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20 Alpha Road, Chelmsford, MA 01824 Phone: 978-250-3343 Fax: 978-250-3373

Order On-line at www.hittite.com

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ESD: On-chip ESD protection networks are incorporated on the terminals, but the RF/microwave compatible interfaces

provide minimal protection and ESD precautions should be used.

Power Supply Sequencing: The recommended power supply startup sequence is Vcc2, Vcc1, VccSH, VccCLK,

(CLKDCP, CLKDCN) if biased from independent supplies. Vcc1, VccSH, VccCLK can be connected to one +5V supply

if desired.

Input Signal Drive: For best results, the inputs should be driven differentially. The input circuit has an on-chip

resistive bias-T for separating the DC and RF components of the input. The low frequency corner of the RF path is

approximately 16 MHz (Ccoupling=10 pF, Rdc=1.28 K ohm). The input can be driven single-ended but the linearity of

the device will be degraded somewhat.

Clock Input: The clock inputs should be driven differentially if possible. The device is in track-mode when (CLKP

– CLKN) is high and it is in hold-mode when (CLKP-CLKN) is low.

The T/H-mode 2nd order linearity of the device varies somewhat with clock slew rate as shown in the performance

data plots. Because of the slew rate dependence, the 2nd order linearity will vary somewhat with clock power for

sinusoidal signals. For optimal linearity, a clock zero-crossing slew rate of roughly 4 V/ns (per clock input) or more is

recommended. For sinusoidal clock inputs, this corresponds to a sinusoidal clock power per differential half-circuit

input of -3.5 dBm at 3 GHz, 0 dBm at 2 GHz, and 6 dBm at 1 GHz.

At clock frequencies lower than 1 GHz, a square wave clock will provide the best 2nd order linearity performance.

The clock input circuit has an on-chip resistive bias-T for separating the DC and RF components of the input. The low

frequency corner of the RF path is approximately 9 MHz (Ccoupling=10 pF, Rdc = 1.7 K ohm with clock DC shorted).

Outputs: The outputs should be sensed differentially for the cleanest output waveforms. The output impedance is

output amplifier (beyond the hold-node) is approximately 7 GHz. This produces approximately a 1 dB roll-off at the 3

dB bandwidth of the hold node (4.5 GHz) resulting in an overall track-mode 3 dB bandwidth of approximately 3.9 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.

The output amplifier noise contribution to the total output noise is substantial. If desired, a significant reduction in

output noise can be achieved by filtering the output to a lower bandwidth than the output amplifier bandwidth of 7

GHz. This is particularly effective if operating at lower clock rates. For example, the output noise can be reduced by

approximately 4 dB if the output bandwidth is reduced by a factor of four from 7 GHz down to 1.75 GHz.

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.

Application Notes

0.02 - 4.5 GHz WIDEBAND,

3 GS/s TRACK-AND-HOLD AMPLIFIER

v02.1208

HMC660LC4B