Guest columnist Harry Holt, applications engineer at Analog Devices, believes that two into one is a relevant question when designs need more than one operational amplifier.
In designs requiring multiple op amps, the offhand tendency is to use duals or quads, and allocate the various sections based on printed circuit board layout considerations.
In many cases, no harm is done, but for some circuits, a careful choice of singles, duals or quads, and proper partitioning can improve circuit performance. This article discusses whether two singles or one dual op amp is the proper choice.
Is a dual op amp really two op amps or a piece of silicon with two functions?
There is a general belief that a dual op amp is the same as two singles, but there are some subtle differences between a monolithic dual IC and two singles on your board that may cause problems in your next design.
Because the two op amps are side by side in the same monolithic piece of silicon, there are electrical and thermal factors to consider when using a dual.
As the op amp’s output voltage changes, the thermal dissipation changes, and a thermal wave propagates across the chip to the input stage, unbalancing it and appearing as an electrical signal. The thermal wave can affect both sections, even if they are electrically separate.
There are also electrical effects. In the desire to reduce die size, and cost, some circuits, such as bias circuits and the associated start-up circuit may be shared by both op amps.
If one op amp operates outside of its normal range and causes the bias circuit to malfunction, then the other op amp will malfunction also.
In addition, with a single pair of power pins, the bond wires and some metallisation on the die carry the combined current of both op amps. Current drawn by one section will generate an IR drop that will be seen by the other op amp through the PSRR specification which varies with frequency.
Benefits of dual
For designers, there are a number of advantages to using a dual op amp over other forms. First, a single insertion instead of two insertions will reduce manufacturing cost.
Second, most semiconductor manufacturers generally price duals lower than twice the cost of a single op amp. By merging subcircuits, the die area is generally smaller than two times a single op amp, thus saving money.
Third, high-speed automatic test equipment (ATE) is limited by handling time for simple functions such as op amps, so the test cost per function is also lower. Similar comments can be made for package costs.
Finally, because the two circuits were very close on the wafer, the match of electrical characteristics between the two, unusually not specified, is very close.
Dual disadvantages
There are some disadvantages though. By putting two or four functions in one package, the power dissipation is increased.
For low bandwidth, low voltage op amps, (low power dissipation) this increase results in a minor (5°C) increase in junction temperature. For high speed op amps, driving a low impedance load, such as a coax cable, this can be significant, perhaps 30°C. Due to die stress, the maximum offset voltage for quads will be higher than for duals or singles. In some cases, the dual will have higher offset than the single, and the quad will have higher offset than the dual.
Crosstalk is also a problem and comes from two effects; thermal and electrical. The thermal wave from one section will unbalance the input stage of the other section. This appears as low frequency feedback.
In addition, with one set of power supply pins, the bond wire resistance is common to all sections, so heavy load current from one section will cause an IR drop across the bond wires. The PSRR of the op amp is not infinite, so some portion will be coupled to the other sections. The PSRR decreases with increasing frequency, so look for this above 5-10 kHz.
For the best system performance and lowest system cost, each socket must be evaluated as to the appropriate op amp required. With automatic assembly and reduced package size, using singles and duals instead of quads may have no effect on overall cost. Considering printed circuit board layout, performance over temperature, channel separation, phase matching and cost, the best combination of singles or duals could be chosen.
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