LTC2209 Linear Technology Corporation, LTC2209 Datasheet - Page 18

• Manufacturer Part Number: LTC2209
• Description: 160Msps ADC
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
LTC2209
CONVERTER OPERATION
The LTC2209 is a CMOS pipelined multistep converter
with a front-end PGA. As shown in Figure 1, the converter
has fi ve pipelined ADC stages; a sampled analog input
will result in a digitized value seven cycles later (see the
Timing Diagram section). The analog input is differential for
improved common mode noise immunity and to maximize
the input range. Additionally, the differential input drive
will reduce even order harmonics of the sample and hold
circuit. The encode input is also differential for improved
common mode noise immunity.
The LTC2209 has two phases of operation, determined
by the state of the differential ENC
brevity, the text will refer to ENC
ENC high and ENC
Each pipelined stage shown in Figure 1 contains an ADC,
a reconstruction DAC and an interstage amplifi er. 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 amplifi ed and
output by the residue amplifi er. Successive stages oper-
ate out of phase so that when 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 differen-
tially 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 voltage
on the sample capacitors is held. While ENC is high, the
held input voltage is buffered by the S/H amplifi er which
drives the fi rst pipelined ADC stage. The fi rst stage acquires
the output of the S/H amplifi er during the high phase of
ENC. When ENC goes back low, the fi rst 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 high, the second stage
produces its residue which is acquired by the third stage.
An identical process is repeated for the third and fourth
stages, resulting in a fourth stage residue that is sent to
the fi fth stage for fi nal evaluation.
Each ADC stage following the fi rst has additional range to
accommodate fl ash and amplifi er offset errors. Results
from all of the ADC stages are digitally delayed such that
the results can be properly combined in the correction
logic before being sent to the output buffer.
18
+
less than ENC
+
+
–
greater than ENC
/ENC
as ENC low.
–
input pins. For
–
as
SAMPLE/HOLD OPERATION AND INPUT DRIVE
Sample/Hold Operation
Figure 2 shows an equivalent circuit for the LTC2209 CMOS
differential sample and hold. The differential analog inputs
are sampled directly onto sampling capacitors (C
through NMOS transistors. The capacitors shown attached
to each input (C
capacitance associated with each input.
During the sample phase when ENC is low, the NMOS
transistors connect the analog inputs to the sampling
capacitors and they charge to, and track the differential
input voltage. When ENC transitions from low to high, the
sampled input voltage is held on the sampling capacitors.
During the hold phase when ENC is high, the sampling
capacitors are disconnected from the input and the held
voltage is passed to the ADC core for processing. As ENC
transitions from high to low, the inputs are reconnected to
the sampling capacitors to acquire a new sample. Since
the sampling capacitors still hold the previous sample,
a charging glitch proportional to the change in voltage
between samples will be seen at this time. If the change
between the last sample and the new sample is small,
the charging glitch seen at the input will be small. If the
ENC
ENC
A
A
IN
IN
+
+
–
–
LTC2209
1.6V
1.6V
6k
V
V
6k
Figure 2. Equivalent Input Circuit
DD
DD
PARASITIC
V
DD
) are the summation of all other
C
1.8pF
C
1.8pF
PARASITIC
PARASITIC
C
C
SAMPLE
SAMPLE
4.6pF
4.6pF
SAMPLE
2209 F02
2209f
)