Texas Instruments | How to Properly Configure Unused Operational Amplifiers (Rev. A) | Application notes | Texas Instruments How to Properly Configure Unused Operational Amplifiers (Rev. A) Application notes

Texas Instruments How to Properly Configure Unused Operational Amplifiers (Rev. A) Application notes
____________________________________________________
How to Properly Configure Unused Operational Amplifiers
Edward Mullins, Ian Williams, Giuseppe Lo Voi, Raphael Puzio
Introduction
Multi-channel operational amplifiers (op amps) are
often implemented in circuits that do not require the
use of all channels. Undesired behavior in an unused
amplifier channel can negatively impact system
performance, as well as the performance of the
channels in use. To avoid degradation of both the op
amp and system performance, the unused op amp
channels must be configured properly.
Degradation Conditions
Improperly terminating unused op amp pins has the
potential to lead to degraded the op amp or system
performance. Leaving either the input or output pins
floating will create unexpected voltage shifts, which
leads to unpredictable behavior. Connecting a singlesupply amplifier in a buffer configuration while
grounding the non-inverting input will cause the output
stage to saturate, potentially causing increased current
consumption above the maximum quiescent current
and unwanted dissipation. This configuration can also
cause unpredictable signal corruption induced by
crosstalk interactions between the used and unused
op amps. Another consideration when configuring an
unused op amp is the differential input voltage range.
An op amp typically cannot support a large differential
voltage at its inputs as large as what would be seen
from one supply rail to the other. Connecting the input
pins together or to the supply rails can lead to device
damage. If configured like this, the input stage can
suffer permanent electrical overstress (EOS) damage,
as shown in Figure 1.
Op Amp Specifications to Consider
The desired outcome when configuring an unused op
amp in your system is to maintain linear and
predictable device behavior. The op amps’ specified
input and output ranges, found in the product
datasheet, serves as a guide to proper
implementation. The input of an op amp will have a
specified common mode range, and the output stage
of an op amp will have a specified voltage range. Both
input and output ranges must be met to prevent nonlinear output behavior. In addition, negative feedback
is highly recommended to achieve amplifier stability.
Implementation: Single Supply Rail
Figure 3 shows the dual-channel OPA2325 op amp
powered by a single supply rail. The OPA2325 has a
specified common mode voltage range of (V-)-0.1 V to
(V+)+0.1 V and an output range of (V-) +0.02 V to
(V+)-0.02 V.
Op amp U1A is implemented into a circuit in a basic
non-inverting configuration. Op amp U1B is unused in
the circuit. It is configured as a voltage follower, where
the input is set by a voltage-divider from the supply
rail. Selecting two equivalent resistors (R3 = R4)
ensures that the input voltage will be within the
common mode range by setting it to mid-supply. The
output is connected to the inverting input to provide
negative feedback, allowing the op amp’s output to
follow the input and operate in its linear region.
space
+V
+
+V
VOUT = VIN(GCL)
(GCL) = (R1/R2) + 1
+
R3
OPA2325
_
Figure 1. Scanning Electron Microscope Picture of
EOS Damaged Device
R2
U1A
R3 = R4
VIN~V+/2
+
VOUT = V+/2
OPA2325
_
U1B
R4
R1
Figure 2. Proper Implementation of Unused
OpAmp – Single Supply Rail
SBOA204A – October 2017 – Revised September 2018
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How to Properly Configure Unused Operational Amplifiers Edward Mullins, Ian Williams,
Copyright © 2017–2018, Texas Instruments Incorporated
Giuseppe Lo Voi, Raphael Puzio
1
www.ti.com
If the implementation of extra resistors is undesirable,
the non-inverting input can be connected to any
reference voltage that is within the specified common
mode and output ranges. This technique favors an op
amp with rail-to-rail input and/or output range
characteristics. Rail-to-rail devices offer a wider linear
operating range, which will accept a wider range of
reference voltages. Table 1 shows a couple devices
that are rail-to-rail in/out and a couple that are rail-torail out.
Table 1. Rail-to-Rail Op Amps
Device
Rail-to-Rail
OPA2325
Input/Output
OPA388
Input/Output
OPA145
Output
OPA189
Output
Implementation: Split Supply Rails
An op amp powered by split supply rails can be
implemented as shown in Figure 3 to meet the input
and output specifications while maintaining stability.
Op amp U2A is implemented into a circuit in a basic
non-inverting configuration. Op amp U2B is unused in
the circuit. The OPA2189 op amp has a specified
common mode range of (V-)-0.1V to (V+)-2.5V and an
output range of (V-) +0.3V to (V+) -0.3V.
+V
VIN
+
+V
VOUT
+
OPA2189
_
U2A
+
-V
R6
VOUT~0
OPA2189
_
U2B
-V
R5
Copyright © 2017, Texas Instruments Incorporated
Figure 3. Proper Implementation of Unused Op
Amps - Split Supply Rail
The non-inverting input of the op amp is directly
connected to ground. Connecting the non-inverting
input to ground sets the voltage to mid-supply. The
output is connected to the inverting input to provide
negative feedback. This implementation is simple and
does not require additional resistors, so it may be the
most cost-effective implementation.
Summary
Unused op amps in multi-channel devices must be
configured properly to avoid possible device
degradation, extra power consumption and noise.
These unused channels can cause undesired effects
on other channels. Understanding input and output
voltage limitations will guide effective configuration of
unused op amps.
Table 2. Related Articles and Resources
Media
Title
Author
Article
Properly terminating an unused op
amp
Todd Toporski
Video
How to terminate an unused op amp
Todd Toporski
The Unused Op Amp -what to do?
Bruce Trump
TI Precision Labs-Op Amps: Electrical
Overstress
Art Kay and Ian
Williams
E2E Post
TIPL Video
2
How to Properly Configure Unused Operational Amplifiers Edward Mullins, Ian Williams,
Giuseppe Lo Voi, Raphael Puzio
SBOA204A – October 2017 – Revised September 2018
Submit Documentation Feedback
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