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RTH010

9GHz Bandwidth 1GS/s Dual Track-and-Hold

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RTH010 DATA SHEET REV F

Theory of Operation

The DTH chip contains two TH’s, TH1 and TH2,

in series, together with clock shaping circuitry,

BUFFER1 and BUFFER2, and a 50-ohm output

driver, OUTBUF (Figure 1). To maximize

dynamic range and insensitivity to noise, all non-

DC internal circuits and all non-DC inputs and

outputs are differential. TH1 determines the

dynamic sampled-mode performance of the

DTH. Its sampling bridges exploit the ultra-high

speed of the Schottky diodes available in the

GaAs HBT process. TH1 clock inputs, CLK1 and

CLK1B, should be driven by a low-jitter clock

source. TH2 is similar to TH1, except that its

bandwidth requirement is lower and its gain is

closer to unity.

The DTH receives a differential analog input

signal at inputs INP and INN, which is sampled

on the TH1 hold capacitors upon a falling

transition of its differential clock voltage V(CLK1)

– V(CLK1B), after an aperture delay, ta, see

Figure 2. TH1’s aperture delay is positive,

nominally 60 ps.

The sampling instant is affected by clock source

jitter (off-chip) and aperture jitter (caused by on-

chip noise). From the Definition of Terms, the

required total sampling jitter for sampling a 1-

GHz 1-Vpp sine wave with 10-bit accuracy is

127 fs. The aperture jitter of the RTH010 is less

than 100 fs for a 1-GHz 0.5-Vpp TH1 clock,

CLK1(B). Using rms addition of jitter, the clock

source jitter must be less than 80 fs (over the

measurement record time) for direct 10-bit

sampling of GHz range signals. Given a noise

variance, ∆V, of the on-chip clock buffer, its

aperture jitter, ∆t, is inversely proportional to the

clock buffer gain and the slew rate of the

incoming clock at the zero-crossing point:

∆t

=

∆V

gain× slew rate

For low slew rates or frequencies, the clock

buffer gain is constant and its aperture jitter is

inversely proportional to the input clock slew

rate, improving with increasing slew rate. For

high slew rates or high frequencies, the jitter

increases again, because the buffer gain drops

steeply. For the RTH010, the clock buffer gain is

still roughly constant up to 1 GHz, so that the

aperture jitter is inversely proportional with the

slew rate of the incoming clock. In the above

equation, we have ∆V/gain ≈ 0.15 mV. The

RTH010 aperture jitter at various slew rates can

then be estimated. For example, a 1-GHz 0.5-

Vpp sinusoidal CLK1(B) signal corresponds to a

slew rate ~ 1.6 V/ns, correctly yielding an

aperture jitter < 100 fs.

The held and buffered output of TH1, VTH1, is

sampled on the TH2 hold capacitors upon a

falling transition of its differential clock voltage

V(CLK2) – V(CLK2B), after an aperture delay

closely equal to that of TH1. This allows simple

out-of-phase clocking of TH1 and TH2 by having

opposite phases for CLK1(B) and CLK2(B).

Aperture jitter of TH2 is irrelevant, since the slew

rate of the TH2 input is equal to the TH1

differential droop rate, about 1000x lower than

the input slew rate for TH1 for a 1-GHz 1-Vpp

sine wave. TH2 can be in track mode before

TH1 switches to hold, but a minimum track time

of TH2 after TH1 enters hold mode must be

observed to ensure that TH2 has fully acquired

the TH1 output (ttrack2,min).

Hold mode feedthrough, or in-to-out hold-mode

gain in dB, again is important for TH1 and not for

TH2, since any distortion on the held TH1 signal

by a rapidly varying TH1 input will be sampled

by TH2, and can not be removed. RTH010’s

TH1 hold mode feedthrough performance is

more than sufficient for 10-bit sampling of GHz

range signals.

After a TH1 postamplifier, TH2 produces an

output VTH2. For out-of-phase clocking, the

delay from the hold instant of TH1 to the ideal

sampling time of circuitry after TH2 is close to

one full clock cycle, for example 1 ns at a 1-GHz

sampling rate. The TH2 output is flat for more

than half a clock cycle, which eases the

bandwidth requirement of subsequent circuitry.

This is true, even though a small glitch will be

present at the transition from track to hold of

The product specifications contained in this data sheet are subject to change. Rockwell Scientific Company reserves the right to make changes to its product

specifications at any time without notice. The information furnished herein is believed to be accurate; however, no responsibility is assumed for its use.

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