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LTC1604

APPLICATIONS INFORMATION

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 18a shows a typical spectral content

with a 333kHz sampling rate and a 5kHz input. The

dynamic performance is excellent for input frequencies up

to and beyond the Nyquist limit of 167kHz.

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 333kHz the LTC1604 maintains above 14 bits up to

the Nyquist input frequency of 167kHz (refer to Figure 19).

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

VVV

...

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 20. The LTC1604 has good distortion

performance up to the Nyquist frequency and beyond.

FREQUENCY (kHz)

–60

–40

–20

1604 F18b

–80

–100

–120

–140

060

80 100 120 140 160

fSAMPLE = 333kHz

fIN = 97.152kHz

SINAD = 89dB

THD = –96dB

FREQUENCY (Hz)

10k

100k

1604 F19

Figure 18b. Even with Inputs at 100kHz, the LTC1604’s

Dynamic Linearity Remains Robust

INPUT FREQUENCY (Hz)

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

1604 G04

10k

100k

THD

3RD

2ND

Figure 20. Distortion vs Input Frequency

Figure 19. Effective Bits and Signal/(Noise + Distortion)

vs Input Frequency

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