8

voltage drift in 60µS for various values of coupling capacitor,

In normal circuit operation, the picture content will also

cause a slow change in voltage across the capacitor, so at

every back porch time period, these error terms can be

corrected.

When a signal source is being switched, e.g., from two

different surveillance cameras, it is recommended to

synchronize the switching with the vertical blanking period,

and to drive the HOLD pin (pin 6) low, during these lines.

This will ensure that the system has been completely

restored, regardless of the average intensity of the two

pictures.

Application Hints

Figures 1 & 2 shows a three channel DC-restoring system,

suitable for R-G-B or Y-U-V component video, or three

synchronous composite signals.

Figure 1 shows the amplifiers configured for non-inverting

gain, and Figure 2 shows the amplifiers configured for

inverting gains. Note that since the DC-restoring function is

accomplished by clamping the amplifier’s non-inverting

input, during the back porch period, any signal on the non-

inverting input will be distorted. For this reason, it is

recommended to use the inverting configuration for

composite video, since this avoids the color burst being

altered during the clamp time period.

Since all three amplifiers are monolithic, they run at the

same temperature, and will have very similar input bias

currents. This can be used to advantage, in situations where

the droop voltage needs to be compensated, since a single

trim circuit can be used for all three channels. A 560k

Ω or

similar value resistor helps to isolate each signal. See Figure

2. The advantage of compensating for the droop voltage, is

that a smaller capacitor can be used, which allows a larger

level restoration within one line. See Table 1 for values of

capacitor and charge/droop rates.

TABLE 2. CHARGE STORAGE CAPACITOR VALUE VS.

DROOP AND CHARGING RATES

CAP VALUE

(NF)

DROOP IN

60µS (MV)

CHARGE IN

2µS (MV)

CHARGE IN

4µS (MV)

10

30

400

800

22

13.6

182

364

47

6.4

85

170

100

3.0

40

80

220

1.36

18

36

EL4390