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 openloop 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
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the ground reference may optimize the dynamic range of an op amp whose output is designed to swing to 0 V. On the other hand, the signal may require level shifting in order to be compatible with the input of other devices (such as ADCs) that are not designed to operate at 0 V input. The need for rail-to-rail amplifier output stages is also driven by the need to maintain wide dynamic range in low-supply voltage applications. A single-supply/rail-to-rail amplifier should have output voltage swings that are within at least 100 mV of either supply rail (under a nominal load). The output voltage swing is very dependent on output stage topology and load current. Single-supply op amp design issues are summarized in Figure 1.
Single Supply Offers: Lower Power Battery Operated Portable Equipment Requires Only One Voltage Design Tradeoffs: Reduced Signal Swing Increases Sensitivity to Errors Caused by Offset Voltage, Bias Current, Finite OpenLoop Gain, Noise, etc. Must Usually Share Noisy Digital Supply Rail-to-Rail Input and Output Needed to Increase Signal Swing Precision Less than the best Dual Supply Op Amps but not Required for All Applications Many Op Amps Specified for Single Supply, but do not have Rail-to-Rail Inputs or Outputs
Figure 1: Single-Supply Op Amp Design Issues OP AMP INPUT STAGES It is extremely important to understand input and output structures of op amps in order to properly design the required interfaces. For ease of discussion, the two can be examined separately, as there is no particular reason to relate them at this point. Bipolar Input Stages The very common and basic bipolar input stage of Figure 2 consists of a "long-tailed pair" built with bipolar transistors. It has a number of advantages: it is simple, has very low offset, the bias currents in the inverting and non-inverting inputs are well-matched and do not vary greatly with temperature. In addition, minimizing the initial offset voltage of a bipolar op amp by laser trimming also minimizes its drift over temperature. This architecture was used in the very earliest monolithic op amps such as the µA709. It is also used with modern high speed types. Although NPN bipolars are shown, the concept also applies with the use of PNP bipolars.
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VIN
Low Offset: As low as 10μV Low Offset Drift: As low as 0.1μV/ºC Temperature Stable IB Well-Matched Bias Currents Low Voltage Noise: As low as 1nV/√Hz
High Bias Currents: 50nA - 10μA (Except Super-Beta: 50pA - 5nA, More Complex and Slower) Medium Current Noise: 1pA/√Hz Matching source impedances minimize offset error due to bias current
Figure 2: A Bipolar Transistor Input Stage Bias Current Compensated Bipolar Input Stage
VIN
Low Offset Voltage: As low as 10μV Low Offset Drift: As low as 0.1μV/ºC Temperature Stable Ibias Low Bias Currents:
