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SGM358YMS/TR Datasheet(PDF) 8 Page - SG Micro Corp |
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SGM358YMS/TR Datasheet(HTML) 8 Page - SG Micro Corp |
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8 / 18 page  1MHz, 60μA, Rail-to-Rail I/O SGM321/SGM358/SGM324 CMOS Operational Amplifiers 8 MARCH 2017 SG Micro Corp www.sg-micro.com APPLICATION NOTES Driving Capacitive Loads The SGM321/SGM358/SGM324 can directly drive 250pF in unity-gain without oscillation. The unity-gain follower (buffer) is the most sensitive configuration to capacitive loading. Direct capacitive loading reduces the phase margin of amplifiers and this results in ringing or even oscillation. Applications that require greater capacitive driving capability should use an isolation resistor between the output and the capacitive load like the circuit in Figure 1. The isolation resistor RISO and the load capacitor CL form a zero to increase stability. The bigger the RISO resistor value, the more stable VOUT will be. Note that this method results in a loss of gain accuracy because RISO forms a voltage divider with the RLOAD. VIN VOUT SGM321 CL RISO Figure 1. Indirectly Driving Heavy Capacitive Load An improved circuit is shown in Figure 2. It provides DC accuracy as well as AC stability. RF provides the DC accuracy by connecting the inverting input with the output. CF and RIso serve to counteract the loss of phase margin by feeding the high frequency component of the output signal back to the amplifier’s inverting input, thereby preserving phase margin in the overall feedback loop. VIN VOUT SGM321 CL RISO RL RF CF Figure 2. Indirectly Driving Heavy Capacitive Load with DC Accuracy For non-buffer configuration, there are two other ways to increase the phase margin: (a) by increasing the amplifier’s closed-loop gain or (b) by placing a capacitor in parallel with the feedback resistor to counteract the parasitic capacitance associated with inverting node. Power-Supply Bypassing and Layout The SGM321/SGM358/SGM324 can operate from either a single +2.1V to +5.5V supply or dual ±1.05V to ±2.75V supplies. For single-supply operation, bypass the power supply +VS with a 0.1µF ceramic capacitor which should be placed close to the +VS pin. For dual-supply operation, both the +VS and the -VS supplies should be bypassed to ground with separate 0.1µF ceramic capacitors. 2.2µF tantalum capacitor can be added for better performance. SGM321 Vn Vp +VS -VS (GND) VOUT 10 μF 0.1 μF SGM321 +VS Vn Vp -VS VOUT 10 μF 0.1 μF 10 μF 0.1 μF Figure 3. Amplifier with Bypass Capacitors |
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