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GB4570 Datasheet(PDF) 3 Page - Gennum Corporation |
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GB4570 Datasheet(HTML) 3 Page - Gennum Corporation |
3 / 5 page 3 521 - 18 - 01 With this automatic adjustment system, the need for an external sync separator circuit to enable the clamping action is eliminated. The input to the GB4570 must be AC coupled using an appropriately sized capacitor. This capacitor acts as a DC reservoir for corrective level shifts. Under equilibrium conditions, the average current supplied by the comparator output is just sufficient to balance the current discharging the input coupling capacitor. Under dynamic conditions, where the system is correcting for a change in the signal level, the charging current may be in the milliamp range. Since the corrective current is small under equilibrium conditions, the error voltage at the comparator input is also small making clamping to within ± 2 mV achievable. The circuit makes use of a ‘peak hold’ capacitor, C HOLD , at the output of the comparator. This gives rise to a more constant voltage at the comparator output. This constant voltage is translated to a more constant corrective current by an internal 50 k Ω resistor between the comparator output and the signal input. To avoid excessive phase shifts and consequent instability of the clamp feedback loop, the peak hold capacitor should be considerably smaller (eg. 1000 times smaller) than the input coupling capacitor. If faster clamping is desirable, the peak hold capacitor can be removed and a smaller input coupling capacitor employed. In this application, some distortion of the signal tip will occur. DETAILED DESCRIPTION The GB4570 is intended for video applications requiring DC restoration. The GB4570 signal path consists of a wideband Darlington emitter follower for maximum bandwidth performance. With this configuration, the GB4570’s small signal frequency response remains within ±0.1 dB of 0 dB out to 80 MHz. Optimal frequency response for the GB4570 occurs with load capacitance in the range of 15 pF. For smaller loads, an external capacitor can be added to maximize the bandwidth of the device. As shown in Figure 1, a small resistor, R OUT , should be included in series with the GB4570 output to obtain optimal response flatness. For a nominal load of 15 pF, R OUT should be chosen to be approximately 40 Ω. The DC restoration function is achieved through the use of a simple comparator. The non-inverting input of the comparator is connected to the clamp reference voltage, while the inverting input is connected to the GB4570 output. For output signals more positive than the clamp reference voltage, the comparator output is essentially open-circuit, while signal voltages more negative than the clamp reference voltage result in the charging of C HOLD , an external non-polar capacitor connected to pin 4. The action of the comparator is to provide a positive current which is fed back to the input under conditions where the device output is more negative than the clamp reference voltage. This negative feedback raises the DC level of the input signal to the point where all signal fluctuations occur at voltages above the clamp reference voltage. All resistors are in ohms, all capacitors in microfarads unless otherwise stated Fig. 1 Typical Application Circuit 75 VIDEO 1.1nF VCC VEE 8 6 4 1 2 7 3 CLOAD CHOLD VCLAMP VIN ROUT CLAMPED VIDEO OUTPUT 22 0.1 0.1 GB4570 SYNC TIP CLAMP |
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Similar Description - GB4570 |
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