5
\$\begingroup\$

I build a mic amplifier for a standard hands free (Apple 4-pole jack). However, even without anything plugged into the jack there's audible noise at the output (MIC_IN in diagram). It sounds like white noise to my ears.

Increasing C13 to 100 uF almost completely eliminates the noise. However, to my surprise, removing C7 also does that. Please help me understand the forces at play here and what a proper fix for the noise is.

The VCC3 supply has lots of digital noise in it. The circuit is using 0805 / 0603 resistors and capacitors.

enter image description here

\$\endgroup\$
6
  • \$\begingroup\$ Whats the purpose of the R9? \$\endgroup\$ – Marko Buršič Jul 24 '20 at 19:05
  • \$\begingroup\$ C7 can allow noise from GND to bypass R11 and into the amp. Depends on how your grounds are laid out. \$\endgroup\$ – Aaron Jul 24 '20 at 19:05
  • \$\begingroup\$ @MarkoBuršič R9 is supplying power to the condenser mic. \$\endgroup\$ – roc1 Jul 24 '20 at 19:23
  • \$\begingroup\$ @Aaron I use a mostly uninterrupted ground plane for the entire board bottom side. Everything you see on this circuit is laid out close together. The digital bits are away. \$\endgroup\$ – roc1 Jul 24 '20 at 19:23
  • 1
    \$\begingroup\$ Removing C7 allows common noise on both inputs which cancel out for some happenstance values of source impedance. \$\endgroup\$ – user_1818839 Jul 24 '20 at 21:22
6
\$\begingroup\$

You might want to consider these things: -

enter image description here

Without the input resistor the 220 nF has an impedance of 723 ohms and this means that your mid band gain (1 kHz) is 150,000/723 = 207. At 10 kHz the gain will be 2070 and you will hear a lot of noise. Try a 10 kohm resistor in series.

\$\endgroup\$
2
  • \$\begingroup\$ I believe an input resistance is not required, because this is a trans-impedance amplifier - the mic is generating AC current. I got the original circuit from a TI application note, and they don't add an input resistor: electronics.stackexchange.com/questions/292534/… \$\endgroup\$ – roc1 Jul 25 '20 at 20:37
  • \$\begingroup\$ The noise is coming from the power supply via the 3k3 resistor. If you are correct about the microphone amplifier being a current source AND, when you removed C7 the noise got less then the modification called "a tad more filtering" will improve this. I would still use an input resistor though. \$\endgroup\$ – Andy aka Jul 27 '20 at 22:20
0
\$\begingroup\$

Don't supply the OPAMP and its input circuitry from voltage reference, latter has to have a high feedback gain to remain low drift, so it picks up noise if you extend a long wire from it. Use AVCC instead.

\$\endgroup\$
1
  • 1
    \$\begingroup\$ Could you elaborate? The opamp is powered from AVCC, only the input is biased from the voltage reference (that is also used by the following ADC). \$\endgroup\$ – roc1 Jul 24 '20 at 19:27
0
\$\begingroup\$

I figured out the issue. The circuit amplifies the power supply noise (in Vcc) by R13 / R9 times. Therefore, it is advantageous to have to have good power supply filtering and/or higher mic supply voltage.

@Andy aka's to add more filtering in the supply will help. An input resistor is not required. More details here: https://www.ti.com/lit/ug/tidu765/tidu765.pdf

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.