OP471Noise Measurement - Current Noise Density adds phase shift in the feedback network and reduces stability. A The test circuit shown in Figure 10 can be used to measure current simple circuit to eliminate this effect is shown in Figure 11. The noise density. The formula relating the voltage output to current added components, C1 and R3, decouple the amplifier from the noise density is: load capacitance and provide additional stability. The values of C1 and R3 shown in Figure 11 are for load capacitances of up Ê 2 to 1,000 pF when used with the OP471. e ˆ 2 nOUT 40nV / Hz ËÁ In applications where the OP471’s inverting or noninverting inputs G ¯˜ - ( ) i = are driven by a low source impedance (under 100 W) or connected n RS to ground, if V+ is applied before V–, or when V– is disconnected, excessive parasitic currents will flow. where: Most applications use dual tracking supplies and with the device G = gain of 10,000 supply pins properly bypassed, power-up will not present a RS = 100 kW source resistance problem. A source resistance of at least 100 W in series with all Capacative Load Driving and Power Supply Considerations inputs (Figure 11) will limit the parasitic currents to a safe level The OP471 is unity-gain stable and is capable of driving large if V– is disconnected. It should be noted that any source resistance, capacitive loads without oscillating. Nonetheless, good supply even 100 W, adds noise to the circuit. Where noise is required to bypassing is highly recommended. Proper supply bypassing be kept at a minimum, a germanium or Schottky diode can be reduces problems caused by supply line noise and improves the used to clamp the V– pin and eliminate the parasitic current capacitive load driving capability of the OP471. flow instead of using series limiting resistors. For most applica- tions, only one diode clamp is required per board or system. R31.24kRfR1R25100kOP471 DUTOP27Een OUT TO SPECTRUM ANALYZEROP471R58.06k8V/sR4200GAIN = 10,000 Figure 12. Pulsed Operation VS =15VUnity-Gain Buffer Applications Figure 10. Current Noise Density Test Circuit When Rf £ 100 W and the input is driven with a fast, large signal pulse (>1 V), the output waveform will look as shown in Figure 12. V+C2 During the fast feedthrough-like portion of the output, the input 10F+ protection diodes effectively short the output to the input, and a C3 current, limited only by the output short-circuit protection, will 0.1F be drawn by the signal generator. With Rf ≥ 500 W, the output is capable of handling the current requirements (IL £ 20 mA at R2 10 V); the amplifier will stay in its active mode and a smooth transition will occur. C1R1200pFVINR3 When R 50 f > 3 kW, a pole created by Rf and the amplifier’s input OP471VOUT capacitance (2.6 pF) creates additional phase shift and reduces 100*C410FCL phase margin. A small capacitor (20 pF to 50 pF) in parallel with +1000pF Rf helps eliminate this problem. C5*0.1FAPPLICATIONS Low Noise Amplifier*SEE TEXTV– A simple method of reducing amplifier noise by paralleling PLACE SUPPLY DECOUPLING CAPACITORS AT OP471 amplifiers is shown in Figure 13. Amplifier noise, depicted in Figure 14, is around 5 nV/÷Hz @ 1 kHz (R.T.I.). Gain for each paralleled amplifier and the entire circuit is 100. The 200 W Figure 11. Driving Large Capacitive Loads resistors limit circulating currents and provide an effective output In the standard feedback amplifier, the op amp’s output resistance resistance of 50 W. The amplifier is stable with a 10 nF capacitive combines with the load capacitance to form a lowpass filter that load and can supply up to 30 mA of output drive. –10– REV. A
