LTC1414IGN#PBF Datasheet LTC1414CGN#PBF, LTC1414IGN#PBF, LTC1414IGN#TRPBF | P10-P12

LTC1414

APPLICATIONS INFORMATION

WUU

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 settling time can be provided by increasing

the time between conversions.

The best choice for an op amp to drive the LTC1414 will

depend on the application. Generally applications fall into

two categories: AC applications where dynamic specifica-

tions 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 LTC1414. More detailed information is

available in the Linear Technology Databooks and on the

LinearView

CD-ROM.

1223: 100MHz Video Current Feedback Amplifier.

6mA supply current. ±5V to ±15V supplies. Low noise.

Good for AC applications.

LT1227: 140MHz Video Current Feedback Amplifier. 10mA

supply current. ±5V to ±15V supplies. Low noise. Best for

AC applications.

LT1229/LT1230: Dual and Quad 100MHz Current Feed-

back Amplifiers. ±2V to ±15V supplies. Low noise. Good

AC specifications, 6mA supply current each amplifier.

LT1360: 50MHz Voltage Feedback Amplifier. 3.8mA sup-

ply current. Good AC and DC specs. ±5V to ±15V supplies.

70ns settling to 0.5LSB.

LT1363: 70MHz, 1000V/µs Op Amps. 6.3mA supply cur-

rent. Good AC and DC specifications. 60ns settling to

0.5LSB.

LT1364/LT1365: Dual and Quad 70MHz, 1000V/µs Op

Amps. 6.3mA supply current per amplifier. 60ns settling

to 0.5LSB.

LinearView is a trademark of Linear Technology Corporation.

AC Coupled Inputs

In applications where only the AC component of the analog

input is important, it may be desirable to AC couple the

input. This is easily accomplished by DC biasing the

LTC1414 analog input with a resistor to ground and using

a coupling capacitor to the input. Figure 7 shows a simple

AC coupled input circuit for the LTC1414 using only two

additional components. C1 is a 10µF ceramic capacitor

and R1 is a 1000Ω resistor to ground. R1 and C1 form a

highpass filter with a lower cut off frequency of 1/2π(C1)R1

or 15.9Hz.

Differential Drive

In some applications the ADC drive circuitry is differential.

The differential drive can be applied directly to the LTC1414

without any special translation circuitry. Differential drive

can be advantageous at high frequencies (>1MHz) since it

provides improved THD and SFDR. Transformers can be

used to provide AC coupling, input scaling and single

ended to differential conversion as shown in Figure 8. The

resistor R

across the secondary will determine the input

impedance on the primary. The input impedance of the

primary R

will be related to the secondary load resistor R

by the equation

= R

For example, if a Minicircuits T4-6T transformer is used,

the turns ratio is 2; if R

is 200Ω then R

is equal to 50Ω.

The center tap of the secondary will set the common

mode voltage and should be grounded for optimal AC

performance.

LTC1414

ANALOG INPUT

–

REF

REFCOMP

AGND

LTC1414 • F07

10µF

1µF

Figure 7. AC Coupled Input

LTC1414

APPLICATIONS INFORMATION

WUU

Input Range

The ±2.5V input range of the LTC1414 is optimized for low

noise and low distortion. Most op amps also perform best

over this same range, allowing direct coupling to the

analog inputs and eliminating the need for special transla-

tion circuitry.

Some applications may require other input ranges. The

LTC1414 differential inputs and reference circuitry can

accommodate other input ranges often with little or no

additional circuitry. The following sections describe the

reference and input circuitry and how they affect the input

range.

Internal Reference

The LTC1414 has an on-chip, temperature compensated,

curvature corrected, bandgap reference that is factory

trimmed to 2.500V. It is connected internally to a reference

amplifier and is available at V

REF

(Pin 3), see Figure 10. A

2k resistor is in series with the output so that it can be

easily overdriven by an external reference or other cir-

cuitry. The reference amplifier multiplies the voltage at the

REF

pin by 1.625 to create the required internal reference

voltage. This provides buffering between the V

REF

pin and

the high speed capacitive DAC. The reference amplifier

compensation pin, REFCOMP (Pin 4) 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 10µF tantalum in parallel

with a 0.1µF ceramic is recommended.

