Datasheet OP470 (Analog Devices) - 9

OP470 5k
The total noise is referred to the input and at the output would be amplified by the circuit gain. Figure 4 shows the relationship
500
between total noise at 1 kHz and source resistance. For RS < 1 kW
1/4 V
the total noise is dominated by the voltage noise of the OP470.
1 20V p-p OP470
As RS rises above 1 kW, total noise increases and is dominated by resistor noise rather than by voltage or current noise of the OP470. When R
50k
S exceeds 20 kW, current noise of the OP470 becomes the major contributor to total noise.
50
Figure 5 also shows the relationship between total noise and
1/4 V
source resistance, but at 10 Hz. Total noise increases more
2 OP470
quickly than shown in Figure 4 because current noise is inversely proportional to the square root of frequency. In Figure 5, current
V
noise of the OP470 dominates the total noise when R
1
S > 5 kW.
CHANNEL SEPARATION = 20 LOG V2/1000
From Figures 4 and 5 it can be seen that to reduce total noise, Figure 2. Channel Separation Test Circuit source resistance must be kept to a minimum. In applications with a high source resistance, the OP400, with lower current noise than the OP470, will provide lower total noise.
+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 C D L OP471 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
The OP470 is a very low-noise quad op amp, exhibiting a typi- Figure 4. Total Noise vs. Source Resistance (Including cal voltage noise of only 3.2 nV÷Hz @ 1 kHz. The exceptionally Resistor Noise) at 1 kHz low-noise characteristics of the OP470 are in part achieved by
100
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 propor- tional to the square root of the collector current. As a result, the outstanding voltage noise performance of the OP470 is gained
OP11
at the expense of current noise performance, which is typical for
OP400
low noise amplifiers.
10 OISE – nV/ Hz OP471
To obtain the best noise performance in a circuit, it is vital to
N L A
understand the relationship between voltage noise (en), current
TOT
noise (i
OP470
n), and resistor noise (et).
RESISTOR TOTAL NOISE AND SOURCE RESISTANCE NOISE ONLY
The total noise of an op amp can be calculated by:
1 100 1k 10k 100k R
2 2 2
S – SOURCE RESISTANCE –
E = e i R e ( ) + ( ) + ( ) n n n S t Figure 5. Total Noise vs. Source Resistance (Including where: Resistor Noise) at 10 Hz En = total input referred noise en = up amp voltage noise in = op amp current noise et = source resistance thermal noise RS = source resistance REV. B –9– Document Outline FEATURES GENERAL DESCRIPTION PIN CONNECTIONS SIMPLIFIED SCHEMATIC SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ORDERING GUIDE Typical Performance Characteristics APPLICATIONS INFORMATION Voltage and Current Noise TOTAL NOISE AND SOURCE RESISTANCE NOISE MEASUREMENTS— PEAK-TO-PEAK VOLTAGE NOISE NOISE MEASUREMENT—NOISE VOLTAGE DENSITY NOISE MEASUREMENT—CURRENT NOISE DENSITY CAPACITIVE LOAD DRIVING AND POWER SUPPLY CONSIDERATIONS UNITY-GAIN BUFFER APPLICATIONS 50k APPLICATIONS Low Noise Amplifier DIGITAL PANNING CONTROL SQUELCH AMPLIFIER FIVE-BAND LOW-NOISE STEREO GRAPHIC EQUALIZER OUTLINE DIMENSIONS Revision History