WM8224 datasheet(21/50 Pages) WOLFSON

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WM8224

Production Data

PD, Rev 4.1, June 2012

CDS/NON-CDS PROCESSING

For CCD type input signals, containing a fixed reference/reset level, the signal may be processed

using Correlated Double Sampling (CDS), which will remove pixel-by-pixel common mode noise. With

CDS processing the input waveform is sampled at two different points in time for each pixel, once

during the reference/reset level and once during the video level. To sample using CDS, register bit

CDS must be set to 1 (default). This causes the signal reference to come from the video reference

level as shown in Figure 16.

The video sample is always taken on the falling edge of the input VSMP signal (VS). In CDS-mode

the reset level is sampled on the falling edge of the RSMP input signal (RS).

For input signals that do not contain a reference/reset level (e.g. CIS sensor signals), non-CDS

processing is used (CDS=0). In this case, the video level is processed with respect to the voltage on

pin VRLC/VBIAS. The VRLC/VBIAS voltage is sampled at the same time as VSMP samples the

video level in this mode. Note that if the ac coupling capacitor (CIN) is used in non-CDS mode

(CDS=0), then to minimise code drift, line clamping should be used and internal input voltage buffers

enabled using the FOL_EN register bit.

Alternatively, if the input signal contains a stable

reference/reset level then pixel clamping should be used, and the voltage buffers need not be

enabled.

Figure 16 CDS/non-CDS Input Configuration

OFFSET ADJUST AND PROGRAMMABLE GAIN

The output from the CDS block is a differential signal, which is added to the output of an 8-bit Offset

DAC to compensate for offsets and then amplified by a 9-bit PGA. The gain and offset for each

channel are independently programmable by writing to control bits DAC[7:0] and PGA[8:0].

The gain characteristic of the WM8224 PGA is shown in Figure 17. Figure 18 shows the maximum

device input voltage that can be gained up to match the ADC full-scale input range (default=2V).

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