MAX108 Maxim, MAX108 Datasheet - Page 13

• Manufacturer Part Number: MAX108
• Description: 5V / 1.5Gsps / 8-Bit ADC with On-Chip 2.2GHz Track/Hold Amplifier
• Manufacturer: Maxim

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.
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 (t
(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 (t
(Figure 4) in the time between the samples.
Aperture delay (t
rising edge of the sampling clock and the instant when
an actual sample is taken (Figure 4).
The MAX108 features an on-chip +2.5V precision
bandgap reference that can be used by connecting
AJ
AW
AD
On-Chip Track/Hold Amplifier
______________________________________________________________________________________
) is the sample-to-sample variation
) is the time the T/H circuit requires
) is the time defined between the
On-Chip 2.2GHz Track/Hold Amplifier
Principle of Operation
Internal Reference
Aperture Width
Aperture Delay
Aperture Jitter
±5V, 1.5Gsps, 8-Bit ADC with
Figure 2. Transfer Function
Figure 3. Internal Structure of the 2.2GHz T/H Amplifier
Figure 4. T/H Aperture Timing
ALL INPUTS ARE ESD PROTECTED
(NOT SHOWN IN THIS
SIMPLIFIED DRAWING).
CLKCOM
OVERRANGE +
CLK+
CLK-
VIN+
VIN-
50
50
GNDI
DATA (T/H)
255
255
254
129
128
127
126
SAMPLED
ANALOG
3
2
1
0
INPUT
CLK
CLK
T/H
50
50
t
AD
AMPLIFIER
SPLITTER
CLOCK
INPUT
TRACK
ANALOG INPUT
SAMPLING
APERTURE DELAY (t
APERTURE WIDTH (t
APERTURE JITTER (t
BRIDGE
0
t
t
HOLD
AW
AJ
GNDI
AMPLIFIER
C
BUFFER
TO
COMPARATORS
HOLD
AD
AW
AJ
)
)
)
TRACK
TO
COMPARATORS
13