LTC2355CMSE-14#PBF Datasheet LTC2355IMSE-14#PBF, LTC2355IMSE-12#PBF, LTC2355CMSE-12#PBF | P10-P12

LTC2355-12/LTC2355-14

2355fb

For more information www.linear.com/LTC2355-12

DRIVING THE ANALOG INPUT

The differential analog inputs of the LTC2355-12/LTC2355-14

may be driven differentially or

as a single-ended input (i.e.,

the A

–

input is grounded). Both differential analog in-

puts, A

and A

–

, are sampled at the same instant. Any

unwanted signal that is common to both inputs of each

input pair will be reduced by the common mode rejection

of the sample-and-hold circuit. The inputs draw only one

small current spike while charging the sample-and-hold

capacitors at the end of conversion. During conversion,

the analog inputs draw only a small leakage current. If the

source impedance of the driving circuit is low, then the

LTC2355-12/LTC2355-14 inputs can be driven directly. As

source impedance increases, so will acquisition time. For

minimum acquisition time with high source impedance,

a buffer ampliﬁer must be used. The main requirement is

that the ampliﬁer driving the analog input(s) must settle

after the small current spike before the next conversion

starts (settling time must be 39ns for full throughput rate).

Also keep in mind while choosing an input ampliﬁer the

amount of noise and harmonic distortion added by the

ampliﬁer.

CHOOSING AN INPUT AMPLIFIER

Choosing an input ampliﬁer is easy if a few requirements

are taken into consideration. First, to limit the magnitude of

the voltage spike seen by the ampliﬁer from charging the

sampling capacitor, choose an ampliﬁer that has a low output

impedance (<100Ω) at the closed-loop bandwidth frequency.

For example, if an ampliﬁer is used in a gain of 1 and has a

unity-gain bandwidth of 50MHz, then the output impedance at

50MHz must be less than 100Ω. The second requirement is

that the closed-loop bandwidth must be greater than 40MHz

to ensure adequate small-signal settling for full throughput

rate. If slower op amps are used, more time for settling can

be provided by increasing the time between conversions.

The best choice for an op amp to drive the LTC2355-12/

LTC2355-14 will depend on the application. Generally, ap

plications fall into two categories: AC applications where

dynamic speciﬁcations are most critical and time domain

applications where DC accuracy and settling time are most

critical. The following list is a summary of the op amps that

are suitable for driving the LTC2355-12/LTC2355-14. (More

APPLICATIONS INFORMATION

detailed information is available in the Linear Technology

Databooks and on the LinearView™ CD-ROM.)

LTC1566-1: Low Noise 2.3MHz Continuous Time Low-

Pass Filter.

1630: Dual 30MHz Rail-to-Rail Voltage FB Ampliﬁer.

2.7V to ±15V supplies. Very high A

VOL

, 500µV offset and

520ns settling to 0.5LSB for a 4V swing. THD and noise

are –93dB to 40kHz and below 1LSB to 320kHz (A

= 1,

P-P

into 1kΩ, V

= 5V), making the part excellent for AC

applications (to 1/3 Nyquist) where rail-to-rail performance

is desired. Quad version is available as LT1631.

LT1632: Dual 45MHz Rail-to-Rail Voltage FB Ampliﬁer.

2.7V to ±15V supplies. Very high A

VOL

, 1.5mV offset and

400ns settling to 0.5LSB for a 4V swing. It is suitable for

applications with a single 5V supply. THD and noise are

–93dB to 40kHz and below 1LSB to 800kHz (A

= 1,

P-P

into 1kΩ, V

= 5V), making the part excellent for

AC applications where rail-to-rail performance is desired.

Quad version is available as LT1633.

LT1813: Dual 100MHz 750V/µs 3mA Voltage Feedback

Ampliﬁer. 5V to ±5V supplies. Distortion is –86dB to 100kHz

and –77dB to 1MHz with ±5V supplies (2V

P-P

into 500Ω).

Excellent part for fast AC applications with ±5V supplies.

LT1801: 80MHz GBWP, –75dBc at 500kHz, 2mA/Ampliﬁer,

8.5nV/√Hz.

LT1806/LT1807: 325MHz GBWP, –80dBc Distortion at 5MHz, Uni

ty-Gain Stable, R-R In and Out, 10mA/Ampliﬁer, 3.5nV/√Hz.

LT1810: 180MHz GBWP, –90dBc Distortion at 5MHz,

Unity-Gain Stable, R-R In and Out, 15mA/Ampliﬁer, 16nV/√Hz.

LT1818/LT1819: 400MHz, 2500V/µs,9mA, Single/Dual

Voltage Mode Operational Ampliﬁer.

LT6200: 165MHz GBWP, –85dBc Distortion at 1MHz,

Unity-Gain Stable, R-R In and Out, 15mA/Amplifier,

0.95nV/√Hz.

LT6203: 100MHz GBWP, –80dBc Distortion at 1MHz,

Unity-Gain Stable, R-R In and Out, 3mA/Amplifier,

1.9nV/√Hz.

LT6600-10: Ampliﬁer/Filter Differential In/Out with 10MHz

Cutoff.

LinearView is a trademark of Linear Technology Corporation.

