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CA5160M96 Scheda tecnica(PDF) 8 Page - Intersil Corporation |
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CA5160M96 Scheda tecnica(HTML) 8 Page - Intersil Corporation |
8 / 19 page 8 to draw modest supply-current (see the lower curve in Figure 21) even through the output stage is strobed off. Figure 4A shows a dual-supply arrangement for the output stage that can also be strobed off, assuming RL = ∞, by pulling the potential of Terminal 8 down to that of Terminal 4. Let it now be assumed that a load-resistance of nominal value (e.g., 2k Ω) is connected between Terminal 6 and ground in the circuit of Figure 4B. Let it further be assumed again that the input terminal bias (Terminals 2 and 3) is such that the output terminal (Number 6) voltage is V+/2. Since PMOS transistor Q8 must now supply quiescent current to both RL and transistor Q12, it should be apparent that under these conditions the supply current must increase as an inverse function of the RL magnitude. Figure 27 shows the voltage drop across PMOS transistor Q8 as a function of load current at several supply voltages. Figure 20 shows the voltage transfer characteristics of the output stage for several values of load resistance. Wideband Noise From the standpoint of low-noise performance considerations, the use of the CA5160 is most advantageous in applications where in the source resistance of the input signal is on the order of 1M Ω or more. In this case, the total input-referred noise voltage is typically only 40 µV when the test-circuit amplifier of Figure 5 is operated at a total supply voltage of 15V. This value of total input-referred noise remains essentially constant, even though the value of source resistance is raised by an order of magnitude. This characteristic is due to the fact that reactance of the input capacitance becomes a significant factor in shunting the source resistance. It should be noted, however, that for values of source resistance very much greater than 1M Ω, the total noise voltage generated can be dominated by the thermal noise contributions of both the feedback and source resistors. Typical Applications Voltage Followers Operational amplifiers with very high input resistances, like the CA5160, are particularly suited to service as voltage followers. Figure 6 shows the circuit of a classical voltage follower, together with pertinent waveforms using the CA5160 in a split supply-configuration. A voltage follower, operated from a single-supply, is shown in Figure 7 together with related waveforms. This follower circuit is linear over a wide dynamic range, as illustrated by the reproduction of the output waveform in Figure 7B with input signal ramping. The waveforms in Figure 7C show that the follower does not lose its input-to-output phase-sense, even though the input is being swung 7.5V below ground potential. This unique characteristic is an important attribute in both operational amplifier and comparator applications. Figure 7C also shows the manner in which the CMOS output stage permits the output signal to swing down to the negative supply rail potential (i.e., ground in the case shown). The digital-to- analog converter (DAC) circuit, described in the following section, illustrates the practical use of the CA5160 in a single- supply voltage follower application. 3 2 8 4 7 6 RL Q8 Q12 + - OUTPUT STAGE FIGURE 4A. DUAL POWER-SUPPLY OPERATION V+ V- 3 2 8 4 7 6 RL Q8 Q12 + - OUTPUT STAGE FIGURE 4B. SINGLE POWER-SUPPLY OPERATION FIGURE 4. CA5160 OUTPUT STAGE IN DUAL AND SINGLE POWER SUPPLY OPERATION V+ 6 7 3 4 2 +7.5V 0.01 µF NOISE VOLTAGE OUTPUT 30.1k Ω 0.01 µF 1k Ω -7.5V RS 1M Ω + - BW (-3dB) = 200kHz TOTAL NOISE VOLTAGE (INPUT REFERRED) = 40 µV (TYP) FIGURE 5. TEST-CIRCUIT AMPLIFIER (30dB GAIN) USED FOR WIDEBAND NOISE MEASUREMENTS CA5160 |
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