PCM1861 datasheet(22/115 Pages) TI

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-12dB

+32dB

0dB

+20dB

-12 to +12dB in 1dB Steps

+12dB

0x74.0

0x0C.0

0x14.0

0x40.0

PGA

Value

CIC Filter

ADC

Analog

PGA

Digital

PGA

Decimation

Filter

Digital

Mixer

S-curve

Volume

Mixer

PGA

-12dB

+32dB

0.5dB to +11.5 dB

-100dB to +18 dB

-100dB

0dB

PCM1861, PCM1863, PCM1865

SLAS831C – MARCH 2014 – REVISED AUGUST 2014

www.ti.com

8.8 Programmable Gain Amplifier

The PCM186x has a two-stage programmable gain amplifier (PGA). Coarse gain adjustment is done in the

analog domain, whilst fine gain adjustment is done in the digital domain. The ±12dB Analog gain steps are

designed for varying line level inputs, whilst the +20dB and +32dB are primarily designed for microphone inputs,

which will likely need additional gain that can be done in the digital domain. The analog gain steps between -

12dB and +12dB are in 1dB steps. Half dB steps between those points are done in the digital PGA. Gain steps

between 12dB and 20dB are all done in the digital domain. (for example, 18dB gain = 12dB analog + 6dB

Digital). The gain structure in the PCM186x is shown below.

Figure 23. PCM186x Complete Gain Structure (PGAs and Attenuator)

The analog gain steps within the analog PGA are shown below. Again, from -12dB to +12dB, the steps are 1dB

each. The digital PGA has granularity down to 0.5dB.

Figure 24. Analog Gain Steps with PCM186x Software Programmable Devices

The PGA in the PCM186x is a hybrid analog and digital programmable gain amplifier. The devices integrate a

lookup table with the optimal gain balance between analog and digital gain, allowing the gain to be set in a single

to the digital.

The PGA is a zero crossing detect type, and has the ability to set target gain, and have the device work towards

it (with a timeout if there is no zero crossing). Any changes in the Analog PGA and Digital PGA are designed to

step towards the final level. However, any changes in the Mixer PGA are immediate. Care should be taken when

changing gain levels in the digital mixer PGA. Alternatively, multiple writes can be made of small enough values

that will not cause significant pops/clicks.

For example, Current level 0dB, set target as 3.5dB – PGA increases gain in 0.5dB steps towards 3.5dB.

The Auto Gain Mapping function can by bypassed if required, using Manual Gain Mapping. Manual Gain

Mapping is particularly useful when using digital microphones, as the PDM input signal bypasses the analog

PGA and must be amplified using the digital PGA. (PGA_MODE Register(Page.0, 0x19)

NOTE

Using the device with a differential signal lowers the PGA gain by 6dB. Designers should

account for this in their software, the PCM186x does not compensate for this.

Differential Analog Gain points are –18 to 6dB, 14dB, 26dB.

8.9 Automatic Clipping Suppression

The PCM186x software controlled devices have the ability to automatically lower the gain in 0.5dB steps under

the following conditions if the ADC is clipping.

The device detects clipping after the decimation filter in the signal chain (shown in Figure 25), and counts the

number of successive clips before responding.

The device can also generate an internal interrupt that can be mapped to a GPIO or interrupt pin, allowing the

system microcontroller to make the decision to increase the gain and consider the clipping an isolated event, or

make the decision that the new gain setting is appropriate.

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Product Folder Links: PCM1861 PCM1863 PCM1865

PCM1861 Datasheet(PDF) 22 Page - Texas Instruments

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