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[DPRG] Working with very small voltage margins & a op amp question

Subject: [DPRG] Working with very small voltage margins & a op amp question
From: Rick Bickle rbickle at intconsys.com
Date: Fri Aug 21 14:22:33 CDT 2009

Ray,

 

Ø  Is it OK if I build the prototype on breadboard first, then transfer it to a PCB?

 

Sure, that's how I generally do it.

 

Ø  And I'm just wondering, heck is it really possible to have noise levels below +/- 10mV on voltages by using those techniques?

 

+/- 10mV shouldn't be a problem, depending on your environment. If your device is  going to run in a relatively low noise (electrical noise and RF noise) environment, the techniques I mentioned will be less important. I it will be operating in a noisy industrial environment, then do everything you can to eliminate noise.

 

Any type of differential signal has better noise rejection from common mode noise as the common mode noise is generally induced on both differential conductors equally, which has little effect on the difference between the two lines. RS485 is a differential transmission standard, and as such has better noise rejection than say RS232 which is not differential.

 

For your op-amps though, I don't think it should be necessary. Just keep everything shielded. Keep all low voltage signals short and shielded.

Noise immunity is black magic. In the end you just have to build it and try to foul it up to see how well it's going to work.

Trial and error is the last resort of all engineering.

 

If your signals are going to be close to ground or VCC, you should probably use a bipolar supply to keep the signal several volts away from the op-amps power supply rails.

 

Rick 

 

From: dprglist-bounces at dprg.org [mailto:dprglist-bounces at dprg.org] On Behalf Of Ray Xu
Sent: Thursday, August 20, 2009 3:34 PM
To: dprglist at dprg.org
Subject: Re: [DPRG] Working with very small voltage margins & a op amp question

 

Thank you Rick,

Is it OK if I build the prototype on breadboard first, then transfer it to a PCB?

And I'm just wondering, heck is it really possible to have noise levels below +/- 10mV on voltages by using those techniques?

Also, I just had an idea...  I remember ECL and LVDSlogic levels use differential signaling to avoid noise problems since they are low voltage.  I'm wondering if this would significantly reduce noise by adding a signal splitter (VCC=+11V, virtual ground=+5.5V), then subtract each other (there for canceling out noise) using a differential op amp configuration (VCC=+11V, reference=GND).  Something like this: http://en.wikipedia.org/wiki/Differential_signaling  Since I don't want to "differentialize" my entire voltage reference circuit, I can only add this to the output of the voltage reference generator circuit.  Would this be a good idea to do?

Thanks

Ray Xu

On Thu, Aug 20, 2009 at 2:03 PM, Rick Bickle <rbickle at intconsys.com> wrote:

Ray,

 

Use the large capacitors and inductors on the power supply rails and not on the signal to the laser circuitry.

Use a linear power supply instead of a switching power supply. Add some filtering on the AC side. (i.e. balun or chokes)

Keep the wires or PCB tracks short between the filtered supply and the op-amps.

Keep all associated op-amp components as close to the chip as possible.

For a PCB, put a ground plane underneath the op-amp chips and associated components. Tie all component grounds to the plane locally.

For a prototype, use a solid copper board with the copper as a ground plane. Solder the chip grounds directly to it as well as all component grounds.

Be sure to bypass all high speed switching devices with small monolithic capacitors close to each device.

For really sensitive circuits, you can put a faraday shield (i.e. metal housing) around the op-amp section.

 

Not sure I understand how the op-amps are going to set the current of your diode, but there are op-amps available with high current open collector outputs. These will give a very linear response.

I recommend the book "IC Op-Amp Cookbook" by Walter Jung, available from Amazon. - and of course the bible "The art of Electronics" by Horowitz and Hill.

 

Good Luck,

 

Rick

 

 

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