LTC1282ACSW datasheet(12/24 Pages) LINER

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LTC1282

APPLICATI

I FOR ATIO

Intermodulation Distortion

If the ADC input signal consists of more than one spectral

component, the ADC transfer function nonlinearity can

produce intermodulation distortion (IMD) in addition to

THD. IMD is the change in one sinusoidal input caused by

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 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 include (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 follow-

ing formula:

IMD (fa ± fb) = 20log

Amplitude at (fa ± fb)

Amplitude at fa

Figure 5 shows the IMD performance at a 20kHz input.

FREQUENCY (kHz)

–120

–100

–80

–60

–40

LTC1282 • F05

–20

fSAMPLE = 160kHz

fIN1 = 19.0kHz

fIN2 = 20.6kHz

VDD = 3V

UNIPOLAR

Figure 5. Intermodulation Distortion Plot

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 68dB (11 effective bits). The

LTC1282 has been designed to optimize input bandwidth,

allowing the ADC to undersample input signals with fre-

quencies above the converter’s Nyquist Frequency.

Driving the Analog Input

The analog input of the LTC1282 is easy to drive. It draws

only one small current spike while charging the sample-

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

version the analog input draws no 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 1.14

µs to

small current transients will allow maximum speed opera-

tion. If slower op amps are 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

®1190/LT1191, LT1007, LT1220, LT1223

and LT1224 op amps.

The analog input tolerates source resistance very well.

Here again, the only requirement is that the analog input

must settle before the next conversion starts. For larger

source resistance, full accuracy can be obtained if more

time is allowed between conversions.

Internal Reference

The LTC1282 has an on-chip, temperature compensated,

curvature corrected, bandgap reference which is factory

trimmed to 1.20V. It is internally connected to the DAC and

is available at pin 2 to provide up to 0.3mA current to an

external load.

For minimum code transition noise the reference output

should be decoupled with a capacitor to filter wideband

noise from the reference (10

µF tantalum in parallel with a

0.1

µF ceramic).

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.

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