LTC1404CS8 datasheet(12/24 Pages) LINER

12 / 24 page

LTC1404

APPLICATIONS INFORMATION

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 sum and difference

frequencies of mfa

± nfb, where m and n = 0, 1, 2, 3, etc.

For example, the 2nd order IMD terms include (fa + fb) and

(fa – fb) while the 3rd order IMD terms includes (2fa + fb),

(2fa – fb), (fa + 2fb) and (fa – 2fb). If the two input sine

waves are equal in magnitude, the value (in decibels) of the

2nd order IMD products can be expressed by the following

formula.

IMD fa

() =

20log

Amplitude at (

)

Amplitude at fa

Figure 5 shows the IMD performance at a 100kHz input.

Figure 5. Intermodulation Distortion Plot in Bipolar Mode

FREQUENCY (kHz)

100

140

180

120

160

220

200

240 260 280 300

1404 F05

–10

–20

–30

–40

–50

– 60

–70

–80

–90

–100

–110

–120

fSAMPLE = 600kHz

fa = 99.16992188kHz

fb = 102.6855469kHz

3fa

2fb – fa

2fa – fb

2fa + fb

2fa

3fb

fa + fb

2fb

2fb + 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 the 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 68dB (11 effective bits). The

LTC1404 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 analog input of the LTC1404 is easy to drive. It draws

only one small current spike while charging the sample-

and-hold capacitor at the end of a conversion. During

conversion, the analog input draws only a small leakage

current. The only requirement is that the amplifier driving

the analog input must settle after the small current spike

before the next conversion starts. Any op amp that settles

in 160ns to small load current transient will allow maxi-

mum speed operation. If a slower op amp is used, more

settling time can be provided by increasing the time

between conversions. Suitable devices capable of driving

the ADC’s AIN input include the LT

® 1360 and the LT1363

op amps.

The LTC1404 comes with a built-in unipolar/bipolar detec-

tion circuit. If the VSS potential is forced below GND, the

internal circuitry will automatically switch to bipolar mode.

The following list is a summary of the op amps that are

suitable for driving the LTC1404, more detailed informa-

tion is available in the Linear Technology databooks and

the LinearView

TM CD-ROM.

LT 1215/LT1216: Dual and quad 23MHz, 50V/

µs single

supply op amps. Single 5V to

±15V supplies, 6.6mA

specifications, 90ns settling to 0.5LSB.

LT1223: 100MHz video current feedback amplifier.

±5V

±15V supplies, 6mA supply current. Low distortion up

to and above 600kHz. Low noise. Good for AC applica-

tions.

LT1227: 140MHz video current feedback amplifier.

±5V

±15V supplies, 10mA supply current. Lowest distor-

tion at frequencies above 600kHz. Low noise. Best for AC

applications.

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