LTC1415I 查看數據表（PDF） - Linear Technology

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LTC1415I

12-Bit, 1.25Msps, 55mW Sampling A/D Converter

Linear Technology 

LTC1415

APPLICATIONS INFORMATION

the presence of another sinusoidal input at a different

frequency.

If two pure sine waves of frequencies fa and fb are applied

to the ADC input, nonlinearities in the ADC transfer func-

tion can create distortion products at the sum and differ-

ence frequencies of mfa + – nfb, where m and n = 0, 1, 2,

3, etc. For example, the 2nd order IMD terms include

(fa + fb). If the two input sine waves are equal in magni-

tude, the value (in decibels) of the 2nd order IMD products

can be expressed by the following formula:

IMD(fa

+

fb) =

20Log

Amplitude at

Amplitude

(fa + fb)

at fa

Peak Harmonic or Spurious Noise

The peak harmonic or spurious noise is the largest spec-

tral component excluding the input signal and DC. This

value is expressed in decibels relative to the RMS value of

a full-scale input signal.

Full-Power and Full-Linear Bandwidth

The full-power bandwidth is that input frequency at which

the amplitude of the reconstructed fundamental is

reduced 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 68dB (11 effective bits). The

LTC1415 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

beyond Nyquist.

Driving the Analog Input

The differential analog inputs of the LTC1415 are easy to

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

ended input (i.e., the –AIN input is grounded). The +AIN and

–AIN 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 LTC1415 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 should

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

150ns for full throughput rate).

10

1

0.1

0.01

0.01

0.1

1

10

SOURCE RESISTANCE (kΩ)

100

1415 F06

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 band-

width 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 20MHz 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 LTC1415 will

depend 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.

10

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