I'm working on a PCB that contains nearly 30 resistors in a ladder configuration, each step switched in or out by a SMT solid state relay. It's essentially a decade box for a specific application. The relays are controlled by a micro and all is powered by a boardmount enclosed CUI AC/DC switching converter, supplying up to 400mA. The resistors are electrically isolated from the SMPS, however I see a lot of noise on the resistance output that can be from nothing other than the SMPS. I'm assuming the power planes on the board are inducing all kinds of noise into the traces that connect the resistors and solid state relays. Replacing the SMPS with a linear would be ideal, but the amount of space required is a big problem. So I'm looking at filtering the output of the SMPS. I'm a digital and DC analog guy, so this switching noise scares me. Some research and an educated guess tells me to try an LC filter on the SMPS output. I'm looking for any first hand experience with this and dumbed-down advice that will hopefully get me a solution without days of headaches. The SMPS claims a 100kHz switching frequency, and spikes on the scope confirm that. I've already tried putting several MLCC 1uF caps across the SMPS output but that didn't change anything.
Reducing SMPS noise
Started by ●September 6, 2016
Reply by ●September 6, 20162016-09-06
On Tuesday, September 6, 2016 at 12:46:20 PM UTC-4, hon...@yahoo.com wrote:> I'm working on a PCB that contains nearly 30 resistors in a ladder configuration, each step switched in or out by a SMT solid state relay. It's essentially a decade box for a specific application. The relays are controlled by a micro and all is powered by a boardmount enclosed CUI AC/DC switching converter, supplying up to 400mA. > > The resistors are electrically isolated from the SMPS, however I see a lot of noise on the resistance output that can be from nothing other than the SMPS. I'm assuming the power planes on the board are inducing all kinds of noise into the traces that connect the resistors and solid state relays. > > Replacing the SMPS with a linear would be ideal, but the amount of space required is a big problem. So I'm looking at filtering the output of the SMPS. I'm a digital and DC analog guy, so this switching noise scares me. Some research and an educated guess tells me to try an LC filter on the SMPS output. I'm looking for any first hand experience with this and dumbed-down advice that will hopefully get me a solution without days of headaches. > > The SMPS claims a 100kHz switching frequency, and spikes on the scope confirm that. I've already tried putting several MLCC 1uF caps across the SMPS output but that didn't change anything.You'll want an indcutor in series with the cap... making a low pass. I think I used 10 uF and 22 uH... (You want the LC frequency to be below 100 kHz.) You might also try adding some caps to ground on the resistor ladder. That should help all but the first step. George H.
Reply by ●September 6, 20162016-09-06
On Tuesday, September 6, 2016 at 12:31:40 PM UTC-5, George Herold wrote:> On Tuesday, September 6, 2016 at 12:46:20 PM UTC-4, hon...@yahoo.com wrote: > > I'm working on a PCB that contains nearly 30 resistors in a ladder configuration, each step switched in or out by a SMT solid state relay. It's essentially a decade box for a specific application. The relays are controlled by a micro and all is powered by a boardmount enclosed CUI AC/DC switching converter, supplying up to 400mA. > > > > The resistors are electrically isolated from the SMPS, however I see a lot of noise on the resistance output that can be from nothing other than the SMPS. I'm assuming the power planes on the board are inducing all kinds of noise into the traces that connect the resistors and solid state relays. > > > > Replacing the SMPS with a linear would be ideal, but the amount of space required is a big problem. So I'm looking at filtering the output of the SMPS. I'm a digital and DC analog guy, so this switching noise scares me. Some research and an educated guess tells me to try an LC filter on the SMPS output. I'm looking for any first hand experience with this and dumbed-down advice that will hopefully get me a solution without days of headaches. > > > > The SMPS claims a 100kHz switching frequency, and spikes on the scope confirm that. I've already tried putting several MLCC 1uF caps across the SMPS output but that didn't change anything. > > You'll want an indcutor in series with the cap... making a low pass. > I think I used 10 uF and 22 uH... (You want the LC frequency to be below 100 kHz.) > > You might also try adding some caps to ground on the resistor ladder. > That should help all but the first step. > > George H.I will give the LC filter a shot. When you mention caps to ground on the ladder, are you referring to the power supply ground? If so, I have to keep that isolated.
