| Electronics > Beginners |
| dc-dc converter supply opamp ultra low noise amplifier |
| (1/2) > >> |
| Yrrah:
Hello, I am working on a phase noise measuring system (UKW Berichte 4/2015...). It deploys an ultra low noise amplifier (10 Hz - 90 kHz, measuring range, the amp itself goes to about 500 kHz and beyond). I will use a 12 V lead battery to supply the instrument. I need +/- 8V to supply low noise opamps (e.g. LM4562), obviously of high quality (low noise, no spikes, reasonably stable). I plan to use the well known 723 for that, because of low noise. To generate the negative bias a dc-dc converter is advised, I bought a Mean Well SMU02M-12 for the negative bias. Switching freq over 100 kHz so above the highest measuring frequency of 90 kHz. That will be followed by the 723 neg reg. I have a testboard on the bench. Battery, 47 uF at the input, DC-DC converter; double PI filter at the output: 47 uF, in each + and - lead: 1 mH and 100 uF. Load is 120 ohm resistor. Scope (Tek2235) at battery: every 8usec (about 120 kHz) a big spike of about 800 mV tt. Scope at resistor is almost the same. No 723 involved yet. I am unsure whether this is pure radiation, do I need more filtering? As even the the battery terminals are polluted I am a bit unsure how to move on. I plan to place the dc-dc converter with 723 in one diecast box (12V in, -8V out). Any advice welcome as are pointers to relevant postings or literature. Thanks in advance, Yrrah |
| ogden:
LM723 is old, PSRR specified only up-to 10KHz. You may consider LT3042 and LT3093 instead. This article as well: http://www.wenzel.com/documents/finesse.html |
| Marco:
That's a bit of a ramble. If certainty of low noise is important enough to start using batteries and linear regulators for the positive rails I don't see how it makes sense to suddenly start using a switcher for the negative side. It's not that you can't get interference and ripple arbitrarily low ... but you've already decided to take the easy way out for the positive rail, why suddenly stop for the negative rail? Just use two batteries. |
| ejeffrey:
A schematic would be much more helpful than listing the component values. Even just a photo of napkin sketch. A photo of the actual circuit would also be helpful as the layout is very important for things like this. Make sure you are filtering the input as well as the output, and put the filters as close as possible to the pins of the converter. Your filter components might be too big such that either you can't get them close enough or the parasitic values compromise their behavior. For instance a bunch of small value capacitors in parallel will have less ESL and one of them can be placed right across the pins. For such a small package radiated emissions should be negligible -- even at the 1000'th harmonic (100 MHz) and I doubt that converter has substantial noise power at 1 GHz. However, you might get near field magnetic coupling, so magnetic shielding around it might help. You also have to use good probing techniques. You could be seeing common mode conversion by your probe or magnetic pickup from the ground loop if you are using the little alligator clip lead. Alternately, get a second 12V battery for the negative supply and get rid of the switch mode converter altogether. Or, for a low power application use a capacitive doubler based inverter to generate the negative supply. This can be considerably easier to filter than a standard switching converter. |
| schmitt trigger:
I ignore whether you can answer the following questions by looking at the Meanwell unit, but here they go: Is it operating in DCM or CCM mode? DCM impose demanding switching characteristics that can cause noise spikes. Is the main magnetic component a toroid? Or a shielded core like a PQ core? The reason is that the cheaper EE cores have significant stray fields. If you can slow the main Mosfet's rise and fall times, noise can be reduced, at the expense of lowered efficiency. Lastly, related to the measurement techniques, which you may or may not be aware, but I am going to ask anyways: Are you aware that with a scope probe, the long ground lead picks up a lot of noise? There is an EXCELLENT app note from Jim Williams, and I believe from Bob Pease too, which describes scope probing techniques for ultra-low noise measurements. |
| Navigation |
| Message Index |
| Next page |