REV. B

AD7470/AD7472

–9–

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 + 0.5 LSB.

Gain Error

The last transition should occur at the analog value 1.5 LSB

below the nominal full scale. The first transition is a 0.5 LSB

above the low end of the scale (zero in the case of AD7470/

AD7472). The gain error is the deviation of the actual difference

between the first and last code transitions from the ideal differ-

ence between the first and last code transitions with offset errors

removed.

Track-and-Hold Acquisition Time

The track-and-hold amplifier returns into track mode after the

end of conversion. 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 conversion.

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 sig-

ratio is dependent 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 12-bit converter, this is 74 dB and for a 10-bit con-

verter is 62 dB.

Total Harmonic Distortion (THD)

Total harmonic distortion is the ratio of the rms sum of har-

monics to the fundamental. For the AD7470/AD7472 it is

defined as

THD dB

VVVVV

V

()

log

()

=

++++

20

2

2

3

2

4

2

5

2

6

2

1

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

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, n = 0, 1, 2, 3, etc. Intermodulation distortion terms are

those for which neither m nor n is 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 AD7470/AD7472 are tested using the CCIF standard

where two input frequencies near the top end of the input band-

width 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.

Aperture Delay

In a sample-and-hold, the time required after the hold command

for the switch to open fully is the aperture delay. The sample is,

in effect, delayed by this interval, and the hold command would

have to be advanced by this amount for precise timing.

Aperture Jitter

Aperture jitter is the range of variation in the aperture delay.

In other words, it is the uncertainty about when the sample is

taken. Jitter is the result of noise which modulates the phase

of the hold command. This specification establishes the ulti-

mate timing error, hence the maximum sampling frequency

for a given resolution. This error will increase as the input

dV/dt increases.