Datasheet

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 n S S
+(i R ) +4kTR
2 2 2
ResistorNoise
OPA211
OPA827
OPA827
SBOS376H –NOVEMBER 2006–REVISED MAY 2012 www.ti.com
APPLICATION INFORMATION
The OPA827 is a unity-gain stable, precision The equation in Figure 41 shows the calculation of
operational amplifier with very low noise, input bias the total circuit noise, with these parameters:
current, and input offset voltage. Applications with
• e
n
= voltage noise
noisy or high impedance power supplies require
• i
n
= current noise
decoupling capacitors placed close to the device pins.
• R
S
= source impedance
In most cases, 0.1μF capacitors are adequate.
• k = Boltzmann's constant = 1.38 × 10
–23
J/K
OPERATING VOLTAGE • T = temperature in kelvins
The OPA827 series of op amps can be used with For more details on calculating noise, see the Basic
single or dual supplies from an operating range of Noise Calculations section.
V
S
= +8V (±4V) and up to V
S
= +36V (±18V). This
device does not require symmetrical supplies; it only
requires a minimum supply voltage of 8V. Supply
voltages higher than +40V (±20V) can permanently
damage the device; see the Absolute Maximum
Ratings table. Key parameters are specified over the
operating temperature range, T
A
= –40°C to +125°C.
Key parameters that vary over the supply voltage or
temperature range are shown in the Typical
Characteristics section of this data sheet.
NOISE PERFORMANCE
Figure 41 shows the total circuit noise for varying
source impedances with the operational amplifier in a
unity-gain configuration (with no feedback resistor
network and therefore no additional noise
contributions). The OPA827 (GBW = 22MHz) and
Figure 41. Noise Performance of the OPA827 and
OPA211 (GBW = 80MHz) are both shown in this
OPA211 in Unity-Gain Buffer Configuration
example with total circuit noise calculated. The op
amp itself contributes both a voltage noise
component and a current noise component. The BASIC NOISE CALCULATIONS
voltage noise is commonly modeled as a time-varying
Low-noise circuit design requires careful analysis of
component of the offset voltage. The current noise is
all noise sources. External noise sources can
modeled as the time-varying component of the input
dominate in many cases; consider the effect of
bias current and reacts with the source resistance to
source resistance on overall op amp noise
create a voltage component of noise. Therefore, the
performance. Total noise of the circuit is the root-
lowest noise op amp for a given application depends
sum-square combination of all noise components.
on the source impedance. For low source impedance,
current noise is negligible, and voltage noise
The resistive portion of the source impedance
generally dominates. The OPA827 family has both
produces thermal noise proportional to the square
low voltage noise and lower current noise because of
root of the resistance. This function is plotted in
the FET input of the op amp. Very low current noise
Figure 41. The source impedance is usually fixed;
allows for excellent noise performance with source
consequently, select the op amp and the feedback
impedances greater than 10kΩ. The OPA211 has
resistors to minimize the respective contributions to
lower voltage noise and higher current noise. The low
the total noise.
voltage noise makes the OPA211 a better choice for
low source impedances (less than 2kΩ). For high
source impedance, current noise may dominate, and
makes the OPA827 series amplifier the better choice.
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