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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. A linearity measurement issue unique to the T/H device is the need for output

waveformfrequencyresponsecorrection.InthecaseofadualrankT/H,theoutputwaveformresemblesasquarewavewith

duration equal to the clock period. Mathematically, the output can be viewed as the convolution of an ideal delta-function

sampletrainwithasinglesquarepulseofdurationequaltooneclockperiod.Thisweightstheoutputspectralcontentwitha

SIN(πf/fs )/(πf/fs) (Sinc) function frequency response envelope which has nulls at harmonics of the clock frequency fs

and substantial response reduction beyond half the clock frequency. This spectral content and envelope function are

observed during spectrum analyzer measurement because the analyzer simply reproduces the entire spectrum of the

incoming waveform. However, the spectral content of the held samples without the envelope weighting is required for

proper measurement of the sample’s linearity, as would be measured by a downstream A/D converter that samples

a time instant in the held waveform. Either the impact of the response envelope must be corrected in the data or a

measurement method must be used that heterodynes the relevant nonlinear harmonic products to low frequencies

to avoid significant envelope response weighting. This latter method is referred to as the low frequency beat-product

technique.

tune the output common-mode level to precisely 0 V if desired. Additionally, the common-mode output level may

approximate relation Vocm=(VccOB-2)/2 where Vocm is the output common mode voltage and VccOB can be

varied in the range of +1 V < VccOB < +3 V.

The bandwidth of the output amplifier that buffers the T/H signal between the hold-node and the 50 ohm outputs is

approximately 7 GHz. 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.

HMC1061LC5

v01.0613

DC - 18 GHz, ULTRA-WIDEBAND, DUAL RANK

4 GS/s TRACK-AND-HOLD AMPLIFIER