Datasheet OP471 (Analog Devices) - 7

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