The realities of the maximum-supply-current specification for op amps. Part 2
A hands-on test
A comparison of a bipolar op amp and a CMOS op amp from Analog Devices and three of its major analog competitors quantifies effect described in the previous section (Table 1). The comparison includes National Semiconductor’s venerable, non-rail-to-rail-output LM358 dual op amp and LM393 dual comparator. The tests measure the supply current as a function of supply voltage using three circuits.
Table 1. Bipolar and CMOS OP-Amp Comparison
Figure 3 shows the classic method for measuring supply current. The ammeters connect as the figure shows to exclude the supply current of the resistive divider. Two ammeters verify that the supply current is accurate and excludes any undesired current path through the input pins. The resistor values are noncritical and ensure that the input to the op amp is within the specified input-voltage range from the data sheet.
To measure supply current when the circuit is operating in open-loop mode, such as operation as a comparator, another method is used (Figure 4). Some low-noise, bipolar op amps have diodes between the inputs to protect the differential input pair, so the data sheet lists the maximum differential voltage in the absolute-maximum table as ±0.7V. Any internal series resistors usually have values of 500Ω to 2 kΩ.
The absolute-maximum table may state that the maximum differential voltage is plus or minus the supply voltage, but this specification does not mean that the part operates. You should consult a simplified internal schematic. If you don’t find one in the data sheet, call the manufacturer to obtain it.
In these two configurations, the choice of resistor values is more critical. The resistor values should be low enough to cause the differential input voltage to be at least 0.5V to guarantee that the output drives hard into the rail but high enough to cause no damage to the internal diodes. The chosen values limit the input current to less than 1 mA.
Bipolar rail-to-rail op amps
All of the bipolar rail-to-rail-output op amps have supply current greater than the maximum op-amp supply current in one or both comparator circuits. You can drive the output stage in several ways, and some methods result in a supply-current increase when driving to one rail or the other. For Texas Instruments’ OP284, the data sheet shows a simplified schematic of the second and output stages (Figure 5).
If Q5, Q3, and Q4 drive the output voltage high, the supply current will be a function of the values of R4 and R6. You select these values to maximize the op amp’s performance and minimize die area, not to enhance comparator operation. When Q6, R1, and Q1 drive the output voltage low, R1 will determine the supply current. Again, select the values of R1, I1, and other components for op-amp performance, not comparator performance.
CMOS rail-to-rail op amps
CMOS op amps have interesting behavior. In some cases, the supply current decreases when you drive it to a rail. The output stage of a CMOS op amp comprises common-source PMOS and NMOS transistors (Figure 6), and gain enters during the output stage. The gain is the transconductance times the load resistance. You can get a reasonable value of transconductance because the drive circuit sets the quiescent current to a certain value.
As the output drives into the rail, the drive circuit decreases the drive on the complementary transistor. Depending on the transfer characteristics from the top transistor to the bottom transistor, the current decreases. Note the wide variation in behavior among the four CMOS op amps.
To reduce die size and, therefore, cost, both op amps can share some circuits, such as bias circuits and the associated start-up circuit. If one op amp operates outside its normal range and causes the bias circuit to malfunction, then the other op amp will also malfunction (Reference 7). For battery-operated systems or when using low-current series regulators, you must consider the additional supply current. Battery life may be less than you calculate, or the regulator may not start up under all conditions, especially over temperature.
For new designs, the easiest approach is to avoid using op amps as comparators. If you must use one as a comparator, then you should check the data sheet to see whether the manufacturer has any information on the device’s operation as a comparator. Some manufacturers include this information (Reference 8). If the data sheet omits this information, ask the manufacturer for it. If the manufacturer cannot provide it, measure several date codes yourself using the circuits in the figures and add 50% for a safety factor.
Rail-to-rail-output op amps have unique characteristics when you operate them as comparators. The best ways to improve battery life and increase performance are to use a low-cost comparator when your design requires a comparator, to tie off any used op-amp sections as followers and connecting the noninverting input to a stable voltage within the input-voltage range of the op amp, and to use singles and duals as appropriate instead of quads.
Supply current may greatly exceed the maximum that the data sheet states. Under carefully considered conditions, you can use unused op amps as comparators, but using the proper mix of op amps and comparators will result in lower supply current and well-defined performance.
6. Holt, Harry, “Op Amps: To Dual or Not to Dual,” EE Times, Nov 19, 2010.
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