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
OPA140
OPA140
OPA2140, OPA4140
www.ti.com SBOS498A –JULY 2010–REVISED AUGUST 2010
APPLICATION INFORMATION
with total circuit noise calculated. The op amp itself
The OPA140, OPA2140, and OPA4140 are unity-gain
contributes both a voltage noise component and a
stable, operational amplifiers with very low noise,
current noise component. The voltage noise is
input bias current, and input offset voltage.
commonly modeled as a time-varying component of
Applications with noisy or high-impedance power
the offset voltage. The current noise is modeled as
supplies require decoupling capacitors placed close
the time-varying component of the input bias current
to the device pins. In most cases, 0.1mF capacitors
and reacts with the source resistance to create a
are adequate. Figure 1 shows a simplified schematic
voltage component of noise. Therefore, the lowest
of the OPA140.
noise op amp for a given application depends on the
source impedance. For low source impedance,
OPERATING VOLTAGE
current noise is negligible, and voltage noise
generally dominates. The OPA140, OPA2140, and
The OPA140, OPA2140, and OPA4140 series of op
OPA4140 family has both low voltage noise and
amps can be used with single or dual supplies from
extremely low current noise because of the FET input
an operating range of V
S
= +4.5V (±2.25V) and up to
of the op amp. As a result, the current noise
V
S
= +36V (±18V). These devices do not require
contribution of the OPAx140 series is negligible for
symmetrical supplies; they only require a minimum
any practical source impedance, which makes it the
supply voltage of +4.5V (±2.25V). For V
S
less than
better choice for applications with high source
±3.5V, the common-mode input range does not
impedance.
include midsupply. Supply voltages higher than +40V
can permanently damage the device; see the
The equation in Figure 34 shows the calculation of
Absolute Maximum Ratings table. Key parameters
the total circuit noise, with these parameters:
are specified over the operating temperature range,
• e
n
= voltage noise
T
A
= –40°C to +125°C. Key parameters that vary over
• I
n
= current noise
the supply voltage or temperature range are shown in
the Typical Characteristics section of this data sheet.
• R
S
= source impedance
• k = Boltzmann's constant = 1.38 × 10
–23
J/K
CAPACITIVE LOAD AND STABILITY
• T = temperature in degrees Kelvin (K)
The dynamic characteristics of the OPAx140 have
For more details on calculating noise, see the section
been optimized for commonly encountered gains,
on Basic Noise Calculations.
loads, and operating conditions. The combination of
low closed-loop gain and high capacitive loads
decreases the phase margin of the amplifier and can
lead to gain peaking or oscillations. As a result,
heavier capacitive loads must be isolated from the
output. The simplest way to achieve this isolation is to
add a small resistor (R
OUT
equal to 50Ω, for example)
in series with the output.
Figure 20 and Figure 21 illustrate graphs of
Small-Signal Overshoot vs Capacitive Load for
several values of R
OUT
. Also, refer to Applications
Bulletin AB-028 (literature number SBOA015,
available for download from the TI web site) for
details of analysis techniques and application circuits.
NOISE PERFORMANCE
Figure 34. Noise Performance of the OPA140 and
Figure 34 shows the total circuit noise for varying
OPA211 in Unity-Gain Buffer Configuration
source impedances with the operational amplifier in a
unity-gain configuration (with no feedback resistor
network and therefore no additional noise
contributions). The OPA140 and OPA211 are shown
Copyright © 2010, Texas Instruments Incorporated 13
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