LTC1412IG#TRPBF Datasheet LTC1412IG#PBF, LTC1412CG#TRPBF, LTC1412IG#TRPBF | P10-P12

LTC1412

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quencies above the converter’s Nyquist Frequency. The

noise floor stays very low at high frequencies; S/(N + D)

becomes dominated by distortion at frequencies far

beyond Nyquist.

Driving the Analog Input

The differential analog inputs of the LTC1412 are easy to

drive. The inputs may be driven differentially or as a single-

ended input (i.e., the A

–

input is grounded). The A

and

–

inputs are sampled at the same instant. Any unwanted

signal that is common mode to both inputs 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 LTC1412 inputs can be

driven directly. As source impedance increases so will

acquisition time (see Figure 6). For minimum acquisition

time, with high source impedance, a buffer amplifier must

be used. The only 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 50ns for full throughput rate).

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 should 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 LTC1412 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 LTC1412. 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. ±5V to ±15V supplies. Good AC and DC

specifications. 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.

Input Filtering

The noise and the distortion of the input amplifier and

other circuitry must be considered since they will add to

the LTC1412 noise and distortion. The small-signal band-

SOURCE RESISTANCE (Ω)

0.01

ACQUISITION TIME (µs)

0.1

100 1k

1412 F06

10k 100k

Figure 6. Acquisition Time vs Source Resistance

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

LinearView is a trademark of Linear Technology Corporation.

LTC1412

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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 7 shows a 500pF capacitor from A

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

circuitry. High quality capacitors and resistors should be

used since these components can add distortion. NPO and

silver mica type dielectric capacitors have excellent linear-

ity. Carbon surface mount resistors can also generate

distortion from self heating and from damage that may

occur during soldering. Metal film surface mount resis-

tors 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. Figure 7b shows a simple implementa-

tion using an LTC1560-1 fifth-order elliptic continuous

time filter.

Input Range

The ±2.5V input range of the LTC1412 is optimized for low

noise and low distortion. Most op amps also perform best

over this same range, allowing direct coupling to the ana-

log inputs and eliminating the need for special translation

circuitry.

Some applications may require other input ranges. The

LTC1412 differential inputs and reference circuitry can ac-

commodate other input ranges often with little or no addi-

tional circuitry. The following sections describe the reference

and input circuitry and how they affect the input range.

Internal Reference

The LTC1412 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 8a. A

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

easily overdriven by an external reference or other cir-

cuitry, see Figure 8b. The reference amplifier gains the

voltage at the V

REF

pin by 1.625 to create the required

internal reference voltage. This provides buffering be-

tween 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 refer-

ence 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 7b. 1MHz Fifth-Order Elliptic Lowpass Filter

LTC1412

–

REF

REFCOMP

AGND

LTC1560-1

1412 F07b

10µF

–5V 5V

0.1µF 0.1µF

Figure 7a. RC Input Filter

LTC1412

–

REF

REFCOMP

AGND

1412 F07a

10µF

500pF

100Ω

ANALOG INPUT

Figure 8a. LTC1412 Reference Circuit

40k

64k

REFERENCE

AMP

10µF

REFCOMP

AGND

REF

2.500V

4.0625V

LTC1412

1412 F08a

BANDGAP

REFERENCE

LTC1412

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LTC1412

ANALOG INPUT

–

REF

REFCOMP

AGND

1412 F08b

10µF

OUT

LT1019A-2.5

RIPPLE FREQUENCY (Hz)

–80

AMPLITUDE OF POWER SUPPLY FEEDTHROUGH (dB)

–40

–100

–60

–20

10k 100k 1M 10M

1412 G08

–120

DGND

mode voltage. THD will degrade as the inputs approach

either power supply rail, from –86dB with a common

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

or –2.5V.

Full-Scale and Offset Adjustment

Figure 11a shows the ideal input/output characteristics for

the LTC1412. The code transitions occur midway between

successive integer LSB values (i.e., –FS/2 + 0.5LSB,

–FS/2 + 1.5LSB, –FS/2 + 2.5LSB,...FS/2 – 1.5LSB, FS/2 –

0.5LSB). The output is two’s complement binary with

1LSB = FS – (–FS)/4096 = 5V/4096 = 1.22mV.

Differential Inputs

The LTC1412 has a unique differential sample-and-hold

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

convert the difference of A

– (A

–

) independent of the

common mode voltage. The common mode rejection

holds up to extremely high frequencies, see Figure 10. The

only requirement is that both inputs cannot exceed the

or AV

power supply voltages. Integral nonlinearity

errors (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

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 11b

shows the extra components required for full-scale error

adjustment. Zero offset is achieved by adjusting the offset

applied to the A

–

input. For zero offset error apply

Figure 10. CMRR vs Input Frequency

Figure 8b. Using the LT1019-2.5 as an External Reference

The V

REF

pin can be driven with a DAC or other means

shown in Figure 9. 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 LTC1412 reference amplifier will limit the

bandwidth and settling time of this circuit. A settling time

of 5ms should be allowed for after a reference adjustment.

LTC1412

ANALOG INPUT

1.25V TO 3V

DIFFERENTIAL

–

REF

REFCOMP

AGND

1412 F09

10µF

LTC1450

1.25V TO 3V

Figure 9. Driving V

REF

with a DAC

INPUT VOLTAGE (V)

OUTPUT CODE

1412 F11a

111...111

111...110

111...101

000...000

000...001

000...010

FS – 1LSBFS – 1LSB

Figure 11a. LTC1412 Transfer Characteristics

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

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Analog to Digital Converters - ADC 12-B, 3Msps, Smpl A/D Conv

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