WM8195_05 Datasheet, PDF (13/33 Pages) Wolfson Microelectronics plc – 14-bit 12MSPS CIS/CCD Analogue Front End/Digitiser

Production Data
WM8195
DEVICE DESCRIPTION
INTRODUCTION
A block diagram of the device showing the signal path is presented on Page 1.
The WM8195 samples up to three inputs (RINP, GINP and BINP) simultaneously. The device then
processes the sampled video signal with respect to the video reset level or an internally/externally
generated reference level using either one or three processing channels.
Each processing channel consists of an Input Sampling block with optional Reset Level Clamping
(RLC) and Correlated Double Sampling (CDS), a 8-bit programmable offset DAC and an 8-bit
Programmable Gain Amplifier (PGA).
The ADC then converts each resulting analogue signal to a 14-bit digital word. The digital output from
the ADC is presented on a 14-bit wide bus, with optional 8+6-bit, 7+7-bit or 4+4+4+2-bit multiplexed
formats.
On-chip control registers determine the configuration of the device, including the offsets and gains
applied to each channel. These registers are programmable via a serial or parallel interface.
INPUT SAMPLING
The WM8195 can sample and process one to three inputs through one or three processing channels
as follows:
Colour Pixel-by-Pixel: The three inputs (RINP, GINP and BINP) are simultaneously sampled for
each pixel and a separate channel processes each input. The signals are then multiplexed into the
ADC, which converts all three inputs within the pixel period.
Monochrome: A single chosen input (RINP, GINP, or BINP) is sampled, processed by the
corresponding channel, and converted by the ADC. The choice of input and channel can be changed
via the control interface, e.g. on a line-by-line basis if required.
Colour Line-by-Line: A single chosen input (RINP, GINP, or BINP) is sampled and multiplexed into
the red channel for processing before being converted by the ADC. The input selected can be
switched in turn (RINP → GINP → BINP → RINP…) together with the PGA and offset DAC control
registers by pulsing the RLC/ACYC pin. This is known as auto-cycling. Alternatively, other sampling
sequences can be generated via the control registers. This mode causes the blue and green
channels to be powered down. Refer to the Line-by-Line Operation section for more details.
RESET LEVEL CLAMPING (RLC)
To ensure that the signal applied to the WM8195 lies within its input range (0V to AVDD) the CCD
output signal is usually level shifted by coupling through a capacitor, CIN. The RLC circuit clamps the
WM8195 side of this capacitor to a suitable voltage during the CCD reset period.
A typical input configuration is shown in Figure 7 An internal clamp pulse, CL, is generated from
MCLK and VSMP by the Timing Control Block. When CL is active the voltage on the WM8195 side of
CIN, at RINP, is forced to the VRLC/VBIAS voltage (VVRLC) by closing of switch 1. When the CL pulse
turns off, switch 1 opens, the voltage at RINP initially remains at VVRLC but any subsequent variation
in sensor voltage (from reset to video level) will couple through CIN to RINP.
RLC is compatible with both CDS and non-CDS operating modes, as selected by switch 2. Refer to
the CDS/Non-CDS Processing section.
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PD Rev 4.1 July 2005
13