STV2050A datasheet(55/83 Pages) STMICROELECTRONICS

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STV2050A - ELECTRICAL LOOP

11 ELECTRICAL LOOP

11.1 PRINCIPLE OF OPERATION

It is possible to multiply convergence values with a digital gain value and add a digital offset

value to the convergence values during convergence signal processing (see Figure 26 "Con-

vergence Values Computation Path" on page 45). This can be used to compensate offset cur-

rent and gain differences of the external convergence amplifiers. However, aging and temper-

ature can cause drifting, and the correction values may have to be adjusted. In order to avoid

a manual after-sales re-alignment, the STV2050A can perform an automatic update of the

field offset canceller values, as well as for the gain correction values.

For this purpose a measuring line in the invisible part of the picture (for example, during the

frame retrace procedure) can be used to provide measurement values at convergence out-

puts. (Refer to Figure 31 "Electrical Loop Block Diagram" on page 55.)

Offset current and gain values of the amplifiers are measured with comparators, which are

connected to the PORA, PORB, PORC port pins of the IC. The reference values are available

on the OGAH and OGAV pins.

Figure 31. Electrical Loop Block Diagram

Once the measurement is carried out, the convenient field offset canceller and gain compen-

sation corrections for the convergence values can be controlled by either an embedded

process (“internal loop”), or by an external MCU, when the internal loop is disabled.

The loop is activated using the STL bit in the E3 register.

STL

0 = Internal Loop enabled

1 = Internal Loop disabled

DAC

Electrical

Loop

Convergence

Processing

Amplifier

Yoke

Sense-

Resistor

Measuring Line

OGAH,OGAV

PORA

PORB

PORC

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