Anyone who works with modern single-channel operational amplifiers, op amps, in their analog circuit designs are familiar with the 5 active device pins; the two inputs, the two supply pins and the output. These five pins accommodate the numerous applications where we apply op amps.
Following them is the next group having a sixth active pin function. Most often that additional pin functions as a shutdown, or conversely, a device enable pin. Shown below is the OPA320S that includes shutdown capability.
So what exactly is this shutdown/enable pin supposed to do?
Most often the purpose of the shutdown pin is to idle the amplifier function and reduce its power consumption. When the op amp is shut down it goes into a non-active mode where the quiescent current (Iq) is reduced by many orders of magnitude.
The OPA320S CMOS op amp, shown above, has a typical Iq of 1.5 mA with a 3.3 V supply level. Its power consumption is 4.95 mW when active. When in shutdown mode, however, the typical Iq reduces to 0.1 uA and the power consumption decreases to 330 nW - a decrease in power of 15,000:1.
The shutdown/enable pin makes switching between the two modes easy. One simply applies the correct voltage level on the pin required for the desired mode. The shutdown/enable levels for the OPA320S when using a single Vs+ supply are shown in the diagram.
Where users get into trouble with the shutdown function is making assumptions about what else the amplifier should do besides reducing power consumption. Probably the most common assumption is that the amplifier becomes a high impedance block at the inputs and output, with no internal conduction paths. Another is the amplifier will no longer have any effect on signals and circuits directly connected to its pins. One, or both, may be true but unless there is something in the data sheet stating the specific amplifier characteristics in shutdown mode then it may not be so!
Exactly how the op amp behaves during shutdown is dictated by its electrical design.
Some op amps having shutdown do, by virtue of their design, almost disappear electrically from the circuit. They draw miniscule supply current, their inputs and outputs appear as very high impedances and signals applied to them couple minimally through them. The “off” pins have little effect on circuit signals. This is often the case for modern, CMOS op amps.
Some op amps, especially some based on bipolar designs, may have input protection diode clamps that can be biased “on” when a large signal is applied to the amplifier output while it is in shutdown mode. In that case, the feedback resistor and diodes can provide a signal path from the output back to anything connected to the amplifier’s non-inverting input.
The OPA211 is an example of a precision bipolar op amp having shutdown capability and has the clamping diode protection in place across its inputs. The potential output-to-input signal path is illustrated in the OPA211 data sheet Fig. 45.
Even though an op amp may almost disappear during shutdown when dc levels are applied, ac signals applied to the pins may couple to unexpected points, or become distorted. This is especially true as their amplitude and/or frequency is increased. Each model of op amp should be evaluated in the application circuit where its shutdown mode behavior can be observed first hand.
It is best to seek an op amp where the data sheet spells out and fully discusses the shutdown behaviors. Don’t read anything into the data sheet regarding the shutdown mode behavior that isn’t stated!
Our forum has an old thread that touches on the topic of non-chopper CMOS op amps, here. Learn about TI’s entire portfolio of amplifier ICs and find technical resources.
Some op amps were also programmable, which allowed you to set the quiescent current. Or you could use it similar to a shutdown pin and vary the quiescent current in a variety of ways. Changing the operating current affects the speed of the part.
Yes, for the amplifier types you mention the operating current level is adjustable allowing one to limit the current to no more than is necessary to attain the required ac bandwidth. Some examples of such amplifiers are the LPV531 CMOS op amp, and the BUF634 hi-speed, high current buffer. Their amplifier functions can't be shut down completely.
Thank you for reading and your constructive comment.
So I am admittedly not smart when it comes to analog design... wondering why you wouldn't simply power cycle an opamp with a fet or something of that nature vs. using the shutdown line. Is that an attempt to reduce the number of parts you are using in a circuit? I for example have a circuit that I want to duty cycle the whole circuit so this enable/shutdown line doesn't seem helpful. Just looking for a comparison between the two methods.
Actually, the shutdown feature is a popular feature for amplifiers that provide the capability. Indeed a bipolar or FET series switch in the power supply line can be used to shut down one, or more amplifiers. However, the switch circuit does require additional components which add cost, and those components do take up board space. The latter is often that is an important consideration; especially in portable electronics.
Other, maybe not so obvious benefits of the internal shutdown is enable/shutdown pin readily interfaces with standard logic voltage levels and its turn-on and turn-off times are characterized and provided in the datasheet. Additionally, some amplifiers having internal shutdown have an output circuit that appears as a very high impedance during shutdown which makes them usable in simple, multiplexer applications. These capabilities are sometimes important in applications and can ease the design effort. Those behaviors may not be so easily achieved with a series switch design. For an external shutdown the responsibility for those characteristics would likely fall on the circuit designer.
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