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LTC1411IG

Single Supply 14-Bit 2.5Msps ADC

Linear Technology 

LTC1411

APPLICATIO S I FOR ATIO

Peak Harmonic or Spurious Noise

100

The peak harmonic or spurious noise is the largest spec-

tral component excluding the input signal and DC. This

10

value is expressed in dB relative to the RMS value of a full-

scale input signal.

1

Full-Power and Full-Linear Bandwidth

The full-power bandwidth is that input frequency at which

the amplitude of the reconstructed fundamental is re-

duced by 3db for a full-scale input signal.

The full-linear bandwidth is the input frequency at which

the S/(N + D) has dropped to 74dB (12 effective bits). The

LTC1411 has been designed to optimize input bandwidth,

allowing the ADC to undersample input signals with fre-

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 be-

yond Nyquist.

Driving the Analog Input

The differential analog inputs of the LTC1411 are easy to

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

ended input (i.e., the AIN– input is tied to a fixed DC voltage

such as the REFOUT pin of the LTC1411 or an external

source). Figure 1 shows a simplified block diagram for the

analog inputs of the LTC1411. The AIN+ and AIN– 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 charg-

ing the sample-and-hold capacitors at the end of conver-

sion. During conversion, the analog inputs draw only a

small leakage current. If the source impedance of the

driving circuits is low, then the LTC1411 inputs can be

driven directly. More acquisition time should be allowed

for a higher impedance source. Figure 5 shows the acqui-

sition time versus 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

0.1

0.01

1

10 100 1000 10000 100000

SOURCE RESISTANCE (Ω)

1411 G16

Figure 5. Acquisition Time vs Source Resistance

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 LTC1411 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 LTC1411. More detailed information is

available in the Linear Technology Databooks and on the

LinearViewTM CD-ROM.

LT®1227: 140MHz Video Current Feedback Amplifier.

10mA supply current. ±5V to ±15V supplies. Low noise.

Good for AC applications.

LT1395: 400MHz Current Feedback Amplifier. Single 5V

or ±5V supplies. Good for AC applications.

LT1800: 80MHz, 25V/µs Low Power Rail-to-Rail Input and

Output Precision Op Amp. Specified at 3V, 5V and ±5V

supplies. Excellent DC performance.

LinearView is a trademark of Linear Technology Corporation.

1411f

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