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LM359N Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM359N Datasheet(HTML) 9 Page - National Semiconductor (TI) |
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9 / 24 page  Application Hints (Continued) these parameters are significant in wide band amplifier de- sign. The input stage current is approximately 3 times I SET(IN) and by using this relationship the following first order approximations for these AC parameters are: where C comp is the total capacitance from the compensation pin (pin 3 or pin 13) to ground, A VOL is the low frequency open loop voltage gain in V/V and an ambient temperature of 25˚C is assumed (KT/q = 26 mV and β typ = 150). ISET(IN) also controls the DC input bias current by the expression: which is important for DC biasing considerations. The total device supply current (for both amplifiers) is also a direct function of the set currents and can be approximated by: I supply ≅ 27xISET(OUT) +11xISET(IN) with each set current programmed by individual resistors. PROGRAMMING WITH A SINGLE RESISTOR Operating current programming may also be accomplished using only one resistor by letting I SET(IN) equal ISET(OUT). The programming current is now referred to as I SET and it is cre- ated by connecting a resistor from pin 1 to pin 8 ( Figure 8). This configuration does not affect any of the internal set cur- rent dependent parameters differently than previously dis- cussed except the total supply current which is now equal to: I supply ≅ 37xISET Care must be taken when using resistors to program the set current to prevent significantly increasing the supply voltage above the value used to determine the set current. This would cause an increase in total supply current due to the re- sulting increase in set current and the maximum device power dissipation could be exceeded. The set resistor val- ue(s) should be adjusted for the new supply voltage. One method to avoid this is to use an adjustable current source which has voltage compliance to generate the set current as shown in Figure 9. This circuit allows I SET to remain constant over the entire supply voltage range of the LM359 which also improves power supply ripple rejection as illustrated in the Typical Per- formance Characteristics. It should be noted, however, that the current through the LM334 as shown will change linearly with temperature but this can be compensated for (see LM334 data sheet). Pin 1 must never be shorted to ground or pin 8 never shorted to V + without limiting the current to 2 mA or less to prevent catastrophic device failure. CONSIDERATIONS FOR HIGH FREQUENCY OPERATION The LM359 is intended for use in relatively high frequency applications and many factors external to the amplifier itself must be considered. Minimization of stray capacitances and their effect on circuit operation are the primary requirements. The following list contains some general guidelines to help accomplish this end: 1. Keep the leads of all external components as short as possible. 2. Place components conducting signal current from the output of an amplifier away from that amplifier’s non-inverting input. 3. Use reasonably low value resistances for gain setting and biasing. 4. Use of a ground plane is helpful in providing a shielding effect between the inputs and from input to output. Avoid using vector boards. 5. Use a single-point ground and single-point supply distri- bution to minimize crosstalk. Always connect the two grounds (one from each amplifier) together. 6. Avoid use of long wires (> 2") but if necessary, use shielded wire. 7. Bypass the supply close to the device with a low induc- tance, low value capacitor (typically a 0.01 µF ceramic) to create a good high frequency ground. If long supply leads are unavoidable, a small resistor (z10 Ω) in series with the bypass capacitor may be needed and using shielded wire for the supply leads is also recommended. COMPENSATION The LM359 is internally compensated for stability with closed loop inverting gains of 10 or more. For an inverting gain of less than 10 and all non-inverting amplifiers (the amplifier al- ways has 100% negative current feedback regardless of the DS007788-13 ISET(IN) = ISET(OUT) = ISET FIGURE 8. Single Resistor Programming of I SET DS007788-14 FIGURE 9. Current Source Programming of I SET www.national.com 9 |
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