Reply by ●September 6, 20162016-09-06
In article <83484c52-da61-4712-98b9-16c01338091d@googlegroups.com>, George Herold <gherold@teachspin.com> wrote:>> Replacing the SMPS with a linear would be ideal, but the amount of space required is a big problem. So I'm looking at >> filtering the output of the SMPS. I'm a digital and DC analog guy, so this switching noise scares me. Some research and >> an educated guess tells me to try an LC filter on the SMPS output. I'm looking for any first hand experience with this and >> dumbed-down advice that will hopefully get me a solution without days of headaches. >> >> The SMPS claims a 100kHz switching frequency, and spikes on the scope confirm that. I've already tried putting several >> MLCC 1uF caps across the SMPS output but that didn't change >> anything.>You'll want an indcutor in series with the cap... making a low pass. >I think I used 10 uF and 22 uH... (You want the LC frequency to be below 100 kHz.)If you've got a volt or so to spare, you could use a transistor-based capacitance multiplier, to give you an effective capacitance value quite a bit larger than its physical value. That would make it easier to push the corner frequency of the low-pass filtering down even further. Check the layout of your PC board, and your grounding arrangements. Try to make sure that current pulses from the SMPS (either the input side or the output side) and voltage jumps aren't being coupled into the output circuitry via either capacitive coupling, or inductive-loop coupling. In an extreme case you might need to have a shield around one side or the other, broken only for the relays themselves. Thinking outside the (shielded) box a bit: Using magnetically-loaded "latching" relays might be another helpful option, as their coils could be left un-energized (either open-circuit, or both sides grounded) except for a short pulse to turn them on or off. This would also cut your power supply current demand, and might let you use a smaller/quieter switcher, or a small linear supply with a big output filter/reserve capacitor.
Reply by ●September 6, 20162016-09-06
On Tue, 6 Sep 2016 09:46:13 -0700 (PDT), hondgm@yahoo.com wrote:>I'm working on a PCB that contains nearly 30 resistors in a ladder configuration, each step switched in or out by a SMT solid state relay. It's essentially a decade box for a specific application. The relays are controlled by a micro and all is powered by a boardmount enclosed CUI AC/DC switching converter, supplying up to 400mA. > >The resistors are electrically isolated from the SMPS, however I see a lot of noise on the resistance output that can be from nothing other than the SMPS. I'm assuming the power planes on the board are inducing all kinds of noise into the traces that connect the resistors and solid state relays. > >Replacing the SMPS with a linear would be ideal, but the amount of space required is a big problem. So I'm looking at filtering the output of the SMPS. I'm a digital and DC analog guy, so this switching noise scares me. Some research and an educated guess tells me to try an LC filter on the SMPS output. I'm looking for any first hand experience with this and dumbed-down advice that will hopefully get me a solution without days of headaches. > >The SMPS claims a 100kHz switching frequency, and spikes on the scope confirm that. I've already tried putting several MLCC 1uF caps across the SMPS output but that didn't change anything.If the noise is in-plain-sight conducted, adding a ferrite bead, or a small inductor, and a cap, can help a lot. I just tried that this morning. https://dl.dropboxusercontent.com/u/53724080/Parts/VREGS/CUI_P7805.zip You might need an EMI filter on the switcher input, too. If the coupling is inductive or ground-loop, the fix isn't as simple. We use mostly wall-wart supplies, to keep the AC line out of our boxes and to better manage noise. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Reply by ●September 6, 20162016-09-06
On Tue, 6 Sep 2016 09:46:13 -0700 (PDT), hondgm@yahoo.com wrote:>I'm working on a PCB that contains nearly 30 resistors in a ladder configuration, each step switched in or out by a SMT solid state relay. It's essentially a decade box for a specific application. The relays are controlled by a micro and all is powered by a boardmount enclosed CUI AC/DC switching converter, supplying up to 400mA. > >The resistors are electrically isolated from the SMPS, however I see a lot of noise on the resistance output that can be from nothing other than the SMPS. I'm assuming the power planes on the board are inducing all kinds of noise into the traces that connect the resistors and solid state relays. > >Replacing the SMPS with a linear would be ideal, but the amount of space required is a big problem. So I'm looking at filtering the output of the SMPS. I'm a digital and DC analog guy, so this switching noise scares me. Some research and an educated guess tells me to try an LC filter on the SMPS output. I'm looking for any first hand experience with this and dumbed-down advice that will hopefully get me a solution without days of headaches. > >The SMPS claims a 100kHz switching frequency, and spikes on the scope confirm that. I've already tried putting several MLCC 1uF caps across the SMPS output but that didn't change anything.Proper layout of the SMPS is critical. If that's still a problem, you can LC filter the output or follow it with a small LDO. Be careful with your scope grounding, too. The noise may not be real.
