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acquisitionZONE Products for the week of March 23, 2009
Linear Technology Says…
LTC6416: Differential ADC Buffer Drives 300MHz Signals with Only 1.8nV/√Hz Noise
Linear Technology introduced the LTC6416, a wideband unity gain 2GHz differential buffer aimed at addressing the challenge of driving high resolution ADCs. The LTC6416 provides excellent noise figure and distortion performance at high frequencies. With its programmable output voltage clamps, the LTC6416 limits the maximum voltage levels applied to the ADC inputs, ensuring that there is no concern with ADC overdrive recovery.
At a low 1.8nV/√Hz output referred noise at high frequencies, the LTC6416 does not degrade the incoming signal for higher resolution ADCs. This buffer can be used with a transformer at its input to achieve additional low noise system gain in high bandwidth applications. The buffer’s inputs and outputs can be AC- or DC-coupled. The LTC6416 output common mode voltage is set by the VCM pin to match the ADC’s input range. The LTC6416 is capable of maintaining exceptional performance when driving high-speed ADCs past 300 MHz, where it maintains -72.5dBc of third-order intermodulation distortion and -74dBc/-67.5dBc second- and third-harmonic distortion, respectively.
EN-Genius Says…
As standard ADC ICs have attained higher and higher frequency performance at increasingly higher resolutions the successful implementation of designs using such parts has come down to how to drive them. There have been buffer solutions offered but I have argued with the vendors that in many cases they have not truly allowed for full implementation of 16-bit high-frequency ADCs. In at least two cases vendors of supposedly suitable parts have not even put noise density numbers in their parametric tables!
Here Linear is offering what looks like a well-balanced solution, albeit – but not at all unexpectedly – at the cost of current.
The LTC6416 is differential throughout. It offers a 2 GHz -3 dB bandwidth; 1.4 GHz -0.5 dB bandwidth; 300 MHz -0.1 dB bandwidth. Differential gain is nominally unity.
The data sheet specs the part at both 3.3 V and 3.6 V, single rail, and at both the input noise voltage density (at 100 kHz) is 1.8 nV/rtHz, while the input noise current density is 6.5 pA/rtHz. Those, to me, make the part suitable for 16-bit ADC driving – provided enough dynamic range of the ADC input is spanned. Fortunately the inputs can be dc coupled and have a 12 kΩ differential input impedance (with 1 pF) so that input matching can be obtained with transformers and shunt resistors to ground. If transformer with turns ratios such as 1:4 or 1:8 are used then there is free input signal gain available.
With dc coupling the common-mode output offset voltage is about -40 mV. If ac coupling is used then the Vcm pin sets the common-mode output ofset voltage. That voltage can be obtained, to match, from the following ADC.
It is advantageous if the output matching from the LTC6416 is also transformer coupled and any necessary filtering can be easily added using only a couple of components because of the 9 Ω resistor that is internally provided in each output leg. The output can also be ac-coupled. The output fast-recovery clamping that is used can be set on the two pins that are provided, if the default conditions do not suit.
The LT6416 can be operated on supplies from 2.7 V to 3.9 V and the quiescent is a typical 42 mA. The output voltage low can be within about 200 mV of ground while the output voltage high is a bit over 1 V from the rail (both measured single-ended with ±2.3 V on the input).
Apart from bandwidth, the other ac numbers are extremely good but the designer should evaluate the exact conditions that the part would be used in and note the results from either the data sheet or from a prototype.
This is an extremely well balanced part in its specifications and is the first differential buffer that I have seen that is truly suitable for driving high-speed 16-bit ADCs. It has been priced at a level to deter you from even thinking about putting together a discrete solution. Financially, for Linear, there will be a knock-on effect from the part as its availability will drive further sales from the LTC2208 (16-bit, up to 150 Msample/s) and LTC2209 (up to 185 Msample/s) ADCs, and other LTC22** family members, which is where real money is to be made – or lost if you cannot provide a complete solution. Where will the applications be? The usual high-frequency RF/IF applications plus some lab applications with CCDs.
The LTC6416 is in production in DFN-10 and versions are available for both commercial and industrial temperature ranges. Pricing starts at $3.50 in 1000-piece lots.
Data Sheet
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