Author Topic: Optimal load to minimize picked up noise.  (Read 2110 times)

0 Members and 1 Guest are viewing this topic.

Offline AJolletTopic starter

  • Newbie
  • Posts: 7
Optimal load to minimize picked up noise.
« on: March 23, 2017, 10:17:14 am »
I'm designing an active sensor that is connected to the main system by 4 wires (2 power lines for the sensor's op amp, the system ground, and the amp's output).
Now I'm wondering what would be the ideal load to put on the main system side (between sensor's output and ground) to minimize the voltage noise picked up by the traces.
Thanks
 

Online Zero999

  • Super Contributor
  • ***
  • Posts: 19345
  • Country: gb
  • 0999
Re: Optimal load to minimize picked up noise.
« Reply #1 on: March 23, 2017, 10:48:48 am »
Please provide more information.

What's the sensor?

Schematic?

Part numbers?
 

Offline AJolletTopic starter

  • Newbie
  • Posts: 7
Re: Optimal load to minimize picked up noise.
« Reply #2 on: March 23, 2017, 11:02:53 am »
It's really more of a general question. The only important parameter I can think of is the output impedance of the active sensor.
But here's a basic schematic if it can help you get what I'm talking about.
 

Online Zero999

  • Super Contributor
  • ***
  • Posts: 19345
  • Country: gb
  • 0999
Re: Optimal load to minimize picked up noise.
« Reply #3 on: March 23, 2017, 11:15:08 am »
Where is the cable? Between the sensor and op-amp or is the op-amp on the same board as the sensor and connected to a long cable?

You've missed the decoupling capacitors on the op-amp's power supply. They should be as near to the IC as possible.
 

Offline AJolletTopic starter

  • Newbie
  • Posts: 7
Re: Optimal load to minimize picked up noise.
« Reply #4 on: March 23, 2017, 11:21:01 am »
The "cable" is between the op-amp and the load.
I didn't put the decoupling caps because the power lines have no resistance in my simulation, so no voltage drop, no need for caps.
They'll be here on the real design don't worry.
 

Offline bobaruni

  • Regular Contributor
  • *
  • Posts: 156
  • Country: au
Re: Optimal load to minimize picked up noise.
« Reply #5 on: March 23, 2017, 06:17:21 pm »
Now I'm wondering what would be the ideal load to put on the main system side (between sensor's output and ground) to minimize the voltage noise picked up by the traces.
Your information is still too vague, what frequency/BW and level range is the desired signal?
There is no "ideal" load, the more you load it, the less noise and desired signal you will have.
Unless you are talking about RF and Impedance matching?, the S/N ratio will remain the same regardless of the load.
What will improve things is to drive the cable with lower Impedance, maybe reduce your 100R to a few tens of Ohms on the output of the OPA.
« Last Edit: March 23, 2017, 06:20:38 pm by bobaruni »
 

Online BrianHG

  • Super Contributor
  • ***
  • Posts: 7660
  • Country: ca
Re: Optimal load to minimize picked up noise.
« Reply #6 on: March 23, 2017, 06:35:08 pm »
The "cable" is between the op-amp and the load.
I didn't put the decoupling caps because the power lines have no resistance in my simulation, so no voltage drop, no need for caps.
They'll be here on the real design don't worry.
What about inductance.  The decoupling caps are to prevent ringing oscillation due to switching current at the power input pins.
 

Online Zero999

  • Super Contributor
  • ***
  • Posts: 19345
  • Country: gb
  • 0999
Re: Optimal load to minimize picked up noise.
« Reply #7 on: March 23, 2017, 06:45:08 pm »
The "cable" is between the op-amp and the load.
I didn't put the decoupling caps because the power lines have no resistance in my simulation, so no voltage drop, no need for caps.
They'll be here on the real design don't worry.
What about inductance.  The decoupling caps are to prevent ringing oscillation due to switching current at the power input pins.
I'd suspect there is no inductance in the simulation either.  ;)
 

Online Kleinstein

  • Super Contributor
  • ***
  • Posts: 14073
  • Country: de
Re: Optimal load to minimize picked up noise.
« Reply #8 on: March 23, 2017, 07:25:11 pm »
There is essentially no pickup of noise - if at all it will be pickup of interfering signals. It depends on the type of interference: inductive coupling adds a voltage and thus is best suppressed for a high impedance signal - in the extreme case as a current loop. Capacitive coupling adds a current which has the least effect on a low impedance (can be << 50 Ohms).  It might be a good idea to have one side terminated with a impedance matching the cable, to avoid resonances on the cable.

Having both sides terminated could be a problem if there is a DC current, as than changes in DC resistance (e.g. connectors) can add "noise". So if frequency allows it, I would tend to use termination at the sending side and a high input impedance. For RF signals, it is termination on both sides anyway.
 


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf