LTC1408IUH-12#TRPBF Datasheet LTC1408CUH-12#TRPBF, LTC1408IUH-12#TRPBF | P13-P15

LTC1408-12

140812f

LTC1566-1: Low Noise 2.3MHz Continuous Time

Lowpass 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 perfor-

mance 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.

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

fier, 8.5nV/√Hz.

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

5MHz, unity gain stable, rail-to-rail in and out,

10mA/amplifier, 3.5nV/√Hz.

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

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

0.95nV/√Hz.

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

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

1.9nV/√Hz.

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

Cutoff frequency.

LTC1408-12 inputs can be driven directly. As source

impedance increases, so will acquisition time. For mini-

mum 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 (the time allowed for settling must be at least 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 LTC1408-12 depends on the

application. Generally, applications fall into two catego-

ries: 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 LTC1408-12. (More detailed information is

available in the Linear Technology Databooks and on the

website at www.linear.com.)

APPLICATIO S I FOR ATIO

WUUU

LinearView is a trademark of Linear Technology Corporation.

LTC1408-12

140812f

APPLICATIO S I FOR ATIO

WUUU

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 LTC1408-12 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 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 CHO

to ground and a 51Ω source resistor

to limit the net input bandwidth to 30MHz. The 47pF ca-

pacitor 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 distor-

tion. 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 13pF of

input capacitance, will reduce the rated 50MHz input band-

width and increase acquisition time beyond 39ns.

INPUT RANGE

The analog inputs of the LTC1408-12 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 CH

and CH

–

at any input pair exceeds 2.5V (unipolar) or 1.25V

(bipolar), the output code will stay fixed at positive full-

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

1.25V (bipolar), the output code will stay fixed at negative

full-scale.

INTERNAL REFERENCE

The LTC1408-12 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 ampli-

fier output V

REF

, (Pin 23) must be bypassed with a capaci-

tor to ground. The reference amplifier 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 volt-

age of the external reference must be higher than the 2.5V

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

ence. The recommended 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

LTC1408-12

CH0

–

REF

GND

1408 F01

10µF

47pF*

51Ω*

CH1

–

47pF*

*TIGHT TOLERANCE REQUIRED TO AVOID

APERTURE SKEW DEGRADATION

51Ω*

ANALOG

INPUT

ANALOG

INPUT

Figure 2. Overdriving V

REF

Pin with an External Reference

LTC2351-12

REF

GND

1408 F02

10µF

LT1790-3

3.5V to 18V

LTC1408-12

140812f

APPLICATIO S I FOR ATIO

WUUU

Figure 4 shows the ideal input/output characteristics for

the LTC1408-12 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/4096 =

610µV for the LTC1408-12. The LTC1408-12 has 0.2 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

1408 G11

FREQUENCY (Hz)

–100

CMRR (dB)

–60

–20

–40

–80

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

–120

100 1k

Integral nonlinearity errors (INL) and differential nonlin-

earity errors (DNL) are largely independent of the common

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

CMRR is typically better than –90dB.

Figure 5 shows the ideal input/output characteristics for

the LTC1408-12 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/4096 =

610µV for the LTC1408-12. The LTC1408-12 has 0.2 LSB

RMS of Gaussian white noise.

Figure 5. LTC1408-12 Transfer Characteristic

in Bipolar Mode (BIP = High)

INPUT VOLTAGE (V)

2'S COMPLEMENT OUTPUT CODE

1408 F05

011...111

011...110

011...101

100...000

100...001

100...010

FS – 1LSB–FS

Figure 4. LTC1408-12 Transfer Characteristic

in Unipolar Mode (BIP = Low)

INPUT VOLTAGE (V)

STRAIGHT BINARY OUTPUT CODE

1408 F04

111...111

111...110

111...101

000...000

000...001

000...010

FS – 1LSB0

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LTC1408IUH-12#TRPBF

Mfr. #:

Buy LTC1408IUH-12#TRPBF

Manufacturer:

Analog Devices Inc.

Description:

Analog to Digital Converters - ADC 6 Ch, 12-B, 600ksps Simultaneous Smpl AD

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LTC1408IUH-12#TRPBF

LTC1408IUH-12#TRPBF

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