Electrical – Capacitor selection – Audio decoupling capacitor for microphone

audiocapacitordecoupling-capacitor

We are developing a custom application based on the TLV320AIC3111. According to the datasheet, the typical configuration has a 1uF cap on the MIC1LP line, which I assume is for AC decoupling the audio input. However, on the EVM for the same caps they've used 0.47uF. The impedance of the mic we'll be using is around 6ohms and we need a good low-frequency response (our signal can go as low as 100Hz). I need help choosing the right value and type (MLCC or tantalum).

Best Answer

What you need to do is to consider the input capacitor as part of a high pass filter.

The cutoff frequency of a simple RC high pass filter can be computed using the following equation:

$$f_ = \frac{1}{2{\pi}RC}$$

C is the capacitance of the input capacitor in farads.

R is the input impedance of the IC in ohms.

If you refer to page 26 of the datasheet, you will find this description:

The input feed-forward resistance for the MIC1LP input of the microphone PGA stage has three settings, 10 kΩ, 20 kΩ, and 40 kΩ, which are controlled by writing to page 1 / register 48, bits D7 and D6. The input feed-forward resistance value selected affects the gain of the microphone PGA. The ADC PGA gain for the MIC1LP input depends on the setting of page1 / register 48 and page 1 / register 49, bits D7–D6. If D7–D6 are set to 01, then the ADC PGA has 6 dB more gain with respect to the value programmed using page 1 / register 47. If D7–D6 are set to 10, then the ADC PGA has the same gain as programmed using page 1 / register 47. If D7–D6 are set to 11, then the ADC PGA has 6 dB less gain with respect to the value programmed using page 1 / register 47. The same gain scaling is also valid for the MIC1RP and MIC1LM input, based on the feed-forward resistance selected using page 1 / register 48, bits D5–D2.

The input resistance is selectable through the registers of the IC, and are used to set the gain. R therefore depends on how much gain you will need.

Since you give the impedance of the microphone as 6 ohms, I assume it is a low impedance dynamic microphone rather than an electret microphone.

The signal level from dynamic microphones is rather low, so I will assume you need maximum gain.

According to this chart from page 26, that would be with the input resistor set to 10k:

enter image description here

Assuming 10k, and a cutoff of 100Hz, you would need an input capacitor of about 160nF.

Larger capacitors will push the cutoff frequency down. Selecting lower gain will also push the cutoff frequency down.

A tantalum capacitor shouldn't be needed - the required capacitance isn't high enough to force you to use tantalum.

Ceramic capacitors have enough capacitance. There shouldn't be enough DC involved to cause problems with reduced capacitance. The same applies to microphony effects from the ceramic capacitor - not enough DC voltage for it to be a problem.

A 470nF ceramic capaacitor as on your evaluation board should do fine.

Related Topic