LTC2205 Linear Technology, LTC2205 Datasheet - Page 20

• Manufacturer Part Number: LTC2205
• Description: (LTC2204 / LTC2205) 65Msps/40Msps
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC2205/LTC2204
20
CONVERTER OPERATION
The LTC2205/LTC2204 are CMOS pipelined multistep
converters 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 LTC2205/LTC2204 have two phases of operation,
determined by the state of the differential ENC
put pins. For brevity, the text will refer to ENC
than ENC
ENC low.
Each pipelined stage shown in Figure 1 contains an
ADC, a reconstruction DAC and a residue 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
–
as ENC high and ENC
ENC
ENC
A
A
IN
IN
+
–
+
–
+
less than ENC
LTC2005/LTC2004
1.6V
1.6V
6k
6k
V
V
Figure 2. Equivalent Input Circuit
DD
DD
+
/ENC
+
greater
V
DD
–
–
in-
as
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 iden-
tical 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.
C
1.8pF
C
1.8pF
PARASITIC
PARASITIC
C
C
SAMPLE
SAMPLE
4.9pF
4.9pF
22054 F02
22054fb