Rainbow Electronics MAX104 - Manual (Page 13)

Principle of Operation
The MAX104’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
500Msps per port (depending on how the demultiplex-
er section is set on the MAX104).
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, effective number of bits
(ENOB) and signal-to-noise ratio (SNR) specifications
will degrade. The MAX104’s on-chip, wide-bandwidth
(2.2GHz) T/H amplifier reduces this effect and increases
the ENOB performance 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 MAX104 with controlled impedance lines.
Figure 3 shows a simplified diagram of the T/H amplifier
stage internal to the MAX104.
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 MAX104’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 MAX104 features an on-chip +2.5V precision
bandgap reference, which can be used by connecting
MAX104
±5V, 1Gsps, 8-Bit ADC with
On-Chip 2.2GHz Track/Hold Amplifier
______________________________________________________________________________________ 13
(-FS + 1LSB)
0
+FS
(+FS - 1LSB)
255
255
254
129
128
127
126
3
2
1
0
ANALOG INPUT
OVERRANGE +
DIGITAL OUTPUT
Figure 2. Transfer Function
HOLD
CLK
ANALOG
INPUT
SAMPLED
DATA (T/H)
T/H
t
AW
t AD
t
AJ
TRACK TRACK
APERTURE DELAY (t
AD
)
APERTURE WIDTH (t
AW
)
APERTURE JITTER (t
AJ
)
CLK
Figure 4. T/H Aperture Timing
TO
COMPARATORS
TO
COMPARATORS
BUFFER
AMPLIFIER
INPUT
AMPLIFIER
CLOCK
SPLITTER
ALL INPUTS ARE ESD PROTECTED
(NOT SHOWN IN THIS
SIMPLIFIED DRAWING).
SAMPLING
BRIDGE
GNDI
50 Ω 50 Ω
VIN+
VIN-
GNDI
C HOLD
50 Ω 50 Ω
CLK+
CLK-
CLKCOM
Figure 3. Internal Structure of the 2.2GHz T/H Amplifier