LTC2209 datasheet(18/32 Pages) LINER

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LTC2209

2209fa

CONVERTER OPERATION

The LTC2209 is a CMOS pipelined multistep converter

with a front-end PGA. As shown in Figure 1, the converter

has ﬁve 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+/ENC– input pins. For

brevity, the text will refer to ENC+ greater than ENC– as

ENC high and ENC+ less than ENC– as ENC low.

Each pipelined stage shown in Figure 1 contains an ADC,

a reconstruction DAC and an interstage 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 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 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 voltage on the

sample capacitors 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 ampliﬁer during the 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 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 ﬁfth stage 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 delayed 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 LTC2209 CMOS

differential sample and hold. The differential analog inputs

are sampled directly onto sampling capacitors (CSAMPLE)

through NMOS transistors. The capacitors shown attached

to each input (CPARASITIC) are the summation of all other

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

Figure 2. Equivalent Input Circuit

APPLICATIONS INFORMATION

CSAMPLE

4.6pF

VDD

LTC2209

AIN+

2209 F02

CSAMPLE

4.6pF

VDD

AIN–

ENC–

ENC+

1.6V

CPARASITIC

1.8pF

CPARASITIC

1.8pF

RPARASITIC

3Ω

RPARASITIC

3Ω

RON

20Ω

RON

20Ω

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