Linear regulator

Hi all!

Over in this thread I'm discussing using an accelerometer for highly sensitive readings:

One of the things that came up, was that I should consider using a very low noise voltage supply. This issue has come up a few times in projects, but I'm not sure exactly what is the best cost-effective way of doing this.

I've heard about 'linear regulators' which seem to be the ideal thing (inefficient but accurate)

There are some products like this one for ~69cents / piece LM317 DC Linear Regulator DC-DC 4.5~40V Turn 1.2~37V Step Down Power Module Adjustable

And some from the hifi market which are more expensive at $18 / piece LT3042 Ultra Low Noise Linear Regulator Power suoppy / Single Voltage Output 5V For Amanero XMOS USB DAC Core Power Supply

These more expensive ones have much larger capacitors, and quote noise at 2nV at 10kHz. (The cheaper ones do not quote a noise level).

Would the first type of product have any improved performance over the 5V step down power supply built into an Arduino UNO, or a 240V switched mode 5V 2000mAh (phone charger type) USB power supply?

Thank you Elliot

Switch mode supplies often have 50 to 100mV of noise on the output which is not an issue for digital devices, but a show-stopped for sensitive analog circuitry like a microphone amp or high resolution ADC/DAC circuits. Linear regulators are always better than that, some are way better, some are good when the raw supply is good but let noise through from it.

High frequency switch mode converters are often much better as all the noise is above the frequencies of interest for the analog circuitry.

Most Arduinos use a linear regulator. That's why there's limits on the input voltages. (A switcher can easily have a max input voltage of 35V.)

Most high-accuracy power supplies get close with a switcher and then use a linear regulator operating just above its dropout voltage to get a smooth supply for the analog part of the circuit. Depending on what you're doing, you may need to consider a separate analog ground and pay attention to where the digital currents are flowing underneath your analog components.

Not only can the switching supply put noise on the voltage output, other things can as well. The microprocessor and any other noisy circuits will also cause noise on the supply line. If you are trying to make very accurate analog readings you want to separate the analog circuits from the digital circuits and that means a separate supply in many cases.

From your advice above, it sounds like I should put the regulator after the Arduino rather than before it, in order to cancel out the noise produced by the microprocessor itself on the voltage line.

Is there an example of a low cost component/module which delivers low ripple/noise on a 3.3VDC output? I could then use that to power the MPU6050 directly.

2-3 orders of magnitude less noise than the Arduino Uno 3.3V output would be great

I want to reduce the number of wire connections to the area which is housing the MPU6050, so ideally I can step down from something I have already (+5VDC, +12VDC or +24VDC) as the source.

Thanks! Elliot

That's a pretty steep requirement for noise. It's already a linear regulator with half-decent filtering caps.

Then you're feeding a digital sensor anyway? It is going to inject significant noise into the supply lines as the digital components inside the chip turn on and off. Just put one or two good decoupling caps onto the power supply for the chip and go on to the next design problem.

The MPU6050 does not have any special requirements for power supply noise mentioned in the datasheet. It does give you instructions on not routing anything else underneath the chip and a rather vague instruction on putting the decoupling cap close by.

LearnEMC has a lot of good tutorials on this stuff. One good, simple one is: If this seems relevant to you, look for more on that site.

If you're using a 5V Arduino, I would regulate that down to 3.0 - 3.3V to supply the IMU. If you are using a 3.3V Arduino you might be able to regulate down to 3.0V, but not all regulators can handle that small of a drop.

It looks like the current draw of the IMU is less than 5mA, so a voltage reference will work. Something like a Maxim MAX6103, TI REF3030 or Analog Devices AD1583 should all work. They only need 0.2V headroom, so you could regulate from 3.3V down to 3V. BTW some of the datasheets say the output capacitors are optional. Use them in your circuit. They will help.

...And the datasheets which don't say the capacitors are optional but just draw them in on the suggested circuit, require capacitors. Some of them can even be upset by the wrong type of capacitors.

Thanks all for your advice here Sorry I've been out for a while :)

Current strategy:

Step down +24VDC to +5VDC for an Orange Pi using this buck converter:

Step down +5VDC to +3.3VDC using the linear regulator on the (ahem, unofficial) Arduino Nano: an AMS1117.

The DC motors are on the +24VDC line on L298N module boards which have decoupling caps. I presume that the +5VDC line will be relatively clean since it's a signfiicant drop for the buck converter (19V headroom, so it'll mostly ignore the noise 'up there')

Then stepping down +5VDC to +3.3VDC using a linear regulator should (hopefully) kill a lot of the noise coming from the logic circuits.

I was unaware that the regulator on the arduino is already quite low noise by virtue of it being a linear regulator.