OP270GS(RevA) 데이터 시트보기 (PDF) - Analog Devices
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OP270GS Datasheet PDF : 16 Pages
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SPECIFICATIONS
500⍀
5k⍀
1/2
OP270
V1 20Vp-p
5k⍀
50⍀
1/2
OP270
V2
CHANNEL SEPERATION = 20 log V1
V /1000
2
Figure 1. Channel Separation Test Circuit
+18V
8
100k⍀
2
1/2
1
3
OP270
200k⍀
6
1/2
7
5
OP270
100k⍀
OP270
TOTAL NOISE AND SOURCE RESISTANCE The total
noise of an op amp can be calculated by:
where:
( ) ( ) ( ) 2
2
2
En = en + inRS + et
En = total input referred noise
en = up 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.
Figure 3 shows the relationship between total noise at 1 kHz
and source resistance. For RS< 1 kW the total noise is domi-
nated by the voltage noise of the OP270. As RS rises above
1 kW, total noise increases and is dominated by resistor noise
rather than by voltage or current noise of the OP270. When
RS exceeds 20 kW, current noise of the OP270 becomes the
major contributor to total noise.
100
4
–18V
Figure 2. Burn-In Circuit
APPLICATIONS INFORMATION
VOLTAGE AND CURRENT NOISE
The OP270 is a very low-noise dual op amp, exhibiting atypical
voltage noise of only 3.2 nV/÷Hz @ 1 kHz. The exceptionally
low noise characteristics of the OP270 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 propor-
tional to the square root of the collector current. As a result, the
outstanding voltage noise performance of the OP270 is gained
at the expense of current noise performance, which is normal 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)l and resistor noise (et).
OP200
10
OP270
RESISTOR
NOISE ONLY
1
100
1k
10k
RS – SOURCE RESISTANCE (⍀)
100k
Figure 3. Total Noise vs Source Resistance
(Including Resistor Noise) at 1kHz
Figure 4 also shows the relationship between total noise
and source resistance, but at 10 Hz. Total noise increases
more quickly than shown in Figure 1 because current noise
is inversely proportional to the square root of frequency. In
Figure 4, current noise of the OP270 dominates the total
noise when RS> 5 kW.
From Figures 3 and 4 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 OP200, with
lower current noise than the OP270, will provide lower
total noise.
REV. A
–9–
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