OP270 Datasheet, PDF (8/16 Pages) Analog Devices – Dual Very Low Noise Precision Operational Amplifier

OP270
500⍀
5k⍀
1/2
OP270
V₁ 20V_p-p
5k⍀
50⍀
1/2
OP270
V₂
V
CHANNEL SEPARATION = 20 log
1
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⍀
4
TOTAL NOISE AND SOURCE RESISTANCE
The total noise of an op amp can be calculated by:
( ) ( ) ( ) ^En ^{= e}n ^{2 + i}n^RS ^{2 + e}t ²
where:
E_n = total input referred noise
e_n = op amp voltage noise
i_n = op amp current noise
e_t = source resistance thermal noise
R_S = 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 R_S < 1 kW the total noise is dominated
by the voltage noise of the OP270. As R_S rises above 1 kW, total
noise increases and is dominated by resistor noise rather than by
the voltage or current noise of the OP270. When R_S exceeds
20 kW, current noise of the OP270 becomes the major contributor
to total noise.
100
OP200
10
–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 characteristic of the OP270 is achieved in part 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 (e_n), current
noise (i_n), and resistor noise (e_t).
OP270
RESISTOR
NOISE ONLY
1
100
1k
10k
R_S – SOURCE RESISTANCE (⍀)
100k
Figure 3. Total Noise vs. Source Resistance
(Including Resistor Noise) at 1 kHz
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 3 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 R_S > 5 kW.
Figures 3 and 4 show 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.
–8–
REV. C