Rainbow-electronics MAX1230 Instrukcja Użytkownika Pobierz pdf (Strona 20)

MAX1226/MAX1228/MAX1230

12-Bit 300ksps ADCs with FIFO,

Temp Sensor, Internal Reference

20 ______________________________________________________________________________________

011 . . . 111

011 . . . 110

000 . . . 010

000 . . . 001

000 . . . 000

111 . . . 111

111 . . . 110

111 . . . 101

100 . . . 001

100 . . . 000

- FS

COM*

INPUT VOLTAGE (LSB)

OUTPUT CODE

ZS = COM

+FS - 1 LSB

COM

≥ V

REF

/ 2

COM

+ V

COM

REF

-FS =

-V

REF

1 LSB =

REF

4096

OUTPUT CODE

FULL-SCALE

TRANSITION

11 . . . 111

11 . . . 110

11 . . . 101

00 . . . 011

00 . . . 010

00 . . . 001

00 . . . 000

123

(COM)

FS - 3/2 LSB

FS = V

REF

+ V

COM

ZS = V

COM

INPUT VOLTAGE (LSB)

1 LSB =

REF

4096

Figure 8. Unipolar Transfer Function, Full Scale (FS) = V

REF

Figure 9. Bipolar Transfer Function, Full Scale (±FS) = ±V

REF

/ 2

Differential Nonlinearity

Differential nonlinearity (DNL) is the difference between

an actual step width and the ideal value of 1 LSB. A

DNL error specification of less than 1 LSB guarantees

no missing codes and a monotonic transfer function.

Aperture Jitter

Aperture jitter (t

) is the sample-to-sample variation in

the time between the samples.

Aperture Delay

Aperture delay (t

) is the time between the rising

edge of the sampling clock and the instant when an

actual sample is taken.

Signal-to-Noise Ratio

For a waveform perfectly reconstructed from digital

samples, signal-to-noise ratio (SNR) is the ratio of the

full-scale analog input (RMS value) to the RMS quanti-

zation error (residual error). The ideal, theoretical mini-

mum analog-to-digital noise is caused by quantization

error only and results directly from the ADC’s resolution

(N bits):

SNR = (6.02 x N + 1.76)dB

In reality, there are other noise sources besides quanti-

zation noise, including thermal noise, reference noise,

clock jitter, etc. Therefore, SNR is calculated by taking

the ratio of the RMS signal to the RMS noise, which

includes all spectral components minus the fundamen-

tal, the first five harmonics, and the DC offset.

Signal-to-Noise Plus Distortion

Signal-to-noise plus distortion (SINAD) is the ratio of the

fundamental input frequency’s RMS amplitude to the

RMS equivalent of all other ADC output signals:

SINAD (dB) = 20 x log (Signal

RMS

/ Noise

RMS

)

Effective Number of Bits

Effective number of bits (ENOB) indicates the global

accuracy of an ADC at a specific input frequency and

sampling rate. An ideal ADC error consists of quantiza-

tion noise only. With an input range equal to the full-

scale range of the ADC, calculate the effective number

of bits as follows:

ENOB = (SINAD - 1.76) / 6.02

1 2 ... 15 16 17 18 19 20 21 22 23 24

MAX1226/MAX1228/MAX1230 1
5x5x0.90 MM 24

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