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Differential nonlinearity, Aperture jitter, Aperture delay – Rainbow Electronics MAX1230 User Manual

Page 20: Signal-to-noise ratio, Signal-to-noise plus distortion, Effective number of bits

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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

*V

COM

≥ V

REF

/ 2

+

V

COM

+

V

COM

FS

=

V

REF

2

-FS =

-V

REF

2

1 LSB =

V

REF

4096

OUTPUT CODE

FULL-SCALE

TRANSITION

11 . . . 111

11 . . . 110

11 . . . 101

00 . . . 011

00 . . . 010

00 . . . 001

00 . . . 000

1

2

3

0

(COM)

FS

FS - 3/2 LSB

FS = V

REF

+ V

COM

ZS = V

COM

INPUT VOLTAGE (LSB)

1 LSB =

V

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

AJ

) is the sample-to-sample variation in

the time between the samples.

Aperture Delay

Aperture delay (t

AD

) 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