wirelessZONE Products for the week of May 28, 2007

Linear Technology Says . . .
LTC6400/01-20: 1 GHz & 2 GHz Fully-Differential Amplifiers Enable High-Speed ADC Performance

Linear Technology Corporation announced the LTC6400-20 and LTC6401-20, ADC drivers that achieve unprecedented performance on a single 3V supply. These fully differential amplifiers include gain setting resistors, simplifying the challenge of driving the highest performance high-speed ADCs. The LTC6400-20 provides a fixed gain of 20dB with -93dBc third-order intermodulation distortion (IMD3) performance and noise figure of 6.5dB for a 140MHz input frequency. At 240MHz, IMD3 is better than -70dBc. Other family members with fixed gains ranging from 8dB to 26dB will follow.

The LTC6400-20 achieves this outstanding performance operating on a 3V supply voltage, providing a differential output voltage swing of 4.4VP-P. In addition to minimizing power consumption, this eliminates the need for a separate 5V supply in applications using the latest 3V and 3.3V ADCs. To further simplify interfacing to high speed ADCs such as the LTC2208, a VOCM pin sets the output common mode voltage of the LTC6400-20 to match the input range of the ADC. An optional on-chip filter helps to handle charge injection common to the capacitive inputs characteristic of pipelined ADCs. Inputs may be AC- or DC-coupled. While fully differential, the LTC6400-20 can be used to easily convert single-ended inputs to differential outputs.

The LTC6401-20 is a lower-power part optimized for lower input frequencies. Consuming just 50mA, half that of the LTC6400-20, the LTC6401-20 maintains the same low noise performance and -90dBc distortion performance for input frequencies up to 70MHz.

"The LTC6400-20 is the first of a family of parts that achieves excellent noise and distortion performance from DC to hundreds of megahertz," says Kris Lokere, design manager. "In addition to driving ADCs in communications and instrumentation applications, it will find many applications as a high performance differential driver and receiver."

EN-Genius Says . . .

There's no point in having a high-speed ADC, with terrific specifications, if you cannot drive it properly. With the fight on between three vendors -- with one more rapidly coming to speed in the wings -- the long term winner of the very high value communications ADC markets is going to be the one who is able to offer a solution that requires little to no analog/RF design skills from the system designer.

Enter the LTC6400.

This driver -- an op amp, really -- doesn't have the lowest noise in the business, but it comes darn close with 1 nV/rtHz for internal noise and 2.2 nV/rtHz typical total input referred voltage noise density (at 140 MHz) with a 1:4 input balun. Noise figure is 6.2 dB. At 240 MHz the numbers are 1.9 nV/rtHz and 7.1 dB, and at the perennially popular 70 MHz they are 2.1 nV/rtHz and 6.2 dB.

Distortion numbers worsen with input frequency, of course, but at 70 MHz, with a 400 ? load, second-order harmonic distortion is -86 dBc and third is -85 dBc, while IMD3 (with a 1 MHz carrier separation) is -93 dBc, and OIP3 is 51 dBm. All extremely satisfactory numbers and they would be difficult to get discretely.

-3 dB bandwidth is a typical 1.84 GHz (with the 1:4 input balun) while 0.1 dB flatness is 300 MHz. Not surprisingly, slew rate is high at 4500 V/µs. Although the input and output of the parts are differential they can also use a single-ended input, of course, and produce a differential output -- but single-ended driven parts have poorer characteristics. The filter outputs have 50 Ω series resistors and a shunt 1.7 pF capacitor between them, giving a -3 dB bandwidth of 590 MHz. Input impedance magnitude is 200 Ω up to about 100 MHz (with phase being relatively flat at about 120°) and that needs to be taken into account when driving the input -- particularly from a 50 Ω RF source. The differential output impedance is 25 Ω.

At 70 MHz S11 is about -30 dB, S22 is -35 dB and S21 (reverse isolation) is about -67 dB. Typical quiescent with the 3 V rail is 90 mA while the lower bandwidth LTC6401 (for which I have not yet seen the data sheet) is quoted with a 50 mA quiescent. The output common-mode voltage can be set by the following ADC to between 1.0 V to 1.6 V and the differential output voltage can be as high as 4.4 V p-p.

I'm not usually in favor of fixed-gain amplifier offerings because of their limitations of use over a variety of applications; but when it comes to offering specifications that you can really achieve there is a lot to be said in letting the vendor solve the problems rather than trying to do it with external components. Particularly in the noise arena you want to ensure that how you control gain is not at the cost of noise. Designers know how to get the circuit gain they want in terms of resistor ratios and feedback equations, but ask them if they remember that thermal noise is a function of the square-root of the resistance value and most will go glassy eyed on you. Linear is attacking the fixed-gain "problem" by offering a range of other gains in the family at later dates. (The range will be 8 dB, 14 dB, 20 dB and 26 dB -- all single-ended input gains.)

I have already expressed some concern to Linear, when I was first briefed on these parts, that the two sets of outputs -- the direct and filtered -- are going to cause their applications engineers some headaches when users decide they would like to use both. The immediate thought is that this is a really good place in the signal chain to put an RSSI…and the vast majority of design solutions will not use the filtered outputs, so they are creating a problem for themselves. Nonetheless, this family will be hugely successful and they are great tools for the FAEs for design in, with the high-speed ADCs out of the same stable; Linear, however, needs to realize that they have to get themselves into the high-speed DAC business as well if they really want to dominate this market. The lack of such products is used as a sales tool by other vendors looking to take this business.

These parts will find the majority of their sockets in the communications field but they will also be successful in general broadband applications and there are probably a number of wideband video applications and SAW filter interfacing in low-frequency radar.

The LTC6400-20 and LTC6401-20 are in production in QFN-16 with the former priced at $3.68 and the latter at $2.95, both in 1000-piece lots.

Data Sheet