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Smaller analog ICs are great but do they really mean smaller PCB circuits

By Bonnie C. Baker,
Senior Applications Engineer, Data Acquisition Products Division,
Burr-Brown Corporation

Here is a familiar scene: An analog IC manufacturer's strategic marketeer visits his customer. His hope is to zero in on the right specifications for a device that many of his customers will use. The challenge for the strategic marketeer is to truly hear and understand the customer's requirements.

I once spent an afternoon with a magazine editor who continually asked, "Why do Analog Houses miss the boat more often than Digital Houses with product introductions?" I don't think that there is a quick easy answer to the question. At the time, I was tempted to blurt out, "Analog is more complex and subtle than digital." That's one way of saying I'm not sure.

So here is the challenge for the analog IC manufacturer. What does it take to make a single-supply, low-power op amp or ADC that is truly small? Remember the quad-amplifier? This product was a spin-off from the dual op-amp that was a spin off from the single op amp. The idea of putting multiple devices in one package is good. So good that one would think the quad op amp would be the most popular of the three. Not so. The most-voiced criticism of the quad-amplifier is not its size or its specifications, but the awkwardness of the layout when the external resistors and capacitors are put in the circuit.

It's Easy, Just Define What Small Is And You're Done

Analog IC manufacturers are continually introducing the "smallest" on the market in efforts to meet their customersı requests to reduce their board real-estate. The idea of smallest may have a different meaning for the analog IC manufacturer as opposed to the analog systems designer. It is analogous to the story about the guy who wanted to build a boat. He decided he would build it in the basement so the project would be near his workbench. And he did. And the boat was wonderful and beautifully constructed. Then it was finally time to go to the lake. But how was he going to get a 9-ft. wide boat through a 3-ft. wide basement door?!

Operational amplifiers and ADCs are appearing on the market in smaller packages than ever before. A 12-bit ADC is now available in an 8-lead MSOP (0.118 x 0.118 in.) The single op amp is available in a 5-lead SOT-23 and the dual in an 8-lead SOT-23 (0.116 x 0.064 in.) For these chip functions, the package sizes are the smallest on the market to date. But what really makes the ADC or the operational amplifier small?

How Small Can An ADC Really Get

Looking beyond ADC specifications, every converter needs a voltage reference. If it is internal to the ADC that is a plus. This removes the requirements for another chip, i.e. a voltage reference. Nice idea, but take a look at the product's application circuits. Usually the internal reference of a converter requires an external capacitor. A 10 mF capacitor is just about equivalent to the size of the SOT-23 package (Panasonic, tantalum electrolytic low-voltage chip capacitor.) External capacitors can quickly consume quite a bit of real-estate.

Alternatively, if the voltage reference is not internal to the ADC, another chip may be required. An external reference in a small package is easy to find, but the addition of this reference will easily double the amount of board space required for your ADC solution and once again the size of the total application solution is enlarged. The best scenario would be if the ADC used the power supply voltage for its reference. No additional capacitors or reference chips are then required.

Now the ADC and the system solution is SMALL.

With Amplifiers, Don't Lose Track Of The External Details

Single-supply, low-power operational amplifiers can be found in almost every analog IC manufacturer's product portfolio. This makes it particularly difficult to find the right amplifier for the application. On the other hand, most amplifiers will faithfully give the user the advertised small size for the application -- as long as the amplifier is not being used in circuits where capacitors are required, like a filter circuit. In filter applications, the specification that separates the small amplifiers from the SMALLEST amplifiers is the input impedance.

A high input impedance -- which implies a low-input bias current -- allows the designer to use high-value resistors. This doesn't sound like an advantage until the capacitors are added to the circuit. Since the filter frequency is reliant on the multiple of resistor/capacitor pairs, higher-value resistors mean lower-value capacitors for the same frequency response. Consequently, through-hole capacitors are not necessary. Chip capacitors will easily fit the application. Now the single-supply, active-filter application is really small thanks to the low-input bias current of the op amp.

Problem Solved ?!

There is always a way to solve a problem, as long as the problem is well defined. In the digital world, the name of the game for the IC manufacturer is higher integration and for the systems designer programming creativity. In contrast, analog designs are still entrenched in the optimization of specialized building-blocks; like op amps and ADCs. The idea of moving the circuit conveniently close to your bench may not be the best solution.

When small is the goal in the analog circuit, the size of the primary chip is only part of the big picture. The best of both worlds is a chip that minimizes the need or size of the external support circuitry. It is refreshing to know that today's smallest ADCs and the smallest operational amplifiers measure up to these tough requirements (or expectations).

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