WM8144-10 Wolfson Microelectronics Ltd., WM8144-10 Datasheet - Page 12

• Manufacturer Part Number: WM8144-10
• Description: Integrated 10-bit Data Acquisition System For Imaging Applications
• Manufacturer: Wolfson Microelectronics Ltd.

Example of Gain and Offset Operation
Input Video polarity
Input sampling
Input voltage amplitude (V
Programmable gain
Clamping
After the input capacitor the input to the WM8144-10 can
be represented as:
Figure 7
For a black pixel:
Assuming that the offset DAC is set to 00dec:
An input voltage of V
from the ADC.
To maximise the dynamic range of the ADC input it is
necessary to program the offset DAC code to move the
ADC code corresponding to the black level towards code
1024(dec).
Hence set the offset DAC to 164(dec) with the sign bit not
set.
When the VMID is 2.5v, the ADC input voltage becomes
3.3 volts which will result in an ADC code of 921(dec).
This is near the ideal full-scale of 1023(dec).
V
V
V
VADC = 1* Vcl - Vcl
VADC 0 + 0 + VMID
VADC = VMID
Theory of Operation (Contd)
ADC
ADC
ADC
WM8144-10
12
= 1*(V
= 0 +
=
337
255
CL
255
82
* V
- V
MID
* V
RS
CL
MID
1 2 0
) + (1 - 2*0) *
MID
+ V
*
VS
corresponds to a code of 512(dec)
V
V
*
MID
RS
VS
255
VS
0
= V
= V
VMID
- V
164
255
CL
CL
RS
*
)
VMID
*
2
V
MID
2
CDS
1.6V
x1
Yes, V
negative
Wolfson Microelectronics
VMID
Vrs
Vvs
+ V
MID
CL
= 3.5V
V
V
V
For a white pixel:
For the white pixel, using the same offset DAC value, the
ADC input can be expressed as:
When the VMID is 2.5V, the ADC input voltage becomes
1.7 volts which will result in a code of 102(dec). This is
near the ideal full-scale of 000(dec).
Therefore the output codes from the ADC are between
921(dec) and 102(dec), which implies that the ADC input
has been set up to maximise the dynamic range avail-
able. If a digital representation of the ADC output with a
black level near 000(dec) and a white level near 1023(dec)
is required then the INVOP control bit should now be set
to ONE.
ADC
ADC
ADC
= 1*(V
= -1.6 +
=
337
255
CL
* V
255
- 1.6 - V
82
MID
* V
- 1.6
MID
CL
) + (1 - 2*0) *
+ V
V
V
RS
VS
MID
= V
= V
CL
CL
- 1.6
164
255
*
V
MID
2
+ V
MID