LTC2355-12/LTC2355-14

2355fb

For more information www.linear.com/LTC2355-12

APPLICATIONS INFORMATION

INPUT FILTERING AND SOURCE IMPEDANCE

The noise and the distortion of the input ampliﬁer and

other circuitry must be considered since they will add to

the LTC2355-12/LTC2355-14 noise and distortion. The

small-signal bandwidth of the sample-and-hold circuit is

50MHz. Any noise or distortion products that are pres

ent at the analog inputs will be summed over this entire

bandwidth. Noisy input circuitry should be ﬁltered prior

to the analog inputs to minimize noise. A simple 1-pole

RC ﬁlter is sufﬁcient for many applications. For example,

Figure 1 shows a 47pF capacitor from A

to ground and a

51Ω source resistor to limit the input bandwidth to 47MHz.

The 47pF capacitor also acts as a charge reservoir for the

input sample-and-hold and isolates the ADC input from

sampling-glitch sensitive circuitry. High quality capacitors

and resistors should be used since these components

can add distortion. NPO and silvermica type dielectric

capacitors have excellent linearity. Carbon surface mount

resistors can generate distortion from self heating and

from damage that may occur during soldering. Metal ﬁlm

surface mount resistors are much less susceptible to both

problems. When high amplitude unwanted signals are close

in frequency to the desired signal frequency, a multiple

pole ﬁlter is required. High external source resistance,

combined with the 13pF of input capacitance, will reduce

the rated 50MHz bandwidth and increase acquisition time

beyond 39ns.

INPUT RANGE

The analog inputs of the LTC2355-12/LTC2355-14 may be

driven fully differentially with a single supply. Each input

may swing up to 2.5V

P-P

individually. When using the

internal reference, the noninverting input should never be

more than 2.5V more positive than the inverting input. The

0V to 2.5V range is also ideally suited for single-ended

input use with single supply applications. The common

mode range of the inputs extend from ground to the sup

ply voltage V

. If the difference between the A

and

–

inputs exceeds 2.5V, the output code will stay ﬁxed

at all ones and if this difference goes below 0V, the ouput

code will stay ﬁxed at all zeros.

INTERNAL REFERENCE

The LTC2355-12/LTC2355-14 has an on-chip, temperature

compensated, bandgap reference that is factory trimmed

to 2.5V to obtain a unipolar 0V to 2.5V input span. The

reference ampliﬁer output V

REF

, (Pin 3) must be bypassed

with a capacitor to ground. The reference ampliﬁer is

stable with capacitors of 1µF or greater. For the best noise

performance, a 10µF ceramic or a 10µF tantalum in parallel

with a 0.1µF ceramic is recommended. The V

REF

pin can be

overdriven with an external reference as shown in Figure

2. The voltage of the external reference must be higher

than the 2.5V output of the internal reference. The recom

mended range for an external reference is 2.55V to V

An external reference at 2.55V will see a DC quiescent load

of 0.75mA and as much as 3mA during conversion.

Figure 1. RC Input Filter Figure 2. Overdriving V

REF

Pin with an External Reference

10µF

–

LTC2355-12/

LTC2355-14

47pF

51Ω

GND

REF

2355 F01

GND

LTC2355-12/

LTC2355-14

LT1790-3

REF

10µF

3.5V TO 18V

2355 F02

LTC2355-12/LTC2355-14

2355fb

For more information www.linear.com/LTC2355-12

APPLICATIONS INFORMATION

INPUT SPAN VERSUS REFERENCE VOLTAGE

The differential input range has a 0V to V

REF

unipolar voltage

span that equals the difference between the voltage at the

reference buffer output V

REF

at Pin 3, and the voltage at

the ground (Exposed Pad Ground). The differential input

range of the ADC is 0V to 2.5V when using the internal

reference. The internal ADC is referenced to these two

nodes. This relationship also holds true with an external

reference.

DIFFERENTIAL INPUTS

The LTC2355-12/LTC2355-14 has a unique differential

sample-and-hold circuit that measures input voltages from

ground to V

. The ADC will always convert the unipolar

difference of A

– A

–

, independent of the common

mode voltage at the inputs. The common mode rejection

holds up at extremely high frequencies, see Figure 3. The

only requirement is that both inputs not go below ground

or exceed V

. Integral nonlinearity errors (INL) and dif-

ferential nonlinearity errors (DNL) are largely independent

of the common mode voltage. However, the offset error

will vary. The change in offset error is typically less than

0.1% of the common mode voltage.

Figure 4 shows the ideal input/output characteristics for

the LTC2355-12/LTC2355-14. The code transitions occur

midway between successive integer LSB values (i.e.,

0.5LSB, 1.5LSB, 2.5LSB, FS – 1.5LSB). The output code

is straight binary with 1LSB = 2.5V/16384 = 153µV for

the LTC2355-14, and 1LSB = 2.5V/4096 = 610µV for the

LTC2355-12. The LTC2355-14 has 1LSB RMS of random

white noise.

Figure 3. CMRR vs Frequency Figure 4. LTC2355-12/LTC2355-14 Transfer Characteristic

FREQUENCY (Hz)

100

CMRR (dB)

–20

–40

–60

–80

–100

–120

10k 100k 1M

2355 F03

10M 100M

INPUT VOLTAGE (V)

UNIPOLAR OUTPUT CODE

2355 F04

111...111

111...110

111...101

000...000

000...001

000...010

FS – 1LSB0

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18

LTC2355CMSE-14#PBF

Mfr. #:

Buy LTC2355CMSE-14#PBF

Manufacturer:

Analog Devices Inc.

Description:

Analog to Digital Converters - ADC 14-Bit, 3.5 Msps Serial ADC Unipolar

Lifecycle:

New from this manufacturer.

Delivery:

DHL FedEx Ups TNT EMS

Payment:

T/T Paypal Visa MoneyGram Western Union

LTC2355CMSE-14#PBF

LTC2355CMSE-14#PBF

Products related to this Datasheet