Input Impedence

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TECH_DD
Posts: 7
Joined: Mon Feb 22, 2016 12:01 pm

Input Impedence

Post by TECH_DD » Fri Dec 30, 2016 2:51 pm

Hi all, I would use the Red Pitaya as Spectrum analyser to do some pre compliance tests.

All the instrumentation and all the references in the ISO and IEC normative are with the impedance of 50 Ohm.
Here: http://redpitaya.com/apps/spectrum-analyzer/ I read that input impedance are of 1M Ohm.

Some one could help me to understand how I can adapt the input at a impedance of 50 Ohm?
Thank you

Nils Roos
Posts: 1441
Joined: Sat Jun 07, 2014 12:49 pm
Location: Königswinter

Re: Input Impedence

Post by Nils Roos » Sat Dec 31, 2016 2:57 am

Hi,
usually you'd terminate the output with a 50ohm load and measure accross that load.

TECH_DD
Posts: 7
Joined: Mon Feb 22, 2016 12:01 pm

Re: Input Impedence

Post by TECH_DD » Thu Jan 12, 2017 4:39 pm

Hi Nils,
thank you for your reply.

But isn't the input of the instrument that need this impedance? All the professional instrumentation (oscilloscope, function generators, analyser, ...) have a 50 ohm input impedance. I would use the RED Pitaya with a LISN for EMC pre-complance, so my doubt is that I could distort the readings of Values.

Many thanks

Nils Roos
Posts: 1441
Joined: Sat Jun 07, 2014 12:49 pm
Location: Königswinter

Re: Input Impedence

Post by Nils Roos » Thu Jan 12, 2017 11:53 pm

I am not an expert in analog measurement circuitry, but my general understanding of the matter is that the actual measuring circuit needs to have a high impedance in order to minimize its own influence on the signal. I'd conclude that the equipment you are familiar with has 50ohm input impedance because they have a built-in load on their inputs and a high impedance probe in parallel.

TECH_DD
Posts: 7
Joined: Mon Feb 22, 2016 12:01 pm

Re: Input Impedence

Post by TECH_DD » Fri Jan 13, 2017 8:32 am

Hi Nils, this is possible and have "a sense". Thank you for your help, I think I'll buy Red Pitaya now!

razvanfatu
Posts: 4
Joined: Wed Jan 11, 2017 10:42 pm

Re: Input Impedence

Post by razvanfatu » Mon Jan 16, 2017 5:58 pm

Matching different impedances also changes the signal's amplitude and probably waveform. You never want that with test equipment.

A 50-ohm resistor in parallel with the input is the right way to do it - some factory equipment has small 50 ohm resistors built in the input BNC connector. Sometimes people feed too much signal to the input and the tiny 50-ohm gets damaged :).

Razvan.

TECH_DD
Posts: 7
Joined: Mon Feb 22, 2016 12:01 pm

Re: Input Impedence

Post by TECH_DD » Mon Jan 16, 2017 6:21 pm

Thank you RAZVAN!

So in your opinion if I have my EUT to test and I need to check the conducted emission I could use a LISN (https://en.wikipedia.org/wiki/Line_Impe ... on_Network )
and connect the REDPitaya directly to it or I need another 50 ohm resistor in parallel to "to EMI receiver" port ?

In the second case impedance are of 25 ohm because there are two 50 ohm impedance in parallel... I don't know if is the right way...

Thank you very much :D

razvanfatu
Posts: 4
Joined: Wed Jan 11, 2017 10:42 pm

Re: Input Impedence

Post by razvanfatu » Tue Jan 17, 2017 5:08 pm

If your EUT requires 50ohm termination then you should have a 50ohm load in parallel with the Red Pitaya input to get the correct measurements. For other equipment (eg. my Tektronix oscilloscope that has 1Mohm+10pF input impedance) I use something like this:

http://www.eevblog.com/forum/blog/eevbl ... 5594;image

Unfortunately I never used a LISN, it would help if you have specs or a diagram for it.

Or maybe you could describe the test setup, as I see a LISN has 3 ports (input, output and measurement port). What do you connect to each port when you take the measurements ?

Razvan.

TECH_DD
Posts: 7
Joined: Mon Feb 22, 2016 12:01 pm

Re: Input Impedence

Post by TECH_DD » Tue Jan 17, 2017 6:32 pm

I need to check the conducted emission for the EMC pre-compliance tests. The test equipment are described by a Normative and the CISPR 12, CISPR 16 and CISPR 25 define the test layout.
To do this is necessary to isolate the test equipment from the DC (or an AC, if needed) line and the LISN are used to do this.
The standard impedance described by the normative are of 50 ohm so my first question was about this.

The link of LISN thant I posted previously are of 3 pole, but my EUT are in DC voltage so I use only two channels, or a two single channel LISN

Here there an explanation better than mine :D :
http://www.emcfastpass.com/test-equipme ... 25-tboh01/

here also the scheme of a LISN compliance with CISPR 25:
http://www.emcfastpass.com/test-equipme ... Manual.pdf

and here an explanation of how to do the test:
http://www.emcfastpass.com/test-equipme ... _Guide.pdf


thank you very much for your help!

razvanfatu
Posts: 4
Joined: Wed Jan 11, 2017 10:42 pm

Re: Input Impedence

Post by razvanfatu » Wed Jan 18, 2017 12:15 am

Great, that helped. It looks like you will need a 50ohm load across the input.

Razvan.

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