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ISL4089IBZ-T7 Folha de dados(PDF) 6 Page - Intersil Corporation |
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ISL4089IBZ-T7 Folha de dados(HTML) 6 Page - Intersil Corporation |
6 / 9 page 6 FN6192.1 June 28, 2006 Figure 12A illustrates the AC test circuit used to operate the video amplifier into a 150 Ω load while providing a 50Ω matched impedance. Figure 12B illustrates the test circuit for impedance matching to 75 Ω test equipment. Application Information General The ISL4089 implements the video DC-restore function using a high performance gain adjustable video amplifier and a nulling, sample-hold amplifier to establish a user defined DC reference voltage at the video amplifier output. A detailed description of the DC-restore function implemented in the ISL4089 can be found in application note AN1089, EL4089 and EL4390 DC-Restored Video Amplifier. The ISL4089 performs the same function with the exception that it is designed for single supply operation. Video Amplifier Operation (Figure 13) The ISL4089 video amplifier (A1) is voltage-feed, high performance video amplifier designed for +5V operation. The output stage is capable of swinging to within 10mV of the negative rail. The differential input stage contains an internal voltage reference that positions the non-inverting input DC level (V1) to ~1.2V higher than the negative supply rail. This offset ensures that the amplifier input DC level is maintained within the common mode input voltage range. The amplifier non-inverting gain is given in Equation 1. DC-Restore Amplifier (Figure 13) The DC-restore circuit contains a voltage reference amplifier and an analog switch function that closes the DC-restore loop under control of the HOLD logic input. The reference amplifier uses an internal 10mV offset voltage (V2) to enable the VREF input to sense down to the negative supply. The A2 amplifier output stage operates in a current-feed mode with a source/sink capability of ±300µA (Typ). A logic “0” at the HOLD input closes switch S1 which closes the DC-restore loop. The video input AC coupling capacitor, CX1, acts as a DC hold capacitor (through the 75 Ω termination resistor RX1) to average the current-source output of amplifier A2. When the DC-restore loop has reached equilibrium, the DC voltage stored on CX1 will the value required to force the output voltages at A1 (VOUT) and A2 (VIN+) according to the following: and; the DC voltage at the non-inverting input of the video amplifier A1 is given in Equation 3: Therefore, if VREF is set to 0V (GND); VOUT = 10mV, and the DC voltage stored on CX1 is ~1.2V. The CX1 capacitor value is chosen from the system requirements. A typical DC-restore application using the horizontal sync to drive the HOLD pin will result in a 62µs hold time. The typical input bias current to the video amplifier is 1.2µA, so for a 62µs hold time, and a 0.01µF capacitor, the output voltage drift is 7.5mV in one line. The restore amplifier can provide a typical current of 300µA to charge capacitor CX1, so with a 1.2µs sampling time, the output can be corrected by 36mV in each line. Using a smaller value of CX1 increases both the voltage that can be corrected, as well as the droop while being held. Likewise, using a larger value of CX1, reduces the correction and droop voltages. A sample of charging and droop rates are shown on the following table. AC Test Circuits FIGURE 12A. VIDEO AMPLIFIER AC TEST CIRCUIT FOR 50 Ω FIGURE 12B. BACKTERMINATED TEST CIRCUIT FOR VIDEO CABLE APPLICATION. RS CL VIN TEST 86.6 Ω 50 Ω 118 Ω RG RF EQUIPMENT 50 Ω + - 1. HOLD INPUT = 1 RS CL VIN TEST 75 Ω 75 Ω RG RF EQUIPMENT 75 Ω + - 1. HOLD INPUT = 1 V OUT V IN+ 1.2V – () 1 R F R G -------- + ⎝⎠ ⎜⎟ ⎛⎞ • = (EQ. 1) TABLE OF CHARGE STORAGE CAPACITOR VS DROOP CHARGING RATES (NOTE) CAP VALUE (if) DROOP IN 62µs (mV) CHARGE IN 1.2µs (mV) CHARGE IN 4µs (mV) 10 7.5 36 120 33 2.3 11 36 100 0.75 3.6 12 NOTE: Basic formulae are: V (droop) = Ib+ * (Line time - Sample time)/Capacitor and V (charge) = IOUT * Sample time/Capacitor V OUT(DC) V REF 10mV + = (EQ. 2) V IN+ V OUT(DC) 1.2V + = (EQ. 3) ISL4089 |
Nº de peça semelhante - ISL4089IBZ-T7 |
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Descrição semelhante - ISL4089IBZ-T7 |
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