Figure 8. If a Transformer Coupled Input is Required,

this Circuit Provides a Simple Solution

LTC1414

INPUT

–

REF

REFCOMP

AGND

LTC1414 • F09

500pF

100Ω

10µF

Figure 9. An RC Filter Reduces the ADC’s 40MHz

Bandwidth to 3.2MHz and Filters Out Wideband Noise

Which May Be Present in the Input Signal

Input Filtering

The noise and the distortion of the input amplifier and

other circuitry must be considered since they will add to

the LTC1414 noise and distortion. The small-signal band-

width of the sample-and-hold circuit is 40MHz. 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 9 shows a 500pF capacitor from

to ground and a 100Ω source resistor to limit the input

bandwidth to 3.2MHz. The 500pF capacitor also acts as a

charge reservoir for the input sample-and-hold and iso-

lates the ADC input from sampling glitch-sensitive cir-

cuitry. High quality capacitors and resistors should be

used since poor quality components can add distortion.

NPO and silver mica type dielectric capacitors have excel-

lent linearity. Carbon surface mount resistors can also

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.

Figure 10. LTC1414 Reference Circuit

40k

64k

REFERENCE

AMP

10µF

REFCOMP

AGND

REF

2.500V

4.0625V

LTC1414

1414 F10

–

BANDGAP

REFERENCE

LTC1414

ANALOG

INPUT

–

REF

REFCOMP

AGND

LTC1414 • F08

50Ω

10µF

1µF

50Ω

500pF

1:N

LTC1414

APPLICATIONS INFORMATION

WUU

The V

REF

pin can be driven with a DAC or other means

shown in Figure 11. This is useful in applications where the

peak input signal amplitude may vary. The input span of

the ADC can then be adjusted to match the peak input

signal, maximizing the signal-to-noise ratio. The filtering

of the internal LTC1414 reference amplifier will limit the

bandwidth and settling time of this circuit. A settling time

of 5ms should be allowed after a reference adjustment.

The output is two’s complement binary with

1LSB = FS – (–FS)/16384 = 5V/16384 = 305.2µV.

In applications where absolute accuracy is important,

offset and full-scale errors can be adjusted to zero. Offset

error must be adjusted before full-scale error. Figure 14

shows the extra components required for full-scale error

adjustment. Zero offset is achieved by adjusting the offset

applied to the

–

input. For zero offset error apply

–152µV (i.e., –0.5LSB) at

and adjust the offset at the

–

input until the output code flickers between 0000

0000 0000 00 and 1111 1111 1111 11. For full-scale

adjustment, an input voltage of 2.499544V (FS – 1.5LSBs)

is applied to

and R2 is adjusted until the output

code flickers between 0111 1111 1111 10 and

0111 1111 1111 11.

Figure 11. Driving V

REF

with a DAC

LTC1414

ANALOG INPUT

±2V TO ±3V

DIFFERENTIAL

–

REF

REFCOMP

AGND

1414 F11

10µF

LTC1450

2V TO 3V

Differential Inputs

The LTC1414 has a unique differential sample-and-hold

circuit that allows rail-to-rail inputs. The ADC will always

convert the difference of

– (

–

) independent of the

common mode voltage. The common mode rejection

holds up to extremely high frequencies, see Figure 12. The

only requirement is that neither input can exceed the AV

or AV

power supply voltages. Integral nonlinearity er-

rors (INL) and differential nonlinearity errors (DNL) are

independent of the common mode voltage, however, the

bipolar zero error (BZE) will vary. The change in BZE is

typically less than 0.1% of the common mode voltage.

Dynamic performance is also affected by the common

mode voltage. THD will degrade as the inputs approach

either power supply rail, from –84dB with a common

mode of 0V to –75dB with a common mode of 2.5V

or –2.5V.

Full-Scale and Offset Adjustment

Figure 13 shows the ideal input/output characteristics for

the LTC1414. The code transitions occur midway between

successive integer LSB values (i.e., –FS + 0.5LSB,

– FS + 1.5LSB, –FS + 2.5LSB,...FS – 2.5LSB, FS – 1.5LSB).

Figure 12. CMRR vs Input Frequency

INPUT FREQUENCY (Hz)

COMMON MODE REJECTION (dB)

10k 100k

LTC1414 • F12

1M 10M

Figure 13. LTC1414 Transfer Characteristics

INPUT RANGE

OUTPUT CODE

LTC1414 • F13

011…111

011…110

011…101

000…000

111…111

100…000

100…001

100…010

FS – 1LSB0–(FS – 1LSB)

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

LTC1414IGN#PBF

Mfr. #:

Buy LTC1414IGN#PBF

Manufacturer:

Analog Devices Inc.

Description:

Analog to Digital Converters - ADC 14-B, 2.2 Msps,Smpl A/D Conv

Lifecycle:

New from this manufacturer.

Delivery:

DHL FedEx Ups TNT EMS

Payment:

T/T Paypal Visa MoneyGram Western Union

LTC1414IGN#PBF

LTC1414IGN#PBF

Products related to this Datasheet