HMC661LC4B Datasheet, PDF (8/14 Pages) Hittite Microwave Corporation – ULTRA-WIDEBAND 4 GS/s TRACK-AND-HOLD AMPLIFIER

HMC661LC4B
v00.0312
ULTRA-WIDEBAND 4 GS/s TRACK-AND-HOLD AMPLIFIER
DC - 18 GHz
Application Notes (Continued)
is approximately 7 GHz. Hence, the output amplitude of the sampled waveform may be somewhat larger than the
track-mode response at high input frequencies due to the effect of the output amplifier bandwidth. In particular,
frequencies beyond the output amplifier bandwidth will be attenuated by the transfer function of the output buffer
while the held signal (which looks like DC to the settled output amplifier) will suffer little attenuation.
The broad output buffer bandwidth is maintained to support the fast settling times required for users operating
at high clock rates. However, because of the broad bandwidth, the output amplifier noise contribution to the total
output noise is significant. Users operating at lower clock rates (such as < 1 GHz) may optimize their signal-to-
noise ratio by filtering the output to a lower bandwidth than the output amplifier bandwidth of 7 GHz. Such an
output filter will not reduce the sampled front-end noise (which is frozen into the signal samples and represents
the majority of the T/H noise because of the wide front-end bandwidth) but it can reduce the output amplifier
noise contribution. The user can filter the output to the lowest bandwidth that still retains the maximum settling
time required to support the chosen clock rate. Typically this optimal bandwidth is of the order of 2 to 3 times the
clock rate and it can be realized with a simple single pole RC filter if desired (for example a shunt capacitance on
the outputs). For example, a user operating at a clock rate of 350 MHz with a 1 GHz noise bandwidth output filter
can achieve approximately 1 dB lower noise relative to the unfiltered output condition.
The output will have very sharp transitions at the clock edges due to the broad output amplifier bandwidth.
The user should be aware that any significant length of cable between the chip output and the load will cause
frequency response roll-off and dispersion that can produce low amplitude tails with relatively long time-constants
in the settling of the output waveform into the load. This effect is most noticeable when operating in a lab setting
with output cables of a few feet length, even with high quality cable. Output cables between the T/H and the load
should be of very high quality and 2 ft or less in length.
Reflections between the load and the device will also degrade the hold mode response. The output cable length
can be adjusted to minimize the reflection perturbations to some extent. In general, the round trip transit time of
the cable should be an integer number of clock periods to obtain the minimal reflection perturbation in the hold
mode portion of the waveform. The optimal performance is obtained when the T/H is within 50 ps or less of the
load since this gives a reflection duration equal to the approximate settling time of the device. In A/D converter
applications the T/H should be placed as close as possible to the A/D converter to minimize reflection effects on
the path between the T/H output and the input of the A/D converter.
Linearity Measurement
When characterizing the linearity of a T/H, the transfer function linearity of the held samples (referred to as T/H-mode
linearity) is usually the quantity of most interest to the user. These samples contain the signal information that is
ultimately digitized by the downstream A/D converter. Since the T/H-mode linearity is often different than the track-
mode linearity, this presents a unique measurement issue for the signal rank T/H in that the linearity of only the hold-
portion of the analog output waveform must be selectively measured.
This issue is aggravated for high speed T/Hs because there are few wide-band time domain instruments (oscilloscopes
or A/D converters) with sufficient linearity to characterize a high linearity T/H operating at high clock rates. Therefore
a frequency domain instrument (spectrum analyzer) and measurement technique are used which allow selective
characterization of the hold-mode portion of the waveform
A common approach to this requirement has been to cascade two T/Hs in a dual rank configuration such that the
second T/H (T/H 2) re-samples the output of the first T/H (T/H 1). The two T/Hs are usually clocked 180 degrees out-
of-phase in master-slave operation to eliminate the track-mode portion of the output waveform from the first T/H. This
arrangement produces an output waveform that consists of two time segments. The first segment is the T/H 1 hold-
mode output as observed through the T/H 2 track-mode transfer function. The second time segment is the T/H 1 hold-
mode output re-sampled and held by the T/H 2 device. This measurement approach is not a perfect representation
of the linearity of a single T/H due to the impact of the second T/H on the overall linearity. However, it does eliminate
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