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 50Ω 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 (t
AW
) 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 (t
AJ
) is the sample-to-sample variation
(Figure 4) in the time between the samples.
Aperture Delay
Aperture delay (t
AD
) 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
MAX108
±5V, 1.5Gsps, 8-Bit ADC with
On-Chip 2.2GHz Track/Hold Amplifier
______________________________________________________________________________________ 13
SIMPLIFIED DRAWING).
(24 Seiten)
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