Rainbow Electronics MAX100 - Manual (Page 10)

MAX100
250Msps, 8-Bit ADC with Track/Hold
10 ______________________________________________________________________________________
______Definitions of Specifications
Signal-to-Noise Ratio and Effective Bits
Signal-to-noise ratio (SNR) is the ratio between the RMS
amplitude of the fundamental input frequency and the
RMS amplitude of all other analog-to-digital (A/D) output
signals. The theoretical minimum A/D noise is caused by
quantization error and is a direct result of the ADC’s reso-
lution: SNR = (6.02N + 1.76)dB, where N is the number
of effective bits of resolution. Therefore, a perfect 8-bit
ADC can do no better than 50dB. The FFT plots in the
Typical Operating Characteristics show the output level in
various spectral bands.
Effective bits is calculated from a digital record taken from
the ADC under test. The quantization error of the ideal
converter equals the total error of the device. In addition
to ideal quantization error, other sources of error include
all DC and AC nonlinearities, clock and aperture jitter,
missing output codes, and noise. Noise on references
and supplies also degrades effective bits performance.
The ADC’s input is a sine wave filtered with an anti-alias-
ing filter to remove any harmonic content. The digital
record taken from this signal is compared against a
mathematically generated sine wave. DC offsets, phase,
and amplitudes of the mathematical model are adjusted
until a best-fit sine wave is found. After subtracting this
sine wave from the digital record, the residual error
remains. The rms value of the error is applied in the fol-
lowing equation to yield the ADC’s effective bits.
measured rms error
Effective bits = N - log
2
(
—————————
)
ideal rms error
where N is the resolution of the converter. In this case,
N = 8.
The worst-case error for any device will be at the con-
verter’s maximum clock rate with the analog input near
the Nyquist rate (1/2 the input clock rate).
Aperture Width and Jitter
Aperture width is the time the T/H circuit takes to dis-
connect the hold capacitor from the input circuit (i.e., to
turn off the sampling bridge and put the T/H in hold
mode). Aperture jitter is the sample-to-sample variation
in aperture delay (Figure 5).
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 for any one of the
input comparators. The resulting output code for many
typical converters can be incorrect, including false full-
or zero-scale output. The MAX100’s unique design
reduces the magnitude of this type of error to 1LSB,
and reduces the probability of the error occurring to
less than one in every 10
15 clock cycles. If the
MAX100 were operated at 250MHz, 24 hours a day,
this would translate to less than one metastable-state
error every 46 days.
Integral Nonlinearity
Integral nonlinearity (INL) is the deviation of the transfer
function from a reference line measured in fractions of
1LSB using a “best straight line” determined by a least
square curve fit.
Differential Nonlinearity
Differential nonlinearity (DNL) is the difference between
the measured LSB step and an ideal LSB step size
between adjacent code transitions. DNL is expressed
in LSBs and is calculated using the following equation:
[V MEAS - (V MEAS-1 )] - LSB
DNL(LSB) = —————————————
LSB
where V MEAS-1 is the measured value of the previous
code.
A DNL specification of less than 1LSB guarantees no
missing codes and a monotonic transfer function.
SAMPLED
DATA (T/H)
T/H
CLK
CLK
ANALOG
INPUT
t AD
TRACK
t AJ
t AW
TRACK HOLD
APERTURE DELAY (t AD )
APERTURE WIDTH (t AW )
APERTURE JITTER (t AJ )
Figure 5. T/H Aperture Timing