Reply by ●September 7, 20162016-09-07
Most likely common mode noise. Whereas for differential noise, you have the right idea (shunt it with a cap, or make an LC filter), it doesn't do anything for common mode. Rotate your perspective a little. Suppose the output DC+/- act, together, in parallel, as a single conductor (which is true, at RF, because there's a huge capacitor between them!). Suppose the mains AC H/N behave the same way (for the same reason, usually). Finally, suppose there were a noise source wired between these two connections. In reality, there are four wires, but it doesn't matter which ones the noise source connects between, because each pair acts the same way at RF. That noise voltage has to go somewhere. If mains is ultimately near or at ground, then the noise stacks on top, and your circuit is riding on the noise voltage. When the noise voltage enters a sensitive circuit, you see interference problems. The hallmark of common mode noise is that it's "noise that's not really there", i.e., the circuit (for the most part) keeps on working as it normally does, yet you measure the same noise waveform /anywhere/ in the circuit. The most illustrative method being: with an oscilloscope and 10x probe, clip the ground clip to the probe tip. Poke the tip into circuit 'GND' (which, because it's being driven by a noise source, is not really "ground", as the oscilloscope sees it!). Note the waveform is not actually zero, despite the probe tip being "shorted" by the ground clip! Ground clips make very poor RF grounds, and the RF voltage dropped across that ~10cm piece of wire is what you measure. What to do? Do the same thing as ever: filter it! You use an LC filter exactly the same way as usual: the prime difference is, you can't simply tie a huge capacitor from output to mains. You must use smaller (~1000pF) Y1 rated capacitors, which will withstand hazardous mains voltages without letting through much leakage current. Since you're filtering pairs of wires, that ultimately aren't identical (they're carrying a DC or low frequency AC voltage, which is the whole point!), you can't simply use one inductor; instead, you use a common mode choke. If the DC output should be isolated, you should use Y1 type capacitors (as small as practical), from AC H/N to safety GND, and from GND to DC output. If the output doesn't need to be isolated, simply ground it, so you need only the mains Y1 caps. The ultimate purpose is to make the mains wires, and the output wires, have the same RF voltage: zero. The power supply is allowed to generate its noise voltage, which is dropped across a common mode choke. (The choke can be on either side of the power supply, but it's usually on the primary side because it can be physically smaller.) Tim -- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com <hondgm@yahoo.com> wrote in message news:dfdba2e2-186a-4ad4-b417-12318f04c1e6@googlegroups.com... I'm working on a PCB that contains nearly 30 resistors in a ladder configuration, each step switched in or out by a SMT solid state relay. It's essentially a decade box for a specific application. The relays are controlled by a micro and all is powered by a boardmount enclosed CUI AC/DC switching converter, supplying up to 400mA. The resistors are electrically isolated from the SMPS, however I see a lot of noise on the resistance output that can be from nothing other than the SMPS. I'm assuming the power planes on the board are inducing all kinds of noise into the traces that connect the resistors and solid state relays. Replacing the SMPS with a linear would be ideal, but the amount of space required is a big problem. So I'm looking at filtering the output of the SMPS. I'm a digital and DC analog guy, so this switching noise scares me. Some research and an educated guess tells me to try an LC filter on the SMPS output. I'm looking for any first hand experience with this and dumbed-down advice that will hopefully get me a solution without days of headaches. The SMPS claims a 100kHz switching frequency, and spikes on the scope confirm that. I've already tried putting several MLCC 1uF caps across the SMPS output but that didn't change anything.
