LM317 & LM337 Current Noise/ Tektronix 7104 and more. - Page 1
Forum section: electronics / projects

author: Rupunzell
author_rank: Frequent Contributor
timestamp: April 03, 2015, 05:34:57 am
content: Some months ago while testing a first pcb run circuit on the bench. It occurred to me that since it was all set up and test gear in place, it would be good to document current noise generated by three terminal regulators. This includes LM317-LM337 and their related fixed voltage parts. The amount of current noise varies among brands, output voltage and other factors, but they all do it do some degree. The current noise generated by these IC regulators tends to appear in the system at unexpected circuit locations resulting in higher system noise than could otherwise be. Since this was a proto test set up, the current noise simply does not matter for this aspect of testing. The most common approach to evaluate power supply noise is to measure the output noise voltage by applying a voltage probe at the test node. This is not too difficult a measurement to do. Measuring current is a bit more difficult for a host of reasons.The scope is a Tektronix 7104 (my daily user) with one 7A26 dual trace 200 Mhz, 7A19 single trace 50 ohm 600 Mhz for the P6042 current probe. This was used as a easy way to get a 50 ohm input for one channel of the 7104. Time bases are 7B15 & 7B10 which pretty much stays in the 7104 most all of the time. Two hewlett packard 10041A probes for the 7A26 and the Tektronix P6042 current probe on the 7A19 for measuring current. Images were taken with an iPhone hand held, notes made using photoshop.The circuit under test cannot be revealed as it is production item. It runs on +/- 30 volts DC power supply made up using an LM317 & LM337, generic configuration with bypass caps in-out, reference bypassed. The mains power is on a variac to allow constantly variable mains voltage by the twist of a knob.Here is what the set up looks like. The probes are clustered together with a single ground point for the system including instrumentation. Power supply comes from three twisted wires to reduce field pick-up and radiation. Power wires are also routed away from the input-outputs. The current probe is on the power supply ground wire.Tek 7104 display with both outputs of this circuit and the ground current probe in the center. The peak ground current goes up and down off the CRT by about 5 divisions to show an expanded area of the current signal. Vertical over drive by this amount has no effect on the displayed signal fidelity as the Tek's vertical system has sufficient dynamic range and fast enough recovery to remain accurate.Annotated image of the CRT. LM317 & LM337 regulators are not regulating. The power mains variac has been adjusted to below the regulators drop out voltage.Another version of this image. At about 8 division of vertical over drive on the 7104, the vertical system begins to complain. This is visible as a curvature on the triangle wave at the two output traces.Main power variac adjusted until the IC regulators are just beginning to regulate. The internal band gap reference is just starting to turn on and the switching transient from this turn on is visible on the current trace. The noise from the band gap starting to work is starting to be visible on the left hand side of the current trace were the IC regulators are beginning to regulate. The band gap on-off transient is visible on both output traces as bumps in the circled area.Adjusting the mains variac to a higher voltage puts both IC regulators into regulation and producing ground current noise.Bernice

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 03, 2015, 07:00:37 am
content: So what were the probes on? You didn't mention.I'm guessing not DC input, as that's a rather fast, sharp and linear triangle ramp for a hand-turned variac.Tim

author: Rupunzell
author_rank: Frequent Contributor
timestamp: April 04, 2015, 01:47:58 am
content: The top and bottom trace are probes on the output of this device which outputs a triangle wave (up to +/- 25 Volts peak) when driven. Those probes happened to be in this test set up at the moment this idea of documenting the current noise from three terminal regulators came to mind.Put a voltage probe at the output of these three terminal regulators and there will not be much beyond the power mains ripple and a bit of noise voltage (few hundred microvots between DC to 30Khz) as the regulators go in and out of regulation.The significant information here is the current noise from three terminal regulates due to the internal band gap reference. The image of most interest is the band gap reference switching on-off and the transient as a result of that. To capture this behavior of the ban gap reference, the variac on the power mains was adjusted until the three terminal band gap reference was at the switching point. The triangle waves on the top and bottom traces have no relationship to the power mains variac. The DUT has no effect on the ground current noise produced. Load resistors can be put in place of the DUT with identical-extremely similar results.Why does this matter, for those who ever wondered why their low noise analog amplifier or A/D was less than low noise, it is due to these three terminal regulators dumping noise current into the system and that current noise re-appearing where is should not.BerniceQuote from: T3sl4co1l on April 03, 2015, 07:00:37 am