8

LTC1406

where ENOB is the effective number of bits and S/(N + D)

is expressed in dB. At the maximum sampling rate of 20MHz

the LTC1406 maintains near ideal ENOBs up to and be-

yond the Nyquist input frequency of 10MHz (see Figure 3).

FREQUENCY (Hz)

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

1406 F02a

0123

4

5

67

8

9

10

SFDR = 64.8dB

SINAD = 48.6dB

Figure 2a. Nonaveraged, 4096 Point FFT

Input Frequency = 1MHz

Signal-to-Noise Ratio

The signal-to-noise plus distortion ratio [S/(N + D)] is the

ratio between the RMS amplitude of the fundamental input

frequency and the RMS amplitude of all other frequency

components at the ADC output. The output is band limited

to frequencies above DC to below half the sampling fre-

quency. The effective number of bits (ENOBs) is a mea-

surement of the resolution of an ADC and is directly related

to the S/(N + D) by the equation:

ENOB = [S/(N + D) – 1.76]/6.02

Figure 2b. Nonaveraged, 4096 Point FFT

Input Frequency = 30MHz

Total Harmonic Distortion

Total harmonic distortion is the ratio of the RMS sum of all

harmonics of the input signal to the fundamental itself. The

out-of-band harmonics alias into the frequency band

between DC and half the sampling frequency. THD is

expressed as:

THD

VV

=

+++

20

3

2

4

2

log

V

. . .V

V

2

2

n

2

1

shown in Figure 4. The LTC1406 has good distortion per-

formance 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 fre-

quency (see Figure 5).

INPUT FREQUENCY (Hz)

2

4

6

100k

1M

10M

100M

1406 TA02

0

3

5

7

1

850

44

38

32

Figure 3. Effective Bits and Signal-to-(Noise + Distortion)

vs Input Frequency

FREQUENCY (Hz)

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

1406 F02b

0123

4

5

67

8

9

10

SFDR = 54.9dB

SINAD = 47.0dB