OPA140
OPA2140, OPA4140
www.ti.com
SBOS498A –JULY 2010–REVISED AUGUST 2010
APPLICATION INFORMATION
with total circuit noise calculated. The op amp itself
contributes both a voltage noise component and a
current noise component. The voltage noise is
commonly modeled as a time-varying component of
the offset voltage. The current noise is modeled as
the time-varying component of the input bias current
and reacts with the source resistance to create a
voltage component of noise. Therefore, the lowest
noise op amp for a given application depends on the
source impedance. For low source impedance,
current noise is negligible, and voltage noise
generally dominates. The OPA140, OPA2140, and
OPA4140 family has both low voltage noise and
extremely low current noise because of the FET input
of the op amp. As a result, the current noise
contribution of the OPAx140 series is negligible for
any practical source impedance, which makes it the
better choice for applications with high source
impedance.
The OPA140, OPA2140, and OPA4140 are unity-gain
stable, operational amplifiers with very low noise,
input bias current, and input offset voltage.
Applications with noisy or high-impedance power
supplies require decoupling capacitors placed close
to the device pins. In most cases, 0.1mF capacitors
are adequate. Figure 1 shows a simplified schematic
of the OPA140.
OPERATING VOLTAGE
The OPA140, OPA2140, and OPA4140 series of op
amps can be used with single or dual supplies from
an operating range of VS = +4.5V (±2.25V) and up to
VS = +36V (±18V). These devices do not require
symmetrical supplies; they only require a minimum
supply voltage of +4.5V (±2.25V). For VS less than
±3.5V, the common-mode input range does not
include midsupply. Supply voltages higher than +40V
can permanently damage the device; see the
Absolute Maximum Ratings table. Key parameters
are specified over the operating temperature range,
TA = –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.
The equation in Figure 34 shows the calculation of
the total circuit noise, with these parameters:
•
•
•
•
•
en = voltage noise
In = current noise
RS = source impedance
k = Boltzmann's constant = 1.38 × 10–23 J/K
T = temperature in degrees Kelvin (K)
CAPACITIVE LOAD AND STABILITY
The dynamic characteristics of the OPAx140 have
been optimized for commonly encountered gains,
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 (ROUT equal to 50Ω, for example)
in series with the output.
For more details on calculating noise, see the section
on Basic Noise Calculations.
10k
EO
OPA211
1k
RS
100
Figure 20 and Figure 21 illustrate graphs of
Small-Signal Overshoot vs Capacitive Load for
several values of ROUT. 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.
OPA140
Resistor Noise
10
2
2
2
EO = en + (in RS) + 4kTRS
1
100
1k
10k
100k
1M
Source Resistance, RS (W)
NOISE PERFORMANCE
Figure 34. Noise Performance of the OPA140 and
OPA211 in Unity-Gain Buffer Configuration
Figure 34 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 OPA140 and OPA211 are shown
Copyright © 2010, Texas Instruments Incorporated
13
Product Folder Link(s): OPA140 OPA2140 OPA4140
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