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LTC1282

APPLICATI

I FOR ATIO

DYNAMIC PERFORMANCE

The LTC1282 has exceptionally high speed sampling capa-

bility. FFT (Fast Fourier Transform) test techniques are

used to characterize the ADC’s frequency response, distor-

tion and noise at the rated throughput. By applying a low

distortion sine wave and analyzing the digital output using

an FFT algorithm, the ADC’s spectral content can be

examined for frequencies outside the fundamental. Figure

2 shows a typical LTC1282 FFT plot.

Signal-to-(Noise + Distortion) 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 LTC1282 FFT plot.

Figure 2. LTC1282 Nonaveraged, 1024 Point FFT Plot

INPUT FREQUENCY (Hz)

100k

10M

LTC1282 • F03

10k

fSAMPLE = 160kHz

VS = ±2.7V BIPOLAR

BIPOLAR

(±1.25V

INPUT)

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 = 20log

√V

2 + V32 + V42 ... + VN2

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through VN are the amplitudes of the

second through Nth harmonics. The typical THD specifi-

cation in the Dynamic Accuracy table includes the 2nd

through 5th harmonics. With a 70kHz input signal, the

LTC1282 has a typical – 82dB THD as shown in Figure 4.

Figure 3. ENOBs and S/(N + D) vs Input Frequency

INPUT FREQUENCY (Hz)

–80

–60

–40

–20

100k

10M

LTC1282 • F04

–100

10k

–90

–70

–50

–30

–10

2nd HARMONIC

THD

fSAMPLE = 140kHz

±3V SUPPLIES

BIPOLAR

3rd HARMONIC

Figure 4. Distortion vs Input Frequency (Bipolar)

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:

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 140kHz the LTC1282 maintains 11.3 ENOBs at

70kHz input frequency. Refer to Figure 3.

FREQUENCY (kHz)

LT1282 • F02

–20

–40

–60

–80

–100

–120

fSAMPLE = 160kHz

VDD = 3V

UNIPOLAR

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