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CA3290 Datasheet(PDF) 5 Page - Intersil Corporation |
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CA3290 Datasheet(HTML) 5 Page - Intersil Corporation |
5 / 10 page 5 elements identified in Figure 4 as I1 through I4, respectively. Their gate source potentials (VGS) are supplied by a common bus from the biasing circuit shown in the right hand portion of the Schematic Diagram. The currents supplied by Q10 and Q12 are twice those supplied by Q9 and Q11. The transistor geometries are appropriately scaled to provide the requisite currents with common VGS applied to Q9 through Q12. Operating Considerations Input Circuit The use of MOS transistors in the input stage of the CA3290 series circuits provides the user with the following features for comparator applications: 1. Ultra high input impedance ( ≅1.7TΩ); 2. The availability of common mode rejection for input signals at potentials below that of the negative power supply rail; 3. Retention of the in phase relationship of the input and out- put signals for input signals below the negative rail. Although the CA3290 employs rugged bipolar (zener) diodes for protection of the input circuit, the input terminal currents should not exceed 1mA. Appropriate series connected limiting resistors should be used in circuits where greater current flows might exist, allowing the signal input voltage to be greater than the supply voltage without damaging the circuit. Output Circuit The output of the CA3290 is the open collector of an n-p-n transistor, a feature providing flexibility in a broad range of comparator applications. An output ORing function can be implemented by parallel connection of the open collectors. An output pull-up resistor can be connected to a power supply having a voltage range within the rating of the particular CA3290 in use; the magnitude of this voltage may be set at a value which is independent of that applied to the V+ terminal of the CA3290. Parasitic Oscillations The ideal comparator has, among other features, ultra high input impedance, high gain, and wide bandwidth. These desirable characteristics may, however, produce parasitic oscillations unless certain precautions are observed to minimize the stray capacitive coupling between the input and output terminals. Parasitic oscillations manifest themselves during the output voltage transition intervals as the comparator switches states. For high source impedances, stray capacitance can induce parasitic oscillations. The addition of a small amount (1mV to 10mV) of positive feedback (hysteresis) produces a faster transition, thereby reducing the likelihood of parasitic oscillations. Furthermore, if the input signal is a pulse waveform, with relatively rapid rise and fall times, parasitic tendencies are reduced. When dual comparators, like the CA3290, are packaged in an 8 lead configuration, the output terminal of each comparator is adjacent to an input terminal. The lead-to-lead capacitance is approximately 1pF, which may be sufficient to cause undesirable feedback effects in certain applications. Circuit factors such as impedance levels, supply voltage, switching rate, etc., may increase the possibility of parasitic oscillations. To minimize this potential oscillatory condition, it is recommended that for source impedances greater than 1k Ω a capacitor ( ≥1pF - 2pF) be connected between the appropriate input terminal and the output terminal. (See Figure 1.) If either comparator is unused, its input terminals should also be tied to either the V+ or V- supply rail. Typical Applications Light Controlled One-Shot Timer In Figure 5 one comparator (A1) of the CA3290 is used to sense a change in photo diode current. The other comparator (A2) is configured as a one-shot timer and is triggered by the output of A1. The output of the circuit will switch to a low state for approximately 60 seconds after the light source to the photo diode has been interrupted. The circuit operates at normal room lighting levels. The sensitivity of the circuit may be adjusted by changing the values of R1 and R2. The ratio of R1 to R2 should be V+ D2 D1 VI+ I1 50 µA QP1 Q2 Q5 I2 100 µA Q3 Q6 I3 50 µA I4 100 µA VI- D3 D4 VO Q8 V- QP4 Q7 FIGURE 4. BASIC CIRCUIT DIAGRAM FOR ONE OF THE TWO COMPARATORS CA3290, CA3290A |
Similar Part No. - CA3290_04 |
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Similar Description - CA3290_04 |
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