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CA3280A Arkusz danych(PDF) 5 Page - Intersil Corporation |
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CA3280A Arkusz danych(HTML) 5 Page - Intersil Corporation |
5 / 10 page 5 Application Information Figures 4 and 5 show the equivalent circuits for the current source and linearization diodes in the CA3280. The current through the linearization network is approximately equal to the programming current. There are several advantages to driving these diodes with a current source. First, only the offset current from the biasing network flows through the input resistor. Second, another input is provided to extend the gain control dynamic range. And third, the input is truly differential and can accept signals within the common mode range of the CA3280. Typical Applications The structure of the variable operational amplifier eliminates the need for matched resistor networks in differential to single ended converters, as shown in Figure 6. A matched resistor network requires ratio matching of 0.01% or trimming for 80dB of common-mode rejection. The CA3280, with its excellent common mode rejection ratio, is capable of converting a small ( ±25mV) differential input signal to a single-ended output without the need for a matched resistor network. Figure 7 shows the CA3280 in a typical gain control application. Gain control can be performed with the amplifier bias current (lABC). With no diode bias current, the gain is merely gMRL. For example, with an lABC of 1mA, the gM is approximately 16mS. With the CA3280 operating into a 5k Ω resistor, the gain is 80. The need for external buffers can be eliminated by the use of low value load resistors, but the resulting increase in the required amplifier bias current reduces the input impedance of the CA3280. The linearization diode impedance also decreases as the diode bias current increases, which further loads the input. The diodes, in addition to acting as a linearization network, also operate as an additional attenuation system to accommodate input signals in the volt range when they are applied through appropriate input resistors. Figure 10 shows a triangle wave to sine wave converter using the CA3280. Two 100k Ω resistors are connected between the differential amplifier emitters and V+ to reduce the current flow through the differential amplifier. This allows the amplifier to fully cut off during peak input signal excursions. THD is appropriately 0.37% for this circuit. FIGURE 4. VOA SHOWING LINEARIZATION DIODES AND CURRENT DRIVE FIGURE 5. BLOCK DIAGRAM OF LINEARIZED VOA FIGURE 6. DIFFERENTIAL TO SINGLE ENDED CONVERTER FIGURE 7. TYPICAL GAIN CONTROL CIRCUIT V+ VOA V- RD = SMALL SIGNAL DIODE IMPEDANCE RD ≈ 52 Ω ID(mA) x 1.34 = 70 ID IABC ID RD VOA ID IABC 4 1/2 + - 13 15 16 3 2k Ω 1 CA3280 14 +15V 68k Ω SINGLE- ENDED OUTPUT DIFFERENTIAL 2k Ω INPUT 10k Ω -15V 4 1/2 + - 13 15 16 14 10k Ω 3 CA3280 1 V+ = +15V 68k Ω OUTPUT 21VP-P 14mV AGC 330k Ω 15k Ω V- = -15V FEEDTHRU 400 µV NOISE AT MAX GAIN 20k Ω 10k Ω 600 Ω 10VP-P INPUT V+ V- 100k Ω VOLTAGE CONTROL CA3280, CA3280A |
Podobny numer części - CA3280A |
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Podobny opis - CA3280A |
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