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Maxim > Design
support > App notes
> Amplifier
and Comparator Circuits > APP 1957

Mar 25, 2003

Keywords: uncommitted op amp, uncommitted
operational amplifier, unused op amp, unused operational
amplifier, quad op amp, uncommitted, unused, opamp

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APPLICATION NOTE 1957

Avoiding Noise and Power Problems with Unused Op
Amps

Abstract: If
connected improperly, an uncommitted op amp leads to problems
such as increased power consumption and added noise. This
application note shows how to properly terminate an
uncommitted op amp.

Sometimes
there are one or more unused operational amplifiers (op amps)
in a circuit design. For example, Figure 2 of Application Note
994: Reference
Voltage for Multiple ADCs uses three of the four op
amps in the MAX4254 to drive the reference inputs of an ADC.
Unused op amps in this situation can be used for other
purposes, but if they are not needed they must be terminated
correctly. If connected improperly, an uncommitted op amp
leads to problems such as increased power consumption and
added noise.

Proper Termination for an Uncommitted Op Amp
Two
conditions are required to properly terminate an uncommitted
op amp:

The op amp inputs must be actively held within the input
common-mode voltage range of the device.

The op amp output must be set within the output voltage
swing range of the device.

The circuit in Figure
1 satisfies both conditions.

If the circuit has
dual supplies, ±3V for example, the two resistors are not
necessary and simply grounding the non-inverting terminal is
sufficient. Additionally, the non-inverting terminal can be
connected to another voltage elsewhere in the circuit that is
within the input common-mode voltage range of the device,
thereby eliminating the need for the two resistors in Figure
1.

Figure
1. A properly terminated uncommitted op amp. The input
common-mode range is satisfied and the output is within the
output voltage swing range.

For prototype circuits
it is a good idea to terminate an uncommitted op amp as shown
in Figure 2. This termination scheme allows the op amp
to be changed into an inverting or non-inverting configuration
if the need arises in a later prototype stage.

Figure
2. A properly terminated uncommitted op amp for prototype
circuits. This termination scheme allows the op amp to be
changed into an inverting or non-inverting configuration if
the need arises in a later prototype stage.

Examples of An Improperly Terminated Operational
Amplifier
Examples of common, yet improper, methods of
terminating an uncommitted op amp are shown in Figure
3.

Figure
3. Improper methods of terminating an uncommitted op amp.
Terminating an uncommitted op amp as shown causes excessive
supply current and additional noise.

A.
This is the worst thing to do with an uncommitted op amp. Both
inputs are floating and will pick up noise and the output may
switch
from rail to rail unpredictably. This configuration will draw
varying amounts of supply current and will couple noise into
the other op amps and may add noise to the power and ground
traces.

B. These two circuits are not
recommended because they are stressful to the op amp inputs.
This circuit applies the maximum potential across the input
and can damage certain op amps such as those with diode
clamps between the inputs. The op amp output will be in a
known state and will be railed low in the first circuit and
railed high in the second. Railing an op amp&'s output low
causes the op amp to consume more supply
current.

C. These two circuits are bad because
the output will be railed in an unknown state. The output
state will be determined by the value of the input stage's
offset voltage. Due to the randomness of input offset voltage
it cannot be predetermined if the output will be railed high
or low. In addition to the output's unknown state there is a
likely chance the output will change states over temperature
inducing unnecessary noise in the circuit. Railing an op amp's
output low causes the op amp to consume more supply
current.

D. These two circuits are the most
common mistake made when terminating an uncommitted op amp.
They are equally bad because the op amp's output will be
railed low, causing the op amp to consume more supply
current.

Avoiding The Uncommitted Amplifier Yields Many
Benefits
In many cases it is beneficial to avoid having an
uncommitted amplifier. The next-generation of sub-micron
technology op amps are affordable and are available in tiny
packages. Instead of using three of four op amps in a TSSOP
package consider using only what you need. Using multiple
single op amps in small SC70 packages or a combination of a
dual op amp in a tiny SOT23 package and a single op amp offers
the following benefits:

Avoids the issue of the uncommitted amplifier

Reduces power dissipation by eliminated extra amplifiers

Reduces PCB
area by using smaller components (Figure 4)

Eases PCB layout and reduces traces length

Less expensive

Figure
4. A comparison of the smallest quad, dual, and single
amplifiers illustrates dramatic size benefits. For example,
four single op amps consume 16mm² whereas one quad op amp
consumes 30mm².

Conclusion
Two conditions are required to properly
terminate an uncommitted op amp:

The op amp inputs must be actively held within the input
common-mode voltage range of the device.

The op amp output must be set within the output voltage
swing range of the device.

In many cases avoiding
the uncommitted op amp by using singles or duals decreases
system size and cost.

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APP 1957: Mar 25, 2003

AN1957, AN 1957, APP1957,
Appnote1957, Appnote 1957

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