Optimal load to minimize picked up noise. - Page 1
Forum section: electronics / projects

author: AJollet
author_rank: Newbie
timestamp: March 23, 2017, 10:17:14 am
content: 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

author: Zero999
author_rank: Super Contributor
timestamp: March 23, 2017, 10:48:48 am
content: Please provide more information.What's the sensor?Schematic?Part numbers?

author: AJollet
author_rank: Newbie
timestamp: March 23, 2017, 11:02:53 am
content: 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.

author: Zero999
author_rank: Super Contributor
timestamp: March 23, 2017, 11:15:08 am
content: 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.

author: AJollet
author_rank: Newbie
timestamp: March 23, 2017, 11:21:01 am
content: 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.

author: bobaruni
author_rank: Regular Contributor
timestamp: March 23, 2017, 06:17:21 pm
content: Quote from: AJollet on March 23, 2017, 10:17:14 amYour 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.

author: BrianHG
author_rank: Super Contributor
timestamp: March 23, 2017, 06:35:08 pm
content: Quote from: AJollet on March 23, 2017, 11:21:01 amWhat about inductance. The decoupling caps are to prevent ringing oscillation due to switching current at the power input pins.

author: Zero999
author_rank: Super Contributor
timestamp: March 23, 2017, 06:45:08 pm
content: Quote from: BrianHG on March 23, 2017, 06:35:08 pmI'd suspect there is no inductance in the simulation either.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 23, 2017, 07:25:11 pm
content: 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.