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Data Acquisition Differential diplexer resolves issues for driving direct sampling ADCs By DDerek Redmayne irect sampling pipeline analog-to-digital converters (ADCs) produce a large collection of mixing products, much like passive mixers. In high speed ADCs, these are sum and difference products that extend out to GHz frequencies, and are the result of transients, glitches, that result from the abrupt connection of the sample capacitors when the ADC resumes tracking. In addition, disturbances conducted into the input network immediately prior to sampling must not be returned during the sampling process. The higher the speed and the greater the resolving power of these ADCs, the more apparent are the effects of poor control of these commutation products, henceforth referred to as “these products.” Simple sum and difference products would fold back onto the frequencies at which these appear in baseband, and with the exception of some gain error, would disappear. However, there are unavoidable, nonlinear products that accompany these simple mixing products, that unless dissipated or absorbed, will result in distortion. There is a time element involved, so distance and settling have a role to play. RF engineers faced with the task of controlling similar products in the case of a passive mixer will recognize the need for an absorptive network on both the RF path and the IF path, if not the LO path. They may not immediately recognize that they are dealing with these same issues in an ADC. Once aware of this similarity, an experienced RF engineer is better prepared to deal with the issues, but the degree to which they now must be absorbed can still be a surprise. The notion of settling is often understood by analog engineers and by signal integrity engineers. But it may not be common knowledge that a network exhibiting complete absorption over the broad range of frequencies necessarily means it will exhibit good settling. The LTC2107, a 210Msps 16-bit ADC, with 80dB SNR, is a case in point where the VSWR of the analog input path must be very good out to GHz frequencies - the frequency range where these mixing products eventually roll off significantly. This point is often not recognized as extending beyond 5GHz in modern high speed ADCs. These products, images of the input signal mirrored around every harmonic of the clock, do not even start to roll off until after 2GHz. The high SNR of the LTC2107 is a double-edged sword in that the larger sample capacitors increase the power in these mixing products, and enhance the ability to resolve the effects of reflections returned from impedance discontinuities. Absorptive filters have proven to produce good results with these ADCs, but are daunting in terms of both complexity of layout and cost. The component count of an absorptive bandpass filter is at least 2x to 3x that of a reflective bandpass of the same order. Even greater complexity may result from measures needed to control the effects of self-resonant frequency (SRF) in final inductors. Lower frequency applications can be addressed by using a high speed feedback amplifier such as Linear Technology’s LTC6409 differential amplifier/ADC driver, source terminated, but with a lumped element differential diplexer to assume the task of absorption at high frequency. As such, this is an absorptive source for the ADC. Simple series source termination fails to produce good return loss (S22) above a certain frequency as the amplifier output impedance rises. At GHz frequencies, the return loss of fully absorptive LC filters, bandpass, or low-pass filters can be a challenge due to limitations imposed by the self-resonant frequencies of inductors, the inductance of capacitors, the effects of short transmission line segments between elements, and the more obvious parasitic pad capacitance under the elements. A Low Temperature Co-fired Ceramic (LTCC) differential diplexer, developed for Linear Technology by Soshin Electric Japan, solves many of these dilemmas by presenting a simple compact way of answering the shortcomings of various input networks. Linear’s LTC6430 high linearity differential RF/IF amplifier/ ADC driver is a case in point. It is a wideband differential gain block, matched to 50 ohms (100 ohms differential). However, above some 500MHz, its return loss is not good enough for the needs of the LTC2107. At 1GHz, the output return loss is about 10dB, whereas the ADC needs better than 20dB return loss to produce full spurious free dynamic range (SFDR). A 10dB return loss at 1GHz and above can produce more than 20dB loss of SFDR. The HMD6117J-T from Soshin handles the mixing products above 400MHz-500MHz, and presents to the ADC an absorptive character extending to 3GHz and beyond. In the process, it diverts the higher frequency mixing products into a pair of termination resistors placed on the high-band ports. In doing so, it also reduces the high frequency disturbances seen by the amplifier. Used in conjunction with the LTC6430, as shown in Figure 1, this combination produces a compact inexpensive driver solution, suitable for frequencies extending from some 20MHz to 400MHz. Anti-aliasing should be done prior to the amplifier for frequency domain applications. In conjunction with the LTC6409, as shown in Figure 2, this produces a complete solution for DC to 100MHz applications. For time domain applications in which spectral power distribution is essentially flat, with no risk of strong interferers above some 100MHz, this topology may be used to 200MHz, or beyond if spectral power distribution rolls off with frequency. The diplexer can be used in a similar manner with the LTC6417, a unity gain differential buffer, and possibly with other feedback amplifiers. For applications requiring as much SNR as possible, the HMD6117J-T can also be used after an absorptive bandpass filter to improve return loss at high frequency, simplifying the Derek Redmayne is Staff Scientist for the Mixed Signal Products at Linear Technology Corp. – www.linear.com 40 Electronic Engineering Times Europe December 2015 www.electronics-eetimes.com


EETE DEC 2015
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