EVAL-AD7482CB 查看數據表（PDF） - Analog Devices

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EVAL-AD7482CB

3 MSPS, 12-Bit SAR ADC

Analog Devices 

AD7482

TERMINOLOGY

Integral Nonlinearity

This is the maximum deviation from a straight line passing

through the endpoints of the ADC transfer function. The end-

points of the transfer function are zero scale, a point 1/2 LSB

below the first code transition, and full scale, a point 1/2 LSB

above the last code transition.

Differential Nonlinearity

This is the difference between the measured and ideal 1 LSB

change between any two adjacent codes in the ADC.

Offset Error

This is the deviation of the first code transition (00 . . . 000) to

(00 . . . 001) from the ideal, i.e., AGND + 0.5 LSB.

Gain Error

This is the deviation of the last code transition (111 . . . 110) to

(111 . . . 111) from the ideal (i.e., VREF – 1.5 LSB) after the

offset error has been adjusted out.

Track-and-Hold Acquisition Time

Track-and-hold acquisition time is the time required for the

output of the track-and-hold amplifier to reach its final value,

within ±1/2 LSB, after the end of conversion (the point at

which the track-and-hold returns to track mode).

Signal-to-(Noise + Distortion) Ratio

This is the measured ratio of signal-to-(noise + distortion) at

the output of the A/D converter. The signal is the rms amplitude

of the fundamental. Noise is the sum of all nonfundamental

signals up to half the sampling frequency (fS/2), excluding dc.

The ratio is dependent on the number of quantization levels in

the digitization process; the more levels, the smaller the quanti-

zation noise. The theoretical signal-to-(noise + distortion) ratio for

an ideal N-bit converter with a sine wave input is given by:

( ) Signal−to−(Noise + Distortion) = 6.02N + 1.76 dB

Thus, for a 12-bit converter this is 74 dB.

Total Harmonic Distortion

Total harmonic distortion (THD) is the ratio of the rms sum

of the harmonics to the fundamental. For the AD7482, it is

defined as:

THD(dB) = 20 log V22 +V32 +V42 +V52 +V62

V1

where V1 is the rms amplitude of the fundamental and V2, V3,

V4, V5, and V6 are the rms amplitudes of the second through the

sixth harmonics.

Peak Harmonic or Spurious Noise

Peak harmonic or spurious noise is defined as the ratio of the

rms value of the next largest component in the ADC output spec-

trum (up to fS/2 and excluding dc) to the rms value of the

fundamental. Normally, the value of this specification is deter-

mined by the largest harmonic in the spectrum, but for ADCs

where the harmonics are buried in the noise floor, it will be a

noise peak.

Intermodulation Distortion

With inputs consisting of sine waves at two frequencies, fa and

fb, any active device with nonlinearities will create distortion

products at sum and difference frequencies of mfa ± nfb, where

m and n = 0, 1, 2, 3, and so on. Intermodulation distortion

terms are those for which neither m nor n are equal to zero.

For example, the second order terms include (fa + fb) and

(fa – fb), while the third order terms include (2fa + fb),

(2fa – fb), (fa + 2fb), and (fa – 2fb).

The AD7482 is tested using the CCIF standard, where two

input frequencies near the top end of the input bandwidth are

used. In this case, the second order terms are usually distanced

in frequency from the original sine waves, while the third order

terms are usually at a frequency close to the input frequencies.

As a result, the second and third order terms are specified sepa-

rately. The calculation of the intermodulation distortion is as

per the THD specification, where it is the ratio of the rms sum

of the individual distortion products to the rms amplitude of the

sum of the fundamentals expressed in dBs.

–6–

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