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Although selecting a digital device that will operate in a low-power design is fairly straightforward, choosing the right low-power op amp can be somewhat more difficult. In the last few years there has been an increase in the number of operational amplifiers advertised as low voltage devices. Although an op amp may claim to work from a low supply voltage, there can be a gray area of what is defined as "works." Despite what some may have you believe, just because an op amp may draw some current at a low supply voltage does not mean it will be fully functional.

When selecting a low-power op amp, it is useful to know how low the supply voltage can go before the amplifier begins to fail. It is also helpful to understand how the amplifier’s performance will degrade, for example, as the battery approaches its end of life.

Although a datasheet will have characterization data at one or more specific supply voltages, designers should determine for themselves how the amplifier will perform in their low-power application.

How to Determine for Yourself

A procedure originally developed by Steve Sockolov is used by application engineers at Analog Devices in determining the lower operating voltage supply limits of amplifiers. This simple procedure involves graphing three parameters against supply voltage:

1) Supply Current
A datasheet will usually show a graph of I_SY vs. V_SY. Looking at this curve will give a general picture of where the op amp is beginning to fail. As the amplifier’s supply current decreases, a number of parameters will begin to degrade, including open-loop gain, gain-bandwidth product, slew rate, and even phase margin, which could result in the amplifier becoming unstable.

2) Offset Voltage
Typical offset voltages at different supply voltages are usually given in the datasheet. Plotting offset voltage as a function of supply voltage will give a more direct view of where the amplifier begins to fail. When configuring the amplifier to measure offset voltage (see Fig. 1) it is easiest to use a dual power supply with tracking, so both supply voltages can be controlled with a single knob. This allows the non-inverting terminal to be connected straight to ground without the need for an external voltage reference, as would be required with a single power supply.

Fig. 1: Configuration for Measuring Offset Voltage

As the input stage of the op amp begins to turn off, the offset voltage will suddenly increase. Knowing where this knee occurs and how steep the increase in V_OS is can provide insight into how the amplifier’s error will increase as a supply battery approaches its end of life.

3) Distortion
Looking at total harmonic distortion (THD) versus supply voltage will show how the op amp will perform with ac signals in a real-world environment. In a simple configuration for measuring distortion (see Fig. 2) and in the absence of distortion analyzing equipment, like a spectrum analyzer or an Audio Precision system, you can observe the output of the amplifier on an oscilloscope. Although this will not be nearly as precise, you will be able to determine if the output waveform is severely distorted.

To get the most accurate picture, this test should be run with the same load resistance and capacitance at the amplifier’s output as would be used in the actual application. Care should be taken to use a small enough input signal to avoid clipping in the amplifier from the decreasing supply voltage, as this would give a false reading.

Fig. 2: Configuration for Measuring Distortion

A distinct knee will be seen in each of the three graphs, although they may not necessarily occur at the same supply voltage. These inflection points indicate where the amplifier is beginning to fail in each test, and depending on the application, one graph may be of more interest than the others.

For example, in a battery monitoring application where the op amp will be used as a dc comparator, the graph of offset voltage vs. supply voltage will be of more interest. If the low power op amp is to be used as a microphone pre-amplifier or in an active filter, the graph of distortion will be more useful.

A Real-Life Example

The OP186 low-power op amp from Analog Devices claims that it can operate down to a +2 V supply voltage. Although the datasheet shows electrical characteristics at +2.2 V, the previous three tests were run on the OP186 to see if it can indeed perform at +2 V.

Fig. 3 is taken from the datasheet and shows the amplifier’s supply current begins to drop off below +2 V. Offset voltage vs. supply voltage is shown in Fig. 4. The sharp increase of V_OS at +1.9 V clearly shows where the input stage of the device is shutting down. Finally, distortion is plotted as a function of supply voltage (see Fig. 5.) The knee in this curve occurs around +2.0 V.

Fig. 3: Supply Current vs. Supply Voltage

Fig. 4: Offset Voltage vs. Supply Voltage

Fig. 5: THD vs. Supply Voltage

Looking at this data should give the designer a degree of comfort in the claim that the OP186 can function down to a +2 V single supply rail.

Talk is cheap in a datasheet, and some key performance issues may be difficult to find in all the specifications data. If the datasheet does not provide specifications at the supply voltage you require, you should use these three tests to find out for sure. This little bit of precautionary effort can help ensure against potential problems in your low-power applications.

Biography

Troy Murphy is the applications engineer for the Precision Amplifier and Audio group at Analog Devices in Santa Clara, California. For more information on the OP186 or any other device from Analog Devices, please visit our web page at www.analog.com. Tel: 408-562-7520, Fax: 408-727-1550

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