Thinking inside the box – the future of converter designs?

Norwood, MA — Analog Devices, a global provider in high-performance semiconductors for signal processing applications and data converters, introduced the first members of its new nanoDAC family of digital-to-analog converters which provide breakthrough performance in smaller packages. Because of their compact size and low power consumption, these new digital-to-analog converters (DACs) provide significant advantages in space-constrained applications where power consumption is critical. Developed using a combination of innovative design processes and packaging techniques, the converters are ideal for voltage level setting applications used in a wide range of communications, consumer and handheld battery-powered designs including digital cameras, projectors and PDAs.

“With manufacturers designing their products in ever decreasing form factors, especially in the communications field, there has been a continual push for cost-effective components that deliver increased performance in smaller packages,” said Mike Britchfield, product line director for precision converters, Analog Devices. “The devices in the nanoDAC family address these design challenges by consuming considerably less power and space than comparable solutions.”

World's smallest digital-to-analog converter
Leading the nanoDAC family's product offerings is the world's smallest, low-power D/A converter. Available in a compact SC-70 package, the AD5641 consumes 70 percent less board area and 80 percent less power than comparable devices. Delivering 14-bit resolution and guaranteed monotonic behavior with a maximum power consumption of 100 uA at 5 volts, the AD5641 combines fine output control with low power in a compact footprint. The combination of small package and low power consumption make the AD5641 ideal for voltage level setting requirements such as generating bias or control voltages in space-constrained low power wireless applications, including cell phone volume controls and screen lighting.

In addition to the AD5641, Analog Devices is introducing the AD5621 (12 bits), AD5611 (10 bits), and AD5601 (8 bits). These lower resolution devices provide suitable alternatives for applications not requiring the performance of the AD5641.

Leading-edge packaging technology
The AD5641 is designed using a combination of Analog Devices' patented “segmented string” architecture, which offers unmatched accuracy in the smallest die area, and patented advanced chip-on-lead (COL) die packaging technology. The string architecture is segmented in substrings to provide significant die size savings, while the COL assembly technology maximizes package cavity size and therefore minimizes the customer's board footprint. Both innovations enable these products to provide customers with the best value on the market today, in terms of price and performance.

The AD5641 (14 bits), AD5621 (12 bits), AD5611 (10 bits) and AD5601 (8 bits) are sampling now and will be available in production quantities in December 2004. Each part is available in 6-lead SC-70 packaging with 1K prices ranging from $0.95 to $4.75 depending on the resolution and accuracy combination.

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Low power single-supply is what the nanoDAC family addresses. It's for space- and power-constrained products. The applications include level setting — setting the voltage level for control or monitoring functions.

These converters could be used for something other than the main signal chain. For example, basestations would have a primary signal chain with high-speed A/D and D/A converters and you may need auxiliary converters that are used to control devices like the power amp or gain control in systems. As long as the required voltages are achieved the customers don't want to sacrifice any power or board space to achieve that.

The nanoDAC is small, real small. It's offered in a SC70 package, and it has low power consumption offering 100 nA at 3V. What are the trade offs when you go this low in size or power? The two key trade offs are increasing the resolution by minimizing the package size versus reducing power while maintaining the settling time. By reducing the power the settling time of the amp power increases. Another and maybe more important, is trying to increase the number of bits of resolution versus reducing the package size. The number of bits of resolution is determined by the number resistors in the string (this uses a string architecture). For example, for this 14-bit D/A converter you need 214 (16,384) resistors in that string. It's not possible to fit that into an SC70-sized package. So, ADI got creative and segmented the string into two substrings and that allowed them to significantly reduce the number of resistors. Once you reduce the number of resistors, it simplifies the matching of the resistors, and that improves the integral non linearity (INL).

ADI is targeting the nanoDAC for higher performance in a smaller package to meet the significant requirements in the market. This DAC is targeted for space-constrained, low power applications, such as a level-set or control application. For base stations it would be setting the level for variable gain amp, controlling the voltage level in an ATE system.

The name nanoDAC – you might think that it came from power consumption of less than 100 nA at 3V but you would only be partially right. The company was really looking for a name that people would associate with small size and power. So combining small package with increased performance helps get that idea across. Nano is the next generation smaller size that follows micro. The 5641 is industry's smallest D/A converter and is in an SC70 package that uses 2mm x 2.1mm board area and that's 48% smaller than a small outline transistor (SOT) 23 package and 70% smaller than the micro small outline package (MSOP).

ADI is targeting specific applications to ease the tradeoff significance. The tradeoffs are not as significant if they are used in applications such as level setting because it doesn't need to change the output of the D/A converter at a high speed. So, it's not a high-speed device that needs to continuously update. Therefore, settling time isn't critical and can be traded off.

What's really required for the targeted application is the resolution and real estate. “It also needs to be monotonic so that if the user turns the dial the changes will be singular,” said James Caffrey, product marketing manager of the Precision Converter Group at ADI. “You would never have a situation where the user changes a code up that the converter would first go down and then up. That's the benefit of a string D/A converter because it steps through the resistors and voltages and the steps are guaranteed to be monotonic. The requirements for the specific applications that ADI focused on are resolution and monotonicity.”

So, how did ADI manage to get more in a smaller package? Essentially, the company accomplished it by thinking inside the box, or in this case, inside the SC70 package. First, the company designed a creative resistor circuit and then redesigned the interior of the chip to use more of the available space.

The traditional string DAC requires 214 resistors which is impossible to get in a small package. So ADI segmented the strings. The first pass is a course pass at setting the level, and the second string does a fine pass of the initial level. So by using two strings it requires a much smaller size and fewer resistors.

nanoDAC resistor string

For example, the user requests an output from the D/A converter, the initial string will choose the resistor string that will give the closest value to what's required and the second string further subdivides the voltage to come closer to the value that's required.

The second technique used is called chip-on-lead packaging. It is an innovation from ADI. Traditionally, the values placed on the paddles, which are the floating paddles that sit in the cavity of the chip package, and the leads, are on either side. The bond wires connect from the chip to the leads. ADI developed a packaging technology where the chip fits directly on the leads, removing the paddles. That allows a bigger chip in the device.

Efficient use of space with COL packaging

The paddle is used as an isolation platform that the IC sits on so it doesn't contact anything in the package. The bond wires are attached to the chip and to the lead. ADI used a non-conductive epoxy to attach the IC to the leads and then used the bond wires to connect to the leads. This allowed the company to make better use of the space within the package and have a bigger die within a similar package dimension. The bigger die allows more resistors in the same cavity space and thus, improve performance of the part.

I liked ADI's ability to get creative by thinking inside the box and coming up with an innovative design. I hope we will see more use of this technique and maybe even some improvements with future iterations. Impressive, no?

AD5641 14-bit DAC data sheet

AD5621 12-bit DAC data sheet

AD5611 10-bit DAC data sheet

AD5601 8-bit DAC data sheet

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