LTC2254_15 Datasheet, PDF (13/24 Pages) Linear Technology – 14-Bit, 125/105Msps Low Power 3V ADCs

LTC2255/LTC2254
APPLICATIO S I FOR ATIO
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 CLK 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 CLK transitions from low to high, the sampled input is
held. While CLK 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 CLK. When CLK 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 CLK goes back high,
the second stage produces its residue which is acquired
by the third stage. An identical process is repeated for the
third, fourth and fifth stages, resulting in a fifth stage
residue that is sent to the sixth stage ADC for final
evaluation.
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 LTC2255/
LTC2254 CMOS differential sample-and-hold. The analog
inputs are connected to the sampling capacitors (C_SAMPLE)
through NMOS transistors. The capacitors shown at-
tached to each input (C_PARASITIC) are the summation of all
other capacitance associated with each input.
During the sample phase when CLK is low, the transistors
connect the analog inputs to the sampling capacitors and
they charge to and track the differential input voltage.
When CLK transitions from low to high, the sampled input
voltage is held on the sampling capacitors. During the hold
phase when CLK is high, the sampling capacitors are
disconnected from the input and the held voltage is passed
to the ADC core for processing. As CLK 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 input change is large, such as
the change seen with input frequencies near Nyquist, then
a larger charging glitch will be seen.
LTC2255/LTC2254
V_DD
15Ω
A_IN+
V_DD
C_PARASITIC
1pF
15Ω
A_IN–
CLK
C_PARASITIC
1pF
V_DD
C_SAMPLE
3.5pF
C_SAMPLE
3.5pF
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Figure 2. Equivalent Input Circuit
Single-Ended Input
For cost sensitive applications, the analog inputs can be
driven single-ended. With a single-ended input the har-
monic distortion and INL will degrade, but the SNR and
DNL will remain unchanged. For a single-ended input, A_IN+
should be driven with the input signal and A_IN– should be
connected to 1.5V or V_CM.
Common Mode Bias
For optimal performance the analog inputs should be
driven differentially. Each input should swing ±0.5V for
the 2V range or ±0.25V for the 1V range, around a
common mode voltage of 1.5V. The V_CM output pin (Pin
31) may be used to provide the common mode bias level.
V_CM can be tied directly to the center tap of a transformer
to set the DC input level or as a reference level to an op amp
differential driver circuit. The V_CM pin must be bypassed to
ground close to the ADC with a 2.2µF or greater capacitor.
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