9

LTC1279

pared with the binary-weighted charges supplied by the

capacitive DAC. Bit decisions are made by the high speed

comparator. At the end of a conversion, the DAC output

12-bit output latches.

DYNAMIC PERFORMANCE

The LTC1279 has excellent high speed sampling capabil-

ity. FFT (Fast Fourier Transform) test techniques are used

to test the ADC’s frequency response, distortion and

noise at the rated throughput. By applying a low distor-

tion sine wave and analyzing the digital output using an

FFT algorithm, the ADC’s spectral content can be exam-

ined for frequencies outside the fundamental. Figures 2a

and 2b show typical LTC1279 FFT plots.

Figure 2b. LTC1279 Nonaveraged, 4096 Point FFT Plot

with 300kHz 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 frequency

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

to frequencies above DC and below half the sampling

frequency. Figure 2a shows a typical spectral content with

a 600kHz sampling rate and a 100kHz input. The dynamic

performance is excellent for input frequencies up to the

Nyquist limit of 300kHz as shown in Figure 2b.

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 600kHz the LTC1279 maintains verygoodENOBsup

to the Nyquist input frequency of 300kHz. Refer to Figure 3.

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:

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

Input Frequency

FREQUENCY (Hz)

10k

100k

1M

5M

1279 G03

12

11

10

9

8

7

6

5

4

3

2

1

0

74

68

62

56

50

NYQUIST

FREQUENCY

FREQUENCY (kHz)

0

50

100

150

200

1279 F02a

250

300

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

FREQUENCY (kHz)

0

50

100

150

200

1279 F02

250

300

0

–10

–20

–30

–40

–50

–60

–70

–80

–90

–100

–110

–120

Figure 2a. LTC1279 Nonaveraged, 4096 Point FFT Plot

with 100kHz Input Frequency