HMC1061LC5TR-R5 datasheet(14/18 Pages) AD

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HMC1061LC5

Data Sheet

Rev. B | Page 14 of 18

Additionally, the common-mode output level may be adjusted

within the range of approximately ±0.5 V by adjusting the

VCCOB power supply according to the 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 track-

and-hold amplifier signal between the hold node and the 50 Ω

outputs is approximately 7 GHz. The broad output buffer band-

width 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

(SNR) by filtering the output to a lower bandwidth than the output

amplifier bandwidth of 7 GHz. Such an output filter does not

reduce the sampled front-end noise (which is frozen into the

signal samples and represents the majority of the track-and-hold

amplifier 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

resistor circuit (RC) filter if desired (for example, a shunt capaci-

tance on the outputs). A user operating at a clock rate of 350 MHz

with a 1 GHz noise bandwidth output filter can achieve approxi-

mately 1 dB lower noise relative to the unfiltered output condition.

The output has very sharp transitions at the clock edges due to

the broad output amplifier bandwidth. The user must be aware

that any significant length of cable between the chip output and

the load causes 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 track-and-hold amplifier and the load must

be of very high quality and 2 feet or less in length.

Reflections between the load and the part degrades 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 must be an integer number

of clock periods to obtain the minimal reflection perturbation

in the hold mode portion of the waveform. The optimal perfor-

mance is obtained when the track-and-hold amplifier 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 ADC

applications, the track-and-hold amplifier must be placed as

close as possible to the ADC to minimize reflection effects on

the path between the track-and-hold output and the input of

the ADC.

LINEARITY MEASUREMENT

When characterizing the linearity of a track-and-hold amplifier,

the transfer function linearity of the held samples (referred to as

track-and-hold 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 ADC. A linearity

measurement issue unique to the track-and-hold device is the

need for output waveform frequency response correction. In the

case of a dual rank track-and-hold amplifier, the output waveform

resembles a square wave with duration equal to the clock period.

Mathematically, the output can be viewed as the convolution of

an ideal delta function sample train with a single square pulse of

duration equal to one clock period. This weights the output

spectral content with a 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 linearity, as measured by a downstream

ADC 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.

The low frequency beat product technique is commonly used

for high speed track-and-hold amplifier linearity measurements,

although the measurement does impose restrictions on the

specific input signal and clock frequencies that can be used. For

example, with a clock frequency of 512.5 MHz, a single-tone

input at 995 MHz beats with the second harmonic of the

sampling frequency (through the sampling process) to produce

a first-order beat product at 30 MHz. Likewise, the second and

third harmonics of the input signal (generated via distortion in

the track-and-hold amplifier) beat with the fourth and sixth

harmonics of the sampling frequency, respectively, to produce

second and third-order beat products at 60 MHz and 90 MHz.

In this manner, the track-and-hold nonlinearity in the vicinity

of 1 GHz can be measured even though the 995 MHz fundamental

and the 1.99 GHz and 2.985 GHz nonlinear harmonics are well

beyond the 256 MHz 4 dB bandwidth of the sinx/x response

envelope.

The possible input frequency choices are overly limited when

the low frequency beat product technique is used at high clock

rates. A related high frequency beat product measurement using

correction for the sinx/x envelope weighting must be employed

to measure linearity over a wide range of input frequencies.

Analog Devices, Inc., uses both low frequency and high

frequency beat product methods to measure linearity for a wide

range of clock and signal frequencies. High frequency beat

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