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LTC1285/LTC1288

APPLICATION INFORMATION

Figure 12. Effective Bits and S/(N + D) vs Input Frequency

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 fre-

quency components at the ADC’s output. The output is

band limited to frequencies above DC and below one half

the sampling frequency. Figure 12 shows a typical spec-

tral content with a 7.5kHz sampling rate.

INPUT FREQUENCY (kHz)

100

LTC1285/88 • TPC12

TA = 25°C

VCC = 2.7V

fCLK = 120kHz

THD

+++ +

20log

VVV

...

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through VN are the amplitudes of the

second through the Nth harmonics. The typical THD speci-

fication in the Dynamic Accuracy table includes the 2nd

through 5th harmonics. With a 1kHz input signal, the

LTC1285/LTC1288 have typical THD of 80dB with

VCC = 2.7V.

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 3rd order IMD terms include (2fa + fb),

(2fa – fb), (fa + 2fb), and (fa – 2fb). If the two input sine

waves are equal in magnitudes, the value (in dB) of the 2nd

order IMD products can be expressed by the following

formula:

IMD f

mplitude f

()=

()













20log

amplitude at fa

For input frequencies of 2.05kHz and 3.05kHz, the IMD of

the LTC1285/LTC1288 is 72dB with a 2.7V supply.

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 dBs relative to the RMS value of a full-

scale input signal.

Effective Number of Bits

The Effective Number of Bits (ENOBs) is a measurement of

the resolution of an ADC and is directly related to S/(N+D)

by the equation:

ENOB = [S/(N + D) – 1.76]/6.02

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

sampling rate of 7.5kHz with a 2.7V supply, the LTC1285

maintains above 10.7 ENOBs at 10kHz input frequency.

Above 10kHz the ENOBs gradually decline, as shown in

Figure 12, due to increasing second harmonic distortion.

The noise floor remains low.

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 of the sampling frequency. THD

is defined as:

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