filtering noisy power signal


Posted on Feb 5, 2014

Previously I used a linear regulator, now I use Max756 ( ) 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.


filtering noisy power signal
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Here is my scope output (blue is power signal and yellow is mic output while it`s quiet): First, 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...




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