AD7940BRJ-R2 datasheet(8/20 Pages) AD

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AD7940

Rev. 0 | Page 8 of 20

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 the 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 + 1 LSB.

Gain Error

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

(111 . . . 111) from the ideal (i.e., VREF − 1 LSB) after the offset

error has been adjusted out.

Track-and-Hold Acquisition Time

The track-and-hold amplifier returns to track mode at the end

of conversion. The 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 LSB, after the end of the conversion.

See the Serial Interface section for more details.

Signal-to-(Noise + Distortion) Ratio

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

the output of the ADC. 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

depends on the number of quantization levels in the digitization

process; the more levels, the smaller the quantization 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) = (6.02 N + 1.76) dB

Thus, for a 14-bit converter, this is 86.04 dB.

Total Harmonic Distortion (THD)

THD is the ratio of the rms sum of harmonics to the funda-

mental. For the AD7940, it is defined as

log

(dB)

THD

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

spectrum (up to fS/2, excluding dc) to the rms value of the fun-

damental. 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 prod-

ucts at sum and difference frequencies of mfa ± nfb where m, n =

0, 1, 2, 3. 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 AD7940 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

separately. 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.

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