MT-035 Datasheet, PDF (1/12 Pages) Analog Devices – Op Amp Inputs, Outputs, Single-Supply, and Rail-to-Rail Issues

MT-035
TUTORIAL
Op Amp Inputs, Outputs, Single-Supply, and Rail-to-Rail Issues
SINGLE-SUPPLY OP AMP ISSUES
Single-supply operation has become an increasingly important requirement because of market
demands. Automotive, set-top box, camera/camcorder, PC, and laptop computer applications are
demanding IC vendors to supply an array of linear devices that operate on a single-supply rail,
with the same performance of dual supply parts. Power consumption is now a key parameter for
line or battery operated systems, and in some instances, more important than cost. This makes
low-voltage/low supply current operation critical; at the same time, however, accuracy and
precision requirements have forced IC manufacturers to meet the challenge of "doing more with
less" in their amplifier designs.
In a single-supply application, the most immediate effect on the performance of an amplifier is
the reduced input and output signal range. As a result of these lower input and output signal
excursions, amplifier circuits become more sensitive to internal and external error sources.
Precision amplifier offset voltages on the order of 0.1 mV are less than a 0.04 LSB error source
in a 12-bit, 10 V full-scale system. In a single-supply system, however, a "rail-to-rail" precision
amplifier with an offset voltage of 1 mV represents a 0.8 LSB error in a 5 V fullscale system (or
1.6 LSB for 2.5 V fullscale).
Gain accuracy in some low voltage single-supply devices is also reduced, so device selection
needs careful consideration. Many amplifiers with ~120 dB open-loop gains typically operate on
dual supplies—for example OP07 types. However, many single-supply/rail-to-rail amplifiers for
precision applications typically have open-loop gains between 25,000 and 30,000 under light
loading (>10 kΩ). Selected devices, like the OP113/OP213/OP413 family, do have high open-
loop gains (>120 dB), for use in demanding applications. Another example would be the
AD855x chopper-stabilized op amp series.
Besides these limitations, many other design considerations that are otherwise minor issues in
dual-supply amplifiers now become important. For example, signal-to-noise (SNR) performance
degrades as a result of reduced signal swing. "Ground reference" is no longer a simple choice, as
one reference voltage may work for some devices, but not others. Amplifier voltage noise
increases as operating supply current drops, and bandwidth decreases. Achieving adequate
bandwidth and required precision with a somewhat limited selection of amplifiers presents
significant system design challenges in single-supply, low-power applications.
Most circuit designers take "ground" reference for granted. Many analog circuits scale their input
and output ranges about a ground reference. In dual-supply applications, a reference that splits
the supplies (0 V) is very convenient, as there is equal supply headroom in each direction, and 0
V is generally the voltage on the low impedance ground plane. In single-supply/rail-to-rail
circuits, however, the ground reference can be chosen anywhere within the supply range of the
circuit, since there is no standard to follow. The choice of ground reference depends on the type
of signals processed and the amplifier characteristics. For example, choosing the negative rail as
Rev.0, 10/08, WK
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