LTC2233 Linear Technology, LTC2233 Datasheet - Page 15

• Manufacturer Part Number: LTC2233
• Description: 10-Bit 105Msps/80Msps ADCs
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
CONVERTER OPERATION
As shown in Figure 1, the LTC2232/LTC2233 is a CMOS
pipelined multistep converter. The converter has five
pipelined ADC stages; a sampled analog input will result in
a digitized value five cycles later (see the Timing Diagram
section). For optimal AC performance the analog inputs
should be driven differentially. For cost sensitive applica-
tions, the analog inputs can be driven single-ended with
slightly worse harmonic distortion. The encode input is
differential for improved common mode noise immunity.
The LTC2232/LTC2233 has two phases of operation,
determined by the state of the differential ENC
pins. For brevity, the text will refer to ENC
ENC
Each pipelined stage shown in Figure 1 contains an ADC,
a reconstruction DAC and an interstage residue amplifier.
In operation, the ADC quantizes the input to the stage and
the quantized value is subtracted from the input by the
DAC to produce a residue. The residue is amplified and
output by the residue amplifier. Successive stages operate
out of phase so that when the odd stages are outputting
their residue, the even stages are acquiring that residue
and vice versa.
When ENC is low, the analog input is sampled differentially
directly onto the input sample-and-hold capacitors, inside
the “Input S/H” shown in the block diagram. At the instant
that ENC transitions from low to high, the sampled input
is held. While ENC is high, the held input voltage is
buffered by the S/H amplifier which drives the first pipelined
ADC stage. The first stage acquires the output of the S/H
during this high phase of ENC. When ENC goes back low,
the first stage produces its residue which is acquired by
the second stage. At the same time, the input S/H goes
back to acquiring the analog input. When ENC goes back
high, the second stage produces its residue which is
acquired by the third stage. An identical process is re-
peated for the third and fourth stages, resulting in a fourth
stage residue that is sent to the fifth stage ADC for final
evaluation.
–
as ENC high and ENC
U
U
+
less than ENC
W
+
–
+
greater than
as ENC low.
/ENC
U
–
input
Each ADC stage following the first has additional range to
accommodate flash and amplifier offset errors. Results
from all of the ADC stages are digitally synchronized such
that the results can be properly combined in the correction
logic before being sent to the output buffer.
SAMPLE/HOLD OPERATION AND INPUT DRIVE
Sample/Hold Operation
Figure 2 shows an equivalent circuit for the LTC2232/
LTC2233 CMOS differential sample-and-hold. The analog
inputs are connected to the sampling capacitors (C
through NMOS transistors. The capacitors shown at-
tached to each input (C
other capacitance associated with each input.
ENC
ENC
A
A
IN
IN
+
–
+
–
LTC2232/LTC2233
15
15
1.6V
1.6V
6k
V
V
6k
Figure 2. Equivalent Input Circuit
DD
DD
V
LTC2232/LTC2233
DD
PARASITIC
C
1pF
C
1pF
PARASITIC
PARASITIC
) are the summation of all
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C
C
SAMPLE
SAMPLE
1.6pF
1.6pF
SAMPLE
15
22323 F02
22323fa
)