Rainbow Electronics MAX108 - Manual (Page 27)

Bit Error Rates
Errors resulting from metastable states may occur when
the analog input voltage (at the time the sample is
taken) falls close to the decision point of any one of the
input comparators. Here, the magnitude of the error
depends on the location of the comparator in the com-
parator network. If it is the comparator for the MSB, the
error will reach full scale. The MAX108’s unique encod-
ing scheme solves this problem by limiting the magni-
tude of these errors to 1LSB.
Dynamic Parameter Definitions
Signal-to-Noise Ratio
For a waveform perfectly reconstructed from digital
samples, the theoretical maximum SNR is the ratio of
the full-scale analog input (RMS value) to the RMS
quantization error (residual error). The ideal, theoretical
minimum analog-to-digital noise is caused by quantiza-
tion error only and results directly from the ADC’s reso-
lution (N bits):
SNR (MAX) = (6.02 N + 1.76)dB
In reality, there are other noise sources besides quanti-
zation noise: thermal noise, reference noise, clock jitter,
etc. SNR is calculated by taking the ratio of the RMS
signal to the RMS noise, which includes all spectral
components minus the fundamental, the first five har-
monics, and the DC offset.
Effective Number of Bits
ENOB indicates the global accuracy of an ADC at a
specific input frequency and sampling rate. An ideal
ADC’s error consists of quantization noise only. ENOB
is calculated from a curve fit referenced to the theoreti-
cal full-scale range.
Signal-to-Noise Plus Distortion
Signal-to-Noise plus distortion (SINAD) is calculated
from the ENOB as follows:
SINAD = (6.02 ENOB) + 1.76
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the RMS
sum of the first five harmonics of the input signal to the
fundamental itself. This is expressed as:
where V 1 is the fundamental amplitude, and V 2 through
V 5 are the amplitudes of the 2nd- through 5th-order
harmonics.
Spurious-Free Dynamic Range
Spurious-free dynamic range (SFDR) is the ratio,
expressed in decibels, of the RMS amplitude of the fun-
damental (maximum signal component) to the RMS
value of the next largest spurious component, exclud-
ing DC offset.
Intermodulation Distortion
The two-tone intermodulation distortion (IMD) is the
ratio, expressed in decibels, of either input tone to the
worst 3rd-order (or higher) intermodulation products.
The input tone levels are at -7dB full scale.
THD 20 log V V V V / V
2
2
3
2
4
2
5
2
1
= + + +
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MAX108
±5V, 1.5Gsps, 8-Bit ADC with
On-Chip 2.2GHz Track/Hold Amplifier
______________________________________________________________________________________ 27
Chip Information
TRANSISTOR COUNT: 20,486
SUBSTRATE CONNECTED TO V EE