SBOS426B – NOVEMBER 2008 – REVISED AUGUST 2010
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INPUT PROTECTION
The input terminals of the OPA209 are protected from
excessive differential voltage with back-to-back
diodes, as shown in
In most circuit
applications, the input protection circuitry has no
consequence. However, in low-gain or G = 1 circuits,
fast ramping input signals can forward-bias these
diodes because the output of the amplifier cannot
respond rapidly enough to the input ramp. This effect
is illustrated in
and
of the Typical
Characteristics. If the input signal is fast enough to
create this forward-bias condition, the input signal
current must be limited to 10mA or less. If the input
signal current is not inherently limited, an input series
resistor can be used to limit the signal input current.
This input series resistor degrades the low-noise
performance of the OPA209. See the
section for further information on noise
performance.
shows an example
configuration that implements a current-limiting
feedback resistor.
R
F
The equation in
shows the calculation of
the total circuit noise, with these parameters:
• e
n
= voltage noise,
• i
n
= current noise,
• R
S
= source impedance,
• k = Boltzmann's constant = 1.38 × 10
–23
J/K,
• and T = temperature in kelvins.
For more details on calculating noise, see the
section.
VOLTAGE NOISE SPECTRAL DENSITY
vs SOURCE RESISTANCE
10k
Votlage Noise Spectral Density, E
O
E
O
1k
R
S
OPA209
100
OPA827
10
2
2
Resistor Noise
1
-
100
1k
E
O
= e
n
+ (i
n
R
S
) + 4kTR
S
10k
100k
1M
2
Source Resistance, R
S
(
W
)
OPA209
R
I
Input
+
Output
Figure 33. Noise Performance of the OPA209 and
OPA827 in Unity-Gain Buffer Configuration
BASIC NOISE CALCULATIONS
Figure 32. Pulsed Operation
Low-noise circuit design requires careful analysis of
all noise sources. External noise sources can
dominate in many cases; consider the effect of
source resistance on overall op amp noise
performance. Total noise of the circuit is the
root-sum-square
combinations
of
all
noise
components.
The resistive portion of the source impedance
produces thermal noise proportional to the square
root of the resistance. This function is illustrated in
The source impedance is usually fixed;
consequently, select the appropriate op amp and the
feedback resistors to minimize the respective
contributions to the total noise.
NOISE PERFORMANCE
shows the total circuit noise for varying
source impedances with the op amp in a unity-gain
configuration (no feedback resistor network, and
therefore no additional noise contributions). Two
different op amps are shown with the total circuit
noise calculated. The OPA209 has very low voltage
noise, making it ideal for low source impedances
(less than 2kΩ). As a comparable precision FET-input
op amp (very low current noise), the
has
somewhat higher voltage noise, but lower current
noise. It provides excellent noise performance at
moderate to high source impedance (10kΩ and up).
For source impedance lower than 300Ω, the
may provide lower noise.
12
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