LTC2351IUH-14#TRPBF Datasheet LTC2351IUH-14#PBF, LTC2351CUH-14#PBF, LTC2351HUH-14#PBF | P13-P15

LTC2351-14

235114fb

APPLICATIONS INFORMATION

LTC1566-1: Low Noise 2.3MHz Continuous Time

Lowpass Filter.

1630: Dual 30MHz Rail-to-Rail Voltage FB Ampliﬁ er.

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 Ampliﬁ er.

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.

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

8.5nV/√Hz.

LT1806/LT1807: 325MHz GBWP, –80dBc distortion at

5MHz, unity gain stable, rail-to-rail in and out, 10mA/am-

pliﬁ er, 3.5nV/√Hz.

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

unity gain stable, rail-to-rail in and out, 15mA/ampliﬁ er,

16nV/√Hz.

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

Voltage Mode Operational Ampliﬁ er.

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

unity gain stable, rail-to-rail in and out, 15mA/ampliﬁ er,

0.95nV/√Hz.

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

unity gain stable, rail-to-rail in and out, 3mA/ampliﬁ er,

1.9nV/√Hz.

LT6600: Ampliﬁ er/Filter Differential In/Out with 10MHz

Cutoff Frequency.

conversion, the analog inputs draw only a small leakage

current. If the source impedance of the driving circuit is

low, then the LTC2351-14 inputs can be driven directly. As

source impedance increases, so will acquisition time. For

minimum acquisition time with high source impedance,

a buffer ampliﬁ er must be used. The main requirement is

that the ampliﬁ er driving the analog input(s) must settle

after the small current spike before the next conversion

starts (the time allowed for settling must be at least 39ns

for full throughput rate). Also keep in mind while choos-

ing an input ampliﬁ er the amount of noise and harmonic

distortion added by the ampliﬁ er.

CHOOSING AN INPUT AMPLIFIER

Choosing an input ampliﬁ er is easy if a few requirements

are taken into consideration. First, to limit the magnitude

of the voltage spike seen by the ampliﬁ er from charging

the sampling capacitor, choose an ampliﬁ er that has a low

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

frequency. For example, if an ampliﬁ er 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 band-

width 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 LTC2351-14 depends on the

application. Generally, applications fall into two categories:

AC applications where dynamic speciﬁ cations 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

LTC2351-14. More detailed information is available in

the Linear Technology Databooks and on the Web site at

www.linear.com.

LTC2351-14

235114fb

LTC2351-14

CH0

–

REF

GND

235114 F01

10μF

47pF*

51Ω*

CH1

–

47pF*

*TIGHT TOLERANCE REQUIRED TO AVOID

APERTURE SKEW DEGRADATION

51Ω*

ANALOG

INPUT

ANALOG

INPUT

APPLICATIONS INFORMATION

INPUT FILTERING AND SOURCE IMPEDANCE

The noise and the distortion of the input ampliﬁ er and

other circuitry must be considered since they will add to

the LTC2351-14 noise and distortion. The small-signal

bandwidth of the sample-and-hold circuit is 50MHz. Any

noise or distortion products that are present at the analog

inputs will be summed over this entire bandwidth. Noisy

input circuitry should be ﬁ ltered prior to the analog inputs.

A simple 1-pole RC ﬁ lter is sufﬁ cient for many applications.

For example, Figure 1 shows a 47pF capacitor from CH0

to ground and a 51Ω source resistor to limit the net input

bandwidth to 30MHz. 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

ﬁ lm 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 ﬁ lter is required.

High external source resistance, combined with 13pF

of input capacitance, will reduce the rated 50MHz input

bandwidth and increase acquisition time beyond 39ns.

INPUT RANGE

The analog inputs of the LTC2351-14 may be driven fully

differentially with a single supply. Either input may swing

up to V

, provided the differential swing is no greater

than 2.5V with BIP (Pin 29) LOW, or ±1.25V with (BIP Pin

29) HIGH. 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 supply voltage V

. If the difference between the

and CH

–

at any input pair exceeds 2.5V (unipolar) or

1.25V (bipolar), the output code will stay ﬁ xed at positive

full-scale, and if this difference goes below 0V (unipolar) or

–1.25V (bipolar), the output code will stay ﬁ xed at nega-

tive full-scale.

INTERNAL REFERENCE

The LTC2351-14 has an on-chip, temperature compen-

sated, bandgap reference that is factory trimmed to 2.5V

to obtain a precise 2.5V input span. The reference ampliﬁ er

output V

REF

, (Pin 23) must be bypassed with a capacitor

to ground. The reference ampliﬁ er is stable with capaci-

tors 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 of the open-drain P-channel output of the internal

reference. The recommended range for an external refer-

ence 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. External Reference

LTC2351-14

REF

GND

235114 F02

10μF

LT1790-3

3.5V to 18V

LTC2351-14

235114fb

235114 F03

FREQUENCY (Hz)

–100

CMRR (dB)

–60

–20

–40

–80

10k 100k 1M 10M 100M 1G

–120

100 1k

INPUT VOLTAGE (V)

2'S COMPLEMENT OUTPUT CODE

235114 F05

011...111

011...110

011...101

100...000

100...001

100...010

FS – 1LSB–FS

INPUT VOLTAGE (V)

STRAIGHT BINARY OUTPUT CODE

235114 F04

111...111

111...110

111...101

000...000

000...001

000...010

FS – 1LSB0

APPLICATIONS INFORMATION

Figure 5 shows the ideal input/output characteristics for

the LTC2351-14 in bipolar mode (BIP = HIGH). 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 2’s complement with 1LSB = 2.5V/16384 =

153μV for the LTC2351-14. The LTC2351-14 has 0.7 LSB

RMS of gaussian white noise.

INPUT SPAN VERSUS REFERENCE VOLTAGE

The differential input range has a unipolar voltage span

that equals the difference between the voltage at the

reference buffer output V

REF

(Pin 23) and the voltage at

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 ADC will always convert the difference of CH

minus

–

, independent of the common mode voltage at any pair

of inputs. The common mode rejection holds up at high

frequencies (see Figure 3.) The only requirement is that

both inputs not go below ground or exceed V

Figure 3. CMRR vs Frequency

Integral nonlinearity errors (INL) and differential nonlinear-

ity errors (DNL) are largely independent of the common

mode voltage. However, the offset error will vary. DC CMRR

is typically better than –90dB.

Figure 4 shows the ideal input/output characteristics for

the LTC2351-14 in unipolar mode (BIP = LOW). 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 LTC2351-14. The LTC2351-14 has 0.7 LSB

RMS of gaussian white noise.

Figure 5. LTC2351-14 Transfer Characteristic

in Bipolar Mode (BIP = HIGH)

Figure 4. LTC2351-14 Transfer Characteristic

in Unipolar Mode (BIP = LOW)

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P20

LTC2351IUH-14#TRPBF

Mfr. #:

Buy LTC2351IUH-14#TRPBF

Manufacturer:

Analog Devices Inc.

Description:

Analog to Digital Converters - ADC 14-Bit, 6-Channel 1.5Msps simultaneous Sampling ADC

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LTC2351IUH-14#TRPBF

LTC2351IUH-14#TRPBF

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