Reply by ●September 7, 20162016-09-07
On Tuesday, September 6, 2016 at 4:11:46 PM UTC-4, John Larkin wrote:> On Tue, 6 Sep 2016 09:46:13 -0700 (PDT), hondgm@yahoo.com wrote: > > >I'm working on a PCB that contains nearly 30 resistors in a ladder configuration, each step switched in or out by a SMT solid state relay. It's essentially a decade box for a specific application. The relays are controlled by a micro and all is powered by a boardmount enclosed CUI AC/DC switching converter, supplying up to 400mA. > > > >The resistors are electrically isolated from the SMPS, however I see a lot of noise on the resistance output that can be from nothing other than the SMPS. I'm assuming the power planes on the board are inducing all kinds of noise into the traces that connect the resistors and solid state relays. > > > >Replacing the SMPS with a linear would be ideal, but the amount of space required is a big problem. So I'm looking at filtering the output of the SMPS. I'm a digital and DC analog guy, so this switching noise scares me. Some research and an educated guess tells me to try an LC filter on the SMPS output. I'm looking for any first hand experience with this and dumbed-down advice that will hopefully get me a solution without days of headaches. > > > >The SMPS claims a 100kHz switching frequency, and spikes on the scope confirm that. I've already tried putting several MLCC 1uF caps across the SMPS output but that didn't change anything. > > If the noise is in-plain-sight conducted, adding a ferrite bead, or a > small inductor, and a cap, can help a lot. I just tried that this > morning. > > https://dl.dropboxusercontent.com/u/53724080/Parts/VREGS/CUI_P7805.zipWhat's 600 ohms on the FB. Is that it's impedance at 10 or 100 MHz? I figured out yesterday how to do the ground for a board with multiple CUI smps's. Shunting everyone's noise to ground, made the local gnd "bad". A star gnd should work but I want to test that today. George H.> > You might need an EMI filter on the switcher input, too. If the > coupling is inductive or ground-loop, the fix isn't as simple. > > We use mostly wall-wart supplies, to keep the AC line out of our boxes > and to better manage noise. > > > > -- > > John Larkin Highland Technology, Inc > picosecond timing precision measurement > > jlarkin att highlandtechnology dott com > http://www.highlandtechnology.com
Reply by ●September 7, 20162016-09-07
Thanks for everyone's responses. I tried some LC filtering and saw no obvious improvement. At that point it became obvious to me that I have to use a linear power supply since a switcher will never be quiet enough. This device is used to calibrate resistance measuring devices, so a switcher probably should have never been in there in the first place.
Reply by ●September 7, 20162016-09-07
On Wed, 7 Sep 2016 05:35:23 -0700 (PDT), George Herold <gherold@teachspin.com> wrote:>On Tuesday, September 6, 2016 at 4:11:46 PM UTC-4, John Larkin wrote: >> On Tue, 6 Sep 2016 09:46:13 -0700 (PDT), hondgm@yahoo.com wrote: >> >> >I'm working on a PCB that contains nearly 30 resistors in a ladder configuration, each step switched in or out by a SMT solid state relay. It's essentially a decade box for a specific application. The relays are controlled by a micro and all is powered by a boardmount enclosed CUI AC/DC switching converter, supplying up to 400mA. >> > >> >The resistors are electrically isolated from the SMPS, however I see a lot of noise on the resistance output that can be from nothing other than the SMPS. I'm assuming the power planes on the board are inducing all kinds of noise into the traces that connect the resistors and solid state relays. >> > >> >Replacing the SMPS with a linear would be ideal, but the amount of space required is a big problem. So I'm looking at filtering the output of the SMPS. I'm a digital and DC analog guy, so this switching noise scares me. Some research and an educated guess tells me to try an LC filter on the SMPS output. I'm looking for any first hand experience with this and dumbed-down advice that will hopefully get me a solution without days of headaches. >> > >> >The SMPS claims a 100kHz switching frequency, and spikes on the scope confirm that. I've already tried putting several MLCC 1uF caps across the SMPS output but that didn't change anything. >> >> If the noise is in-plain-sight conducted, adding a ferrite bead, or a >> small inductor, and a cap, can help a lot. I just tried that this >> morning. >> >> https://dl.dropboxusercontent.com/u/53724080/Parts/VREGS/CUI_P7805.zip > >What's 600 ohms on the FB. Is that it's impedance at 10 or 100 MHz?The convention is Z at 100 MHz. That bead has a low-frequency inductance of a few uH.> >I figured out yesterday how to do the ground for a board with multiple >CUI smps's. Shunting everyone's noise to ground, made the local gnd >"bad". A star gnd should work but I want to test that today.One can cut a ground peninsula out of the PCB ground plane and locate things such as to keep switcher ground loop currents out of the rest of the board. -- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
