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wirelessZONE Products for the week of May 12, 2008
Linear Technology Says…
LTC6421-20: 1.8 GHz and 1.3GHz, Low Distortion, Dual ADC Drivers Deliver Guaranteed Matching & Low Noise
Linear Technology announced the LTC6420-20 and LTC6421-20, the industry’s first high-speed dual differential ADC drivers with guaranteed gain matching, and exceptional noise and distortion performance. With guaranteed matched gain of ±0.25dB and matched phase of ±0.1° (typ), the LTC6420-20 reduces errors in multichannel systems such as IQ demodulation or diversity receivers. Channel separation is 80dB at 100MHz.
The LTC6420-20 provides a fixed gain of 20dB with -84dBc third-order intermodulation distortion at 100MHz input frequency. Input voltage noise is just 2.2nV/√Hz, including internal gain-setting resistors. These amplifiers achieve their excellent distortion and noise specs on a single 3V supply and have rail-to-rail output swing. This allows the dual ADCs and amplifiers to share the same voltage supply in many applications. These drivers can drive an ADC directly without any external output impedance matching and can convert single-ended inputs into differential outputs.
“The LTC6420-20 and LTC6421-20 are the dual versions of our award-winning LTC6400/LTC6401,” says Brian Black, Product Marketing Manager. “These new products extend the benefits of outstanding noise and distortion performance, low voltage operation, and excellent stability to applications which require close matching between ADC channels.”
EN-Genius Says…
As the release says, the LTC6420-20 and the LTC6421-20 are the dual versions of the fixed 20 dB gain LTC6400 and LTC6401. Those two parts were not only reviewed by EN-Genius; they were also a winner of one of our Product of the Year Awards for 2007.
We all get very smug here at EN-Genius when we call a winner, as these parts have become.
There is little need to repeat our previous review here and we need only comment about the decision to go for the dual version. First, however, I would like to commend Linear for dumping the second outputs on the original parts, the ones that were filtered. I hazarded in the original review, and to Linear earlier, that they could be a problem.
The decision to go to the dual parts is extremely intelligent. As bandwidths increase in some of the strange modulation schemes that have been pushed by digital engineers, all manner of weird things can happen to signal integrity. Many RF engineers have never seen some of these spurious events (although radar engineers often have). I have worked with systems with bandwidths exceeding 100 MHz and, believe me, they produce multiple challenges.
The gain matching between channels at ±0.1 dB typical (with a spread of 70% better than ±0.05% at 100 MHz) is important, as is the input impedance matching of a typical ±0.1% between channels. Even more important, however, is the matching of delay through the channels. Linear is expressing it in terms of phase, characterized at 100 MHz, as ±0.2º typical. I would prefer to see this expressed in differences in group delay against frequency, and although that characterization is plotted in the data sheet it needs to be shown at a much finer granulation, probably in picoseconds. That may not be a question that is asked this year, but it will come up.
Channel separation is a typical 80 dB at 100 MHz falling to 60 dB at 1 GHz. The upper end is a potential problem for some systems; for most applications the numbers are good.
With the removal of the filtered outputs Linear has squeezed the new parts into a QFN with only four more pins. The quiescent is a typical 40 mA with a shutdown current of a typical 1 mA.
Will these parts be a hit? You bet. With these dual drivers and Linear ADCs following a large number of headaches have been removed, at source, for the designer. It makes it all about as simple as it can get at frequencies that some designers never expected to be working at. These will be great business for Linear. The single channel parts were listed for $3.68 and $2.95 in 1000-piece lots. The duals are priced at less than 50% more – so they’re a bargain too!
The LTC4620-20 and the LTC4621-20 are in production in thermally-enhanced QFN-20 and are priced at $5.22 and $4.82, respectively, both in 1000-piece lots.
Data Sheet
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