LTC1286CS8 datasheet(18/24 Pages) LINER

18 / 24 page

LTC1286/LTC1298

0.2V reference. If this offset is unacceptable, it can be

corrected digitally by the receiving system or by offsetting

the “–” input of the LTC1286.

Noise with Reduced VREF

The total input referred noise of the LTC1286 can be

reduced to approximately 400

µV peak-to-peak using a

ground plane, good bypassing, good layout techniques

and minimizing noise on the reference inputs. This noise

is insignificant with a 5V reference but will become a larger

fraction of an LSB as the size of the LSB is reduced.

For operation with a 5V reference, the 400

µV noise is

only 0.33LSB peak-to-peak. In this case, the LTC1286

noise will contribute virtually no uncertainty to the

output code. However, for reduced references the noise

may become a significant fraction of an LSB and cause

undesirable jitter in the output code. For example, with

a 2.5V reference this same 400

µV noise is 0.66LSB

peak-to-peak. This will reduce the range of input volt-

ages over which a stable output code can be achieved by

1LSB. If the reference is further reduced to 1V, the 400

µV

noise becomes equal to 1.65LSBs and a stable code may

be difficult to achieve. In this case averaging multiple

readings may be necessary.

This noise data was taken in a very clean setup. Any setup

induced noise (noise or ripple on VCC, VREF or VIN) will add

to the internal noise. The lower the reference voltage to be

used the more critical it becomes to have a clean, noise free

setup.

Conversion Speed with Reduced VREF

With reduced reference voltages, the LSB step size is

reduced and the LTC1286 internal comparator over-

drive is reduced. Therefore, it may be necessary to

reduce the maximum CLK frequency when low values

of VREF are used.

DYNAMIC PERFORMANCE

The LTC1286/LTC1298 have exceptional sampling capa-

bility. Fast Fourier Transform (FFT) test techniques are

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

APPLICATION INFORMATION

tortion 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

11 shows a typical LTC1286 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 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 12.5kHz sampling rate.

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 12.5kHz with a 5V supply, the LTC1286

maintains above 11 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.

FREQUENCY (kHz)

–60

–40

LTC 1286/98 G21

–80

–100

–120

–140

–20

TA = 25°C

VCC = VREF = 5V

fIN = 5kHz

fCLK = 200kHz

fSMPL = 12.5kHz

Figure 11. LTC1286 Non-Averaged, 4096 Point FFT Plot

LTC1286CS8 датащи(PDF) 18 Page - Linear Technology

LTC1286CS8 датащи(HTML) 18 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

датащи скачать

ссылки URL