–5–

REV. PrE

AD7680

PRELIMINARY TECHNICAL DATA

TERMINOLOGY

Integral Nonlinearity

This is the maximum deviation from a straight line pass-

ing through the endpoints of the ADC transfer function.

The endpoints 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 + 1LSB

Gain Error

This is the deviation of the last code transition (111 . . .

LSB) after the offset error has been adjusted out.

Track/Hold Acquisition Time

The track/hold amplifier returns into track mode at the

end of conversion. Track/Hold acquisition time is the

time required for the output of the track/hold amplifier

to reach its final value, within ±0.5 LSB, after the end

of conversion. See serial interface timing section for

more details.

Signal to (Noise + Distortion) Ratio

This is the measured ratio of signal to (noise + distor-

tion) 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

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 16-bit converter, this is 98 dB.

Total Harmonic Distortion

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

sum of harmonics to the fundamental. For the AD7860, it

is defined as:

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

value of the fundamental. Normally, the value of this

specification is determined 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 are

equal to zero. For example, the second order terms in-

clude (fa + fb) and (fa – fb), while the third order terms

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

The AD7680 is tested using the CCIF standard where two

input frequencies nearthe 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 dis-

tortion products to the rms amplitude of the sum of the

fundamentals expressed in dBs.

THD (dB )

= 