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LTC1412IG

12-Bit, 3Msps, Sampling A/D Converter

Linear Technology 

LTC1412

APPLICATIONS INFORMATION

SAMPLE

AIN+

SAMPLE

AIN–

CSAMPLE+

HOLD

CSAMPLE–

HOLD

CDAC+

VDAC+

CDAC–

ZEROING SWITCHES

HOLD

HOLD

+

COMP

–

VDAC–

12

SAR

OUTPUT

LATCHES

•••

D11

D0

1412 F01

Figure 1. Simplified Block Diagram

sample-and-hold capacitors to acquire the analog signal.

During the convert phase the comparator zeroing switches

open, putting the comparator into compare mode. The

input switches connect the CSAMPLE capacitors to ground,

transferring the differential analog input charge onto the

summing junction. This input charge is successively com-

pared with the binary-weighted charges supplied by the

differential capacitive DAC. Bit decisions are made by the

high speed comparator. At the end of a conversion, the

differential DAC output balances the AIN+ and AIN– input

charges. The SAR contents (a 12-bit data word) which

represents the difference of AIN+ and AIN– are loaded into

the 12-bit output latches.

Dynamic Performance

The LTC1412 has excellent high speed sampling capabil-

ity. FFT (Fast Four Transform) test techniques are used to

test the ADC’s frequency response, distortion and noise at

the rated throughput. By applying a low distortion sine

wave and analyzing the digital output using an FFT algo-

rithm, the ADC’s spectral content can be examined for

frequencies outside the fundamental. Figure 2 shows a

typical LTC1412 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

0

fSMPL = 3Msps

fIN = 97.412kHz

– 20

SFDR = 93.3dB

SINAD = 73dB

– 40

– 60

– 80

– 100

– 120

0

200 400 600 800 1000 1200 1400

FREQUENCY (kHz)

1412 F02a

Figure 2a. LTC1412 Nonaveraged, 4096 Point FFT,

Input Frequency = 100kHz

0

fSMPL = 3Msps

fIN = 1.419kHz

–20 SFDR = 83dB

SINAD = 72.5dB

SNR = 73db

– 40

– 60

– 80

– 100

– 120

0

200 400 600 800 1000 1200 1400

FREQUENCY (kHz)

1412 F02B

Figure 2b. LTC1412 Nonaveraged, 4096 Point FFT,

Input Frequency = 1.45MHz

to frequencies from above DC and below half the sampling

frequency. Figure 2 shows a typical spectral content with

a 3MHz sampling rate and a 100kHz input. The dynamic

performance is excellent for input frequencies up to and

beyond the Nyquist limit of 1.5MHz.

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 3MHz the LTC1412 maintains near ideal ENOBs up

to the Nyquist input frequency of 1.5MHz. Refer to

Figure␣ 3.

8

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