with this system and see what happens. I'll let you know.

 

[John Mulcahy]

I wasn't referring to the sense resistor, I was meaning to take a 100 ohm resistor and measure it with the rig. Should have picked a different value

 

[djcarpentier]

Ahh. Will try that and let you know.

 

[djcarpentier]

Just made a measurement with my Focusrite Get a flat line value of 2.87 ohms. Put in my second precision 100 ohm 0.1% resistor and now i get a value of 102.8 ohms. Somehow my measurement rig has an impedance of just under 3 ohms. Frustrating. I guess i can resolder everything, but i fail to see that their will be a significant improvement.

 

[djcarpentier]

Re-soldered everything with short leads. Exactly the same output. Don't think its worth investing more time and money in parts. Time for plan B.

 

[John Mulcahy]

Strange one. Difficult to think of a mechanism that would produce an offset like that, perhaps something a little odd with the ground connections? Best of luck with plan B in any case.