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LM346M Datasheet(PDF) 8 Page - National Semiconductor (TI) |
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LM346M Datasheet(HTML) 8 Page - National Semiconductor (TI) |
8 / 17 page Application Hints (Continued) Input Capacitance: The input capacitance, C IN,ofthe LM146 is approximately 2 pF; any stray capacitance, C S, (due to external circuit circuit layout) will add to C IN. When resistive or active feedback is applied, an additional pole is added to the open loop frequency response of the device. For instance with resistive feedback ( Figure 2), this pole oc- curs at 1⁄2 π (R1||R2) (C IN +CS). Make sure that this pole oc- curs at least 2 octaves beyond the expected −3 dB fre- quency corner of the closed loop gain of the amplifier; if not, place a lead capacitor in the feedback such that the time constant of this capacitor and the resistance it parallels is equal to the R I(CS +CIN), where RI is the input resistance of the circuit. Temperature Effect on the GBW: The GBW (gain band- width product), of the LM146 is directly proportional to I SET and inversely proportional to the absolute temperature. When using resistors to set the bias current, I SET, of the de- vice, the GBW product will decrease with increasing tem- perature. Compensation can be provided by creating an I SET current directly proportional to temperature (see typical ap- plications). Isolation Between Amplifiers: The LM146 die is isother- mally layed out such that crosstalk between all 4 amplifiers is in excess of −105 dB (DC). Optimum isolation (better than −110 dB) occurs between amplifiers A and D, B and C; that is, if amplifier A dissipates power on its output stage, ampli- fier D is the one which will be affected the least, and vice versa. Same argument holds for amplifiers B and C. LM146 Typical Performance Summary: The LM146 typical behaviour is shown in Figure 3. The device is fully predict- able. As the set current, I SET, increases, the speed, the bias current, and the supply current increase while the noise power decreases proportionally and the V OSremains con- stant. The usable GBW range of the op amp is 10 kHz to 3.5−4 MHz. Low Power Supply Operation: The quad op amp operates down to ±1.3V supply. Also, since the internal circuitry is bi- ased through programmable current sources, no degrada- tion of the device speed will occur. Speed vs Power Consumption: LM146 vs LM4250 (single programmable). Through Figure 4, we observe that the LM146’s power consumption has been optimized for GBW products above 200 kHz, whereas the LM4250 will reach a GBW of no more than 300 kHz. For GBW products below 200 kHz, the LM4250 will consume less power. DS005654-9 FIGURE 2. DS005654-8 FIGURE 3. LM146 Typical Characteristics DS005654-10 FIGURE 4. LM146 vs LM4250 www.national.com 8 |
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