LTC2248 datasheet(14/24 Pages) LINER

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LTC2248/LTC2247/LTC2246

224876fa

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 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 LTC2248/

LTC2247/LTC2246 CMOS differential sample-and-hold.

The analog inputs are connected to the sampling capaci-

tors (CSAMPLE) through NMOS transistors. The capacitors

shown attached to each input (CPARASITIC) are the summa-

tion 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

APPLICATIO S I FOR ATIO

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.

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, AIN+

should be driven with the input signal and AIN– should be

connected to 1.5V or VCM.

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 VCM output pin (Pin

31) may be used to provide the common mode bias level.

VCM 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 VCM pin must be bypassed to

ground close to the ADC with a 2.2

µF or greater capacitor.

Figure 2. Equivalent Input Circuit

VDD

Ω

CPARASITIC

1pF

CPARASITIC

1pF

CSAMPLE

4pF

CSAMPLE

4pF

LTC2248/47/46

AIN+

AIN–

CLK

224876 F02

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