Signal Generators, Voltages, and Connectors

[ccaudle]

You are grounding the cold. It probably won’t work depending on the input topologyand if it does work it won’t be an accurate measurement.

If that is the case then your differential input is completely broken, or incompetently designed.

 

[amplexus]

That match maker box is super cool!

they come up fairly cheaply a lot. Transformer balanced/isolated outputs as well. A VERY useful studio/bench tool for sure.

 

[Gold]

If that is the case then your differential input is completely broken, or incompetently designed.

You would trust a measurement that grounds one side of something like a THAT 124x? That’s not a normal situation tontest it under.

 

[swpaskett]

Matchmaker box? Never knew you could buy something like that. I use a collection of 600:600 transformers and input/output buffer boxes I built up around 5532s years ago; some have transistor driver stages to drive some of the oddball ancient stuff I use like the GenRad Microvolters with something like a 284 ohm input impedance. Who builds stuff like that? Oh, yeah, GenRad did. Emphasis on did.
What I find is that even some of the HP gear I own can't drive 600 ohms. I used to think HP was the bees knees in test gear, then as I got to know it better it wasn't any better than Leader or BK (not to be confused with B&K). Sensing a rant coming on; best stop now.

 

[ccaudle]

You would trust a measurement that grounds one side of something like a THAT 124x? That’s not a normal situation tontest it under.

You would be better off with an instrumentation amp style input that puts high impedance buffers in front of the 124x inputs, but other than poor CMRR with unbalanced drive, sure, there won't be any problems with connecting an unbalanced source to a differential amp input. That is exactly what you would have if you connected a keyboard or drum machine to a balanced input.

What exactly is your concern?

 

[Gold]

What exactly is your concern?

This is a test setup. Unless he wants to know how the DUT behaves under that specific condition (which he doesn't) hobbling the performance of the DUT doesn't seem like a good idea to me. I know a 1646 doesn't output the same exact level when one leg is grounded. IIRC neither does a 124x.

 

[ccaudle]

I know a 1646 doesn't output the same exact level when one leg is grounded.

A ThatCorp 1646 is an output driver. Your previous post:

It probably won’t work depending on the input topology

Of course input circuits and output circuits have different considerations. We were not discussing output drivers, we were discussing connecting a single ended signal generator to a balanced input.
This post is probably not the place to cover basic circuit analysis, but I stand by my previous statement: a proper differential input will work correctly with the cold input connected to the signal generator ground. I'm sure you have some single ended equipment, try it for yourself. Just be aware that if either piece of equipment is floating (i.e. does not have protective earth connection through a 3 wire power cord) then you should connect the device grounds together to keep common mode voltage within acceptable limits.

 

[Gold]

[QUOTE="ccaudle]Just be aware that if either piece of equipment is floating (i.e. does not have protective earth connection through a 3 wire power cord) then you should connect the device grounds together to keep common mode voltage within acceptable limits.
[/QUOTE]


Right, you don’t need to be aware of that or other stuff like an accidental ground loop if you use a balanced source.

 

[warpie]

I understand that 0VU (+4dBu) is 1.28V RMS. When measuring a balanced signal (TRS cable from a balanced output) I have been measuring across positive and ground leads.

Do you mean 1.228 Vrms?