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OP471ARCMDA Datasheet(PDF) 7 Page - Analog Devices |
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OP471ARCMDA Datasheet(HTML) 7 Page - Analog Devices |
7 / 16 page REV. A OP471 –7– 500 5k V1 20V p-p 1/4 OP471 50 50k CHANNEL SEPARATION = 20 LOG V1 V2 / 1000 V2 1/4 OP471 Figure 2. Channel Separation Test Circuit 7 6 5 1 2 3 +1V +18V 4 –18V 11 A +1V B D 14 13 12 –1V C 8 9 10 –1V Figure 3. Burn-In Circuit APPLICATIONS INFORMATION Voltage and Current Noise The OP471 is a very low-noise quad op amp, exhibiting a typical voltage noise of only 6.5 Hz @ 1 kHz. The low noise character- istic of the OP471 is, in part, achieved by operating the input transistors at high collector currents since the voltage noise is inversely proportional to the square root of the collector current. Current noise, however, is directly proportional to the square root of the collector current. As a result, the outstanding voltage noise performance of the OP471 is gained at the expense of current noise performance which is typical for low noise amplifiers. To obtain the best noise performance in a circuit, it is vital to understand the relationship between voltage noise (en), current noise (in), and resistor noise (et). Total Noise and Source Resistance The total noise of an op amp can be calculated by: Ee i R e nn n S t = () +() +() 22 2 where: En = total input referred noise en = op amp voltage noise in = op amp current noise et = source resistance thermal noise RS = source resistance The total noise is referred to the input and at the output would be amplified by the circuit gain. RS – SOURCE RESISTANCE – 100 1 100 100k 10 10k 1k OP11 OP400 OP471 OP470 RESISTOR NOISE ONLY Figure 4. Total Noise vs. Source Resistance (Including Resistor Noise) at 1 kHz RS – SOURCE RESISTANCE – 100 1 100 100k 10 10k 1k OP11 OP400 OP471 OP470 RESISTOR NOISE ONLY Figure 5. Total Noise vs. Source Resistance (Including Resistor Noise) at 10 Hz Figure 4 shows the relationship between total noise at 1 kHz and source resistance. For RS < 1 k W the total noise is domi- nated by the voltage noise of the OP471. As RS rises above 1 k W, total noise increases and is dominated by resistor noise rather than by voltage or current noise of the OP471. When RS exceeds 20 k W, current noise of the OP471 becomes the major contributor to total noise. Figure 5 also shows the relationship between total noise and source resistance, but at 10 Hz. Total noise increases more quickly than shown in Figure 4 because current noise is inversely pro- portional to the square root of frequency. In Figure 5, current noise of the OP471 dominates the total noise when RS > 5 k W. From Figures 4 and 5, it can be seen that to reduce total noise, source resistance must be kept to a minimum. In applications with a high source resistance, the OP400, with lower current noise than the OP471, will provide lower total noise. |
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