PCM186X datasheet(21/115 Pages) TI1

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Mix

Mux

Mix

Mux

Audio

ADC

Audio

ADC

Analog

PGA

Digital

PGA

PGA Zero

Cross

Detect

VINL4 (4M)

VINL3 (4P)

VINL2 (1M)

VINL1 (1P)

VINR1 (2P)

VINR2 (2M)

VINR4 (3M)

VINR3 (3P)

Digital Microphone

PDM Input

(Software programmable devices only)

Decimation and other filters

Mic Bias

Generator

PCM1860, PCM1862, PCM1864, PCM1864-Q1

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SLASE55A – NOVEMBER 2014 – REVISED DECEMBER 2014

10.4 Analog Front End

Figure 21. High Level View of PCM186x Front End Circuitry

The PCM186x has a universal front end that accepts differential or single-ended inputs, from microphone level to

2.1VRMS. The highest performance (up to 103 dB SNR) can be achieved using differential inputs. In differential

mode, the full scale voltage is defined as 4.2VRMS, where each pin is 2.1VRMS, out of phase.

The front-end Mix and MUX circuit allows both differential and single-ended inputs to be used in the products, as

well as direct Input mixing. This feature is mainly enabled on the software-controlled devices, while the hardware-

controlled devices support single-ended or differential inputs. The Mix and MUX circuits are summing circuits,

done pre-PGA. No individual volume controls are available before the PGA.

DC blocking capacitors are required on the inputs, to ensure that the DC bias conditions are known (assumed to

be GND, but not certain). Also, the value of the output short-circuit protection resistor in the source product is not

known, which could cause issues such as gain error and DC shift.

10.5 Microphone Support

The PCM186x supports analog and digital microphones. Analog signals are treated in much the same way as

line-level signals, except for the requirement for mic bias. Digital microphone Inputs (PDM inputs) use GPIOs on

the device. Two-channel ADC variants of the PCM186x family can support two digital microphones using a single

data pin and a single clock pin. The 4-channel variants can support up to 4 digital microphones (2 data pins).

Three gain choices are available in the PCM1860, controlled by pins MD2, MD5 and MD6; 0 dB, 12 dB, or 32

dB.

The PCM186x software programmable devices support electret condenser mics through a mic bias circuit and a

PGA input providing up to 32 dB of gain.

Digital microphones typically have a PDM output that can be brought into an ADC digital decimation filter. PDM

microphones require power and a clock. Power should be handled from an external source.

Digital microphone mode Gain can be added in the digital PGA and in the mixer. The Maximum gain is 30 dB (18

dB in the mixer + 12 dB in the digital PGA).

On the PCM1864, a 2-channel digital mic + 2-channel ADC mode is possible. With the PCM1862, four channels

are only possible with a ADC + I2S input configuration.

10.5.1 Mic Bias

The PCM186x can provide a microphone bias to power and bias microphones at 2.6V on pin 5. Mic Bias should

be decoupled/filtered with an external capacitor. Mic Bias is typically used with a electret microphone. An on-chip

series resistor can be bypassed using register MIC_BIAS_CTRL (Page.3, 0x15). By default, the device is

configured to bypass the on-chip resistor.

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Product Folder Links: PCM1860 PCM1862 PCM1864 PCM1864-Q1

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