[help] Question about filtering noisy power signal - Page 1
Forum section: electronics / projects

author: mribble
author_rank: Regular Contributor
timestamp: June 17, 2011, 07:39:51 pm
content: I am redesigning a circuit. Previously I used a linear regulator, now I use Max756 (http://datasheets.maxim-ic.com/en/ds/MAX756-MAX757.pdf) boost converter. I'm seeing 0.1 - 0.3V output ripple in the current. The Max756 datasheet says I should expect about 50 mA ripple. So my power is a little noisier than expected, but my real problem is when powering a microphone (plugged into sensor1) I see 2.4V of ripple. This just isn't acceptable for my needs.Here is my circuit:http://www.glacialwanderer.com/_blog/blog2011/06_June/CA53_schematic.pngHere is my scope output (blue is power signal and yellow is mic output while it's quiet):http://www.glacialwanderer.com/_blog/blog2011/06_June/IMG_0821_30d.jpgHere is the circuit to the microphone:http://www.sparkfun.com/datasheets/BreakoutBoards/Amplified-Mic-Electret-v14.pdfThe time between peaks in the the ripple is 110 us.Is there another capacitor I could add or some other thing to the circuit that would filter out the noise?Thanks!

author: ejeffrey
author_rank: Super Contributor
timestamp: June 17, 2011, 08:11:52 pm
content: Yeah. You have two problemsFirst, your microphone amplifier has a gain of 100 and sees ripple on the power supply exactly the same as a signal from the microphone. You need to heavily filter the supply going to the microphone and amplifier circuit. First thing, add a bigger capacitor (maybe 10 uF) in parallel with the 100 nF bypass capacitor on the breakout board. Then add a resistor or inductor in series with the Vcc line from the main board to the microphone breakout board.Your second problem is that your switch frequency is too low. The switching frequency is 10 kHz -- you are just never going to get good audio performance with a 10 kHz switcher anywhere nearby. If you look at the data sheet, the pulse frequency drops with load current. It looks like this particular boost converter has a fixed duration 'on' time, and changes the off time to vary the duty cycle as needed. Your circuit is drawing relatively low current so the pulse frequency is very low. You would prefer to have a style of regulator that works by pulse width modulation: the pulse frequency is constant, but the 'on' time is varied to regulate the output voltage. Alternately, look for a regulator of the same style that works at a higher frequency under your actual load current. 100 kHz is easier to filter from the supplies, easier to filter from the audio path, and inaudible even if a bit makes it through.

author: mribble
author_rank: Regular Contributor
timestamp: June 18, 2011, 09:54:58 am
content: Thanks! I will try some of these suggestions and get back to you. I'd really rather avoid replacing the my boost converter at this late stage in the design. I'm not trying to capture decent audio, all I want to do is detect noise. So I'd be willing to accept some ripple in the sound wave.

author: Unknown
author_rank: Unknown
timestamp: June 18, 2011, 11:10:46 am
content: Getting an amplifier with better power supply ripple rejection would also help. Haven't checked the amplifier schematic, but if ejeffrey is correct that the PSRR is essentially zero, you will have a very hard time getting a low noise floor.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 18, 2011, 01:10:53 pm
content: The problem is not with the op-amp -- although the virtual earth is also problematic it is reasonably well bypassed on this schematic. The real problem is the microphone. The electret capsule is a 2-pin device that uses a single pin for power in and signal out with capacitive coupling to separate them. Any noise on the supply will be be coupled via the input capacitor to the opamp.

author: mribble
author_rank: Regular Contributor
timestamp: June 25, 2011, 09:19:20 am
content: Thanks for all your help! After various experimentation I've decided to switch to the TPS61200 boost converter. I have used it in the past and know that it gives a much cleaner signal. I just couldn't get the max chip to give clean enough power for my use case.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 25, 2011, 10:48:15 am
content: Sounds good. The higher and fixed frequency of that device should make it easier to filter the ripple and also keep it out of the audio band. I would still consider additional filtering on the microphone simply because it bothers my sense of aesthetics to allow PS ripple to directly couple to the audio input, but since you aren't doing hi-fi and probably don't want to rework the sparkfun breakout board, it really isn't necessary.

author: dimlow
author_rank: Frequent Contributor
timestamp: June 25, 2011, 01:26:55 pm
content: Your scope probe is a x1 right ? 2.4V or .24V?

author: mribble
author_rank: Regular Contributor
timestamp: June 26, 2011, 11:44:35 am
content: Scope was at 1x. The ripple on the actual power is around 100 mA, but that gets amplified a lot with the mic sensor.Thanks for the feeback ejeffrey. I have experimented with putting a fairly large 100 uF cap on the microphone and that does fix it. I can do that as a backup, but hopefully it won't be needed and at the very least the new higher frequency will let me us a much smaller cap on the microphone. I do plan to make my own microphone sensor someday and I'll be sure to incorporate what was mentioned here.One last note that might be helpful for others. The thing that finally made me make this fairly big change late in the design was the max756 was giving me terrible efficiency at my low current loads. I did some experiments and the TI boost converter gives me nearly twice the efficiency of the max chip at my low loads from a single cell lithpoly. I measured 52 mA with the max chip and 27 mA with the TI chip.