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LTC1278

12-Bit, 500ksps Sampling A/D Converter with Shutdown

Linear Technology 

LTC1278

APPLICATIONS INFORMATION

compare mode. The input switch switches CSAMPLE to

ground, injecting the analog input charge onto the sum-

ming junction. This input charge is successively com-

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

balances the AIN input charge. The SAR contents (a 12-bit

data word) which represent the AIN are loaded into the

12-bit output latches.

DYNAMIC PERFORMANCE

The LTC1278 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. Figure 2

shows a typical LTC1278 FFT plot.

0

fSAMPLE = 500kHz ±5V

fIN = 97.045kHz

–20

–40

–60

–80

–100

–120

0

50k 100k 150k 200k 250k

FREQUENCY (Hz)

LTC1278 F2

Figure 2. LTC1278 Nonaveraged, 4096 Point FFT Plot

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 500kHz sampling rate and a 100kHz input. The dynamic

performance is excellent for input frequencies up to the

Nyquist limit of 250kHz.

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 500kHz the LTC1278 maintains very good ENOBs up

to the Nyquist input frequency of 250kHz. Refer to Figure 3.

12

74

11

68

10

NYQUIST

62

9

FREQUENCY

56

8

7

6

5

4

3

2

1 fSAMPLE = 500kHz

0

10k

100k

1M 2M

INPUT FREQUENCY (Hz)

LT1278 G4

Figure 3. Effective Bits and Signal-to-Noise + Distortion vs

Input Frequency

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√V22

+

V32

+ V42

V1

...

+

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. THD versus input

9

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