5. Acceptable Op Amp Substitution Criteria

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The op amp amplifies the photodiode signal in the color sensor circuit. Because the photodiode (or reversed LED) provides such a small amount of current flow, it is critical that the op amp have ultra-low input current needs. Older generations of op amps won’t work.

Researching op amps, you'll notice that there are a lot of other characteristics specified in their datasheets. Obviously an acceptable op amp must work in the voltage range of the circuit. Although I chose a 5V power supply, some robots and devices use 3.3V or less. So, you'll want to make sure the op amp operates in the range you need.

Because I wanted to drive an LED to indicate color detection, it may be useful for the op amp output to have the ability to drive a reasonable amount of current. The LMC6482 can drive between 11 mA and 15 mA @ 5V.

Op Amp Speed

Since this op amp is being used for a color sensor, speed may be critical for your application. A color line-following robot may veer off course if the op amp takes too long to process the photodiode signal. And, the time it takes for a color sensor to detect candy or LEGO brick color may be the weak link that slows down a sorting machine.

The LMC6482 has a sister, the LMC6462, with nearly identical characteristics. The LMC6462 uses less power in exchange for less speed. Power usage versus speed is a common trade off in chips.

Oscilloscope trace of the fall time of a relatively fast ultra-low input current op amp.

Oscilloscope trace of the fall time of a relatively fast ultra-low input current op amp.

The image above illustrates the trailing edge of the output of the LMC6482. Notice that the edge is clean and square. It takes less than 10 µs (microseconds) to drop from 6 V to 0 V.

Oscilloscope trace of the fall time of a slower ultra-low input current op amp.

Oscilloscope trace of the fall time of a slower ultra-low input current op amp.

The image above illustrates the trailing edge of the output of the LMC6462. Notice that the edge is rounded.

It takes the LMC6462 at least 150 µs (microseconds) to drop the same amount as the LMC6482. That’s 15 times as long. That limits the maximum signal frequency that it can process, in comparison to the LMC6482.

Oscilloscope trace showing ringing on the leading edge of the slower ultra-low input current op amp.

Oscilloscope trace showing ringing on the leading edge of the slower ultra-low input current op amp.

A rapidly rising signal overwhelms the slower op amp, resulting in “ringing” on the rising edge of the output. Well, that’s my hypothesis anyway. Switching to the faster op amp eliminated the problem.

Choosing the Chip

The appropriate chip for your robot or electronic project depends on the individual design needs. If the attached circuit has Schmidt trigger inputs (a 74VHC14 for example) or is not speed critical (an LED indicator for a human to see) then the slow response of the LMC6462 is not necessarily a disadvantage. And if the robot’s power usage is a significant factor, such as if the robot is solar powered, perhaps the LMC6462 is a better choice.

However, for a light detector circuit, the speedier LMC6482 seems more appropriate. This may be especially true if the circuit is part of a tachometer or wheel encoder. After all, a single LED alone use an order of magnitude more power than the core of the color sensor circuit, which uses less than 1 mA. So, what real value is the savings from a lower power op amp, especially in relation to the sluggish responsiveness in light detection?

Although the National Semiconductor LMC6482 is specified in the schematic in this article, the Texas Instruments TLC3702 appears to have compatible specifications. If you have a preference for one manufacturer or another, I’m sure you can find a top performer in their current generation op amp family.

For many readers, some elements of the reversed LED color sensor circuit may be unusual territory, such as picofarad currents, megohm resistors, and ultra-low input current op amps. But, there may be a time when it is critical to measure a peak wavelength that only a commercial color LED can detect at a reasonable price. Or, you may need an op amp to amplify and digitize some other sort of signal for you.