Datasheet

OPA209
Output
R
F
Input
-
+
R
I
VOLTAGENOISESPECTRALDENSITY
vsSOURCERESISTANCE
100k 1M
SourceResistance,R ( )W
S
100
1k 10k
10k
1k
100
10
1
VotlageNoiseSpectralDensity,E
O
R
S
E
O
E =e
O n S
+(i R ) +4kTR
n S
2 2 2
ResistorNoise
OPA827
OPA209
OPA209
OPA2209
OPA4209
SBOS426C –NOVEMBER 2008–REVISED OCTOBER 2013 www.ti.com
INPUT PROTECTION The equation in Figure 33 shows the calculation of
the total circuit noise, with these parameters:
The input terminals of the OPA209 are protected from
• e
n
= voltage noise,
excessive differential voltage with back-to-back
• i
n
= current noise,
diodes, as shown in Figure 32. In most circuit
applications, the input protection circuitry has no
• R
S
= source impedance,
consequence. However, in low-gain or G = 1 circuits,
• k = Boltzmann's constant = 1.38 × 10
–23
J/K,
fast ramping input signals can forward-bias these
• and T = temperature in kelvins.
diodes because the output of the amplifier cannot
respond rapidly enough to the input ramp. This effect
For more details on calculating noise, see the Basic
is illustrated in Figure 25 and Figure 26 of the Typical
Noise Calculations section.
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 Noise
Performance section for further information on noise
performance. Figure 32 shows an example
configuration that implements a current-limiting
feedback resistor.
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
NOISE PERFORMANCE
source resistance on overall op amp noise
performance. Total noise of the circuit is the root-
Figure 33 shows the total circuit noise for varying
sum-square combinations of all noise components.
source impedances with the op amp in a unity-gain
configuration (no feedback resistor network, and
The resistive portion of the source impedance
therefore no additional noise contributions). Two
produces thermal noise proportional to the square
different op amps are shown with the total circuit
root of the resistance. This function is illustrated in
noise calculated. The OPA209 has very low voltage
Figure 33. The source impedance is usually fixed;
noise, making it ideal for low source impedances
consequently, select the appropriate op amp and the
(less than 2kΩ). As a comparable precision FET-input
feedback resistors to minimize the respective
op amp (very low current noise), the OPA827 has
contributions to the total noise.
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 OPA211
may provide lower noise.
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