10

LTC1279

THD = 20log

√V

2

second through Nth harmonics. THD versus input fre-

quency is shown in Figure 4. The LTC1279 has good

distortion performance up to the Nyquist frequency and

beyond.

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

following formula:

IMD (fa ± fb) = 20log

Amplitude at (fa ± fb)

Amplitude at fa

INPUT FREQUENCY (Hz)

10k

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

100k

1M

1279 G06

2M

THD

2ND HARMONIC

3RD HARMONIC

Figure 4. Distortion vs Input Frequency

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

Figure 5. Intermodulation Distortion Plot

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 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

LTC1279 has been designed to optimize input bandwidth,

allowing ADC to undersample input signals with frequen-

cies above the converter’s Nyquist Frequency. The noise

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

comes dominated by distortion at frequencies far beyond

Nyquist.

Driving the Analog Input

The LTC1279’s analog input is easy to drive. It draws only

one small current spike while charging the sample-and-

hold capacitor at the end of conversion. During conversion

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 160ns to small current

transients will allow maximum speed operation. If slower

FREQUENCY (kHz)

0

50

100

150

200

1279 G08

250

300

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

fa = 94.189kHz

fb = 97.705kHz

(fb – fa)

(2fa – fb)

(2fa)

(2fb)

(3fa)

(2fb – fa)

(fa) (fb)

(fa + fb) (2fa + fb)

(fa + 2fb)

(3fb)