MAX108 datasheet(13/32 Pages) MAXIM

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Principle of Operation

The MAX108’s flash or parallel architecture provides

the fastest multibit conversion of all common integrated

ADC designs. The key to this high-speed flash archi-

tecture is the use of an innovative, high-performance

comparator design. The flash converter and down-

stream logic translate the comparator outputs into a

parallel 8-bit output code and pass this binary code on

to the optional 8:16 demultiplexer, where primary and

auxiliary ports output PECL-compatible data at up to

750Msps per port (depending on how the demultiplex-

er section is set on the MAX108).

The ideal transfer function appears in Figure 2.

On-Chip Track/Hold Amplifier

As with all ADCs, if the input waveform is changing

rapidly during conversion, ENOB and signal-to-noise

ratio (SNR) specifications will degrade. The MAX108’s

on-chip, wide-bandwidth (2.2GHz) T/H amplifier

reduces this effect and increases the ENOB perfor-

mance significantly, allowing precise capture of fast

analog data at high conversion rates.

The T/H amplifier buffers the input signal and allows a

full-scale signal input range of ±250mV. The T/H ampli-

fier’s differential 50

Ω input termination simplifies inter-

facing to the MAX108 with controlled impedance lines.

Figure 3 shows a simplified diagram of the T/H amplifier

stage internal to the MAX108.

Aperture width, delay, and jitter (or uncertainty) are

parameters that affect the dynamic performance of

high-speed converters. Aperture jitter, in particular,

directly influences SNR and limits the maximum slew

rate (dV/dt) that can be digitized without contributing

significant errors. The MAX108’s innovative T/H amplifier

design limits aperture jitter typically to less than 0.5ps.

Aperture Width

Aperture width (tAW) is the time the T/H circuit requires

(Figure 4) to disconnect the hold capacitor from the

input circuit (for instance, to turn off the sampling

bridge and put the T/H unit in hold mode).

Aperture Jitter

Aperture jitter (tAJ) is the sample-to-sample variation

(Figure 4) in the time between the samples.

Aperture Delay

Aperture delay (tAD) is the time defined between the

rising edge of the sampling clock and the instant when

an actual sample is taken (Figure 4).

Internal Reference

The MAX108 features an on-chip +2.5V precision

bandgap reference that can be used by connecting

±5V, 1.5Gsps, 8-Bit ADC with

On-Chip 2.2GHz Track/Hold Amplifier

______________________________________________________________________________________

255

254

129

128

127

126

ANALOG INPUT

OVERRANGE +

Figure 2. Transfer Function

COMPARATORS

BUFFER

AMPLIFIER

INPUT

AMPLIFIER

CLOCK

SPLITTER

ALL INPUTS ARE ESD PROTECTED

(NOT SHOWN IN THIS

SIMPLIFIED DRAWING).

SAMPLING

BRIDGE

GNDI

Ω

VIN+

VIN-

GNDI

CHOLD

Ω

CLK+

CLK-

CLKCOM

Figure 3. Internal Structure of the 2.2GHz T/H Amplifier

HOLD

CLK

ANALOG

INPUT

SAMPLED

DATA (T/H)

T/H

tAW

tAD

tAJ

TRACK

APERTURE DELAY (tAD)

APERTURE WIDTH (tAW)

APERTURE JITTER (tAJ)

CLK

Figure 4. T/H Aperture Timing

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