> 3.15.2 | Microphone Circuit

> TC65 has two identical analog microphone inputs. There is no on-board microphone supply circuit, except for the internal voltage supply VMIC and the dedicated audio ground line AGND. Both lines are well suited to feed a balanced audio application or a single-ended audio application.
> The AGND line on the TC65 board is especially provided to achieve best grounding conditions for your audio application. As there is less current flowing than through other GND lines of the module or the application, this solution will avoid hum and buzz problems.
> While TC65 is in Power-down mode, the input voltage at any MIC pin must not exceed ±0.3 V relative to AGND (see also Section 5.1). In any other operating state the voltage applied to any MIC pin must be in the range of +2.4 V to 0 V, otherwise undervoltage shutdown may be caused. Consider that the maximum full scale input voltage is Vpp = 1.6 V.
> If VMIC is used to generate the MICP-pin bias voltage as shown in the following examples consider that VMIC is switched off (0V) outside a call. Audio signals applied to MICP in this case must not fall below -0.3 V.
> If higher input levels are used especially in the line input configuration the signal level must be limited to 600 mVpp outside a call, or AT^SNFM=1 should be used to switch on VMIC permanently.

> 3.15.2.1 | Single-ended Microphone Input

> Figure 25 as well as Figure 51 show an example of how to integrate a single-ended microphone input.

> | R A = typ. 2k R B = typ. 5k R VMIC = typ. 470Ohm C k = typ. 100nF C F = typ. 22µF V MIC = typ. 2.5V V bias = 1.0V … 1.6V, typ. 1.5V
> Figure 25: Single ended microphone input |

> R A has to be chosen so that the DC voltage across the microphone falls into the bias voltage range of 1.0 V to 1.6 V and the microphone feeding current meets its specification.
> The MICNx input is automatically self biased to the MICPx DC level. It is AC coupled via C K to a resistive divider which is used to optimize supply noise cancellation by the differential microphone amplifier in the module.
> The VMIC voltage should be filtered if gains larger than 20 dB are used. The filter can be attached as a simple first order RC-network (RVMIC and CF).
> This circuit is well suited if the distance between microphone and module is kept short. Due to good grounding the microphone can be easily ESD protected as its housing usually connects to the negative terminal.

> 3.15.2.2 | Differential Microphone Input

> Figure 26 shows a differential solution for connecting an electret microphone.

> | R A = typ. 1 k R VMIC = 470 Ω C K = typ. 100 nF C F = typ. 22 µF V MIC = typ. 2.5 V V bias = 1.0V … 1.6 V, typ. 1.5 V
> Figure 26: Differential microphone input |

> The advantage of this circuit is that it can be used if the application involves longer lines between microphone and module.
> While VMIC is switched off, the input voltage at any MIC pin should not exceed ± 0.25 V relative to AGND (see also Section 5.1). In this case no bias voltage has to be supplied from the customer circuit to the MIC pin and any signal voltage should be smaller than Vpp = 0.5 V.
> VMIC can be used to generate the MICP-pin bias voltage as shown below. In this case the bias voltage is only applied if VMIC is switched on.
> Only if VMIC is switched on, can the voltage applied to any MIC pin be in the range of 2.4 V to 0V. If these limits are exceeded undervoltage shutdown may be caused.
> Consider that the maximum full scale input voltage is Vpp = 1.6 V.
> The behavior of VMIC can be controlled with the parameter micVccCtl of the AT command AT^SNFM (see [1]):

> • | micVccCtl=2 (default). VMIC is controlled automatically by the module. VMIC is always switched on while the internal audio circuits of the module are active (e.g., during a call). VMIC can be used as indicator for active audio in the module.

> • | micVccCtl=1. VMIC is switched on continuously. This setting can be used to supply the microphone in order to use the signal in other customer circuits as well. However, this setting leads to a higher current consumption in SLEEP modes.

> • | micVccCtl=0. VMIC is permanently switched off.

> 3.15.2.3 | Line Input Configuration with OpAmp

> Figure 27 shows an example of how to connect an opamp into the microphone circuit.

> | R A = typ. 47 k R VMIC = 470 Ω C k = typ. 100 nF C F = typ. 22 µF V MIC = typ. 2.5 V V bias = typ. ½ VMIC = 1.25 V
> Figure 27: Line input configuration with OpAmp |

> The AC source (e.g. an opamp) and its reference potential have to be AC coupled to the MICPx resp. MICNx input terminals. The voltage divider between VMIC and AGND is necessary to bias the input amplifier. MICNx is automatically self biased to the MICPx DC level.
> The VMIC voltage should be filtered if gains larger than 20dB are used. The filter can be attached as a simple first order RC-network (RVMIC and CF). If a high input level and a lower gain are applied the filter is not necessary.
> Consider that if VMIC is switched off, the signal voltage should be limited to Vpp = 0.5V and any bias voltage must not be applied. Otherwise VMIC can be switched on permanently by using AT^SNFM=,1. In this case the current consumption in SLEEP modes is higher.
> If desired, MICNx via C K can also be connected to the inverse output of the AC source instead of connecting it to the reference potential for differential line input.