8

LTC1419

frequencies outside the fundamental. Figure 2 shows a

typical LTC1419 FFT plot.

Effective Number of Bits

The effective number of bits (ENOBs) is a measurement of

the resolution of an ADC and is directly related to the

S/(N + D) by the equation:

N = [S/(N + D) – 1.76]/6.02

where N is the effective number of bits of resolution and

S/(N + D) is expressed in dB. At the maximum sampling

rate of 800kHz the LTC1419 maintains near ideal ENOBs

up to the Nyquist input frequency of 400kHz (refer to

Figure 3).

FREQUENCY (kHz)

0

–60

–40

–20

300

1419 F02a

–80

–100

100

200

400

250

50

150

350

–120

–140

0

SFDR = 98dB

THD = – 93.3dB

Figure 2a. LTC1419 Nonaveraged, 4096 Point FFT,

Input Frequency = 100kHz

FREQUENCY (kHz)

0

–60

–40

–20

300

1419 F02b

–80

–100

100

200

400

250

50

150

350

–120

–140

0

SFDR = 88.3dB

SINAD = 80.1

Figure 2b. LTC1419 Nonaveraged, 4096 Point FFT,

Input Frequency = 375kHz

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 to the RMS amplitude of all other frequency

components at the A/D output. The output is band limited

to frequencies from above DC and below half the sampling

frequency. Figure 2 shows a typical spectral content with

a 800kHz sampling rate and a 100kHz input. The dynamic

performance is excellent for input frequencies up to and

beyond the Nyquist limit of 400kHz.

Total Harmonic Distortion

Total harmonic distortion (THD) 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

Log

VV

V

Vn

V

=

+++…

20

23

4

1

222

2

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through Vn are the amplitudes of the

second through nth harmonics. THD vs Input Frequency is

shown in Figure 4. The LTC1419 has good distortion

performance up to the Nyquist frequency and beyond.

INPUT FREQUENCY (Hz)

10k

100k

1M 2M

1419 TA02

1k

14

13

12

11

10

9

8

7

6

5

4

3

2

86

80

74

68

62

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

vs Input Frequency