MAX100 查看數據表（PDF） - Maxim Integrated

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MAX100

250Msps, 8-Bit ADC with Track/Hold

Maxim Integrated 

250Msps, 8-Bit ADC with Track/Hold

______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.

( ) Effective bits = N - log2

m—e—as—ur—ed—rm—s—e—rro—r

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

CLK

CLK

tAW

ANALOG

INPUT

tAD

tAJ

SAMPLED

DATA (T/H)

TRACK

T/H

HOLD

TRACK

APERTURE DELAY (tAD)

APERTURE WIDTH (tAW)

APERTURE JITTER (tAJ)

Figure 5. T/H Aperture Timing

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 1015 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:

[VMEAS - (VMEAS-1)] - LSB

DNL(LSB) = —————————————

LSB

where VMEAS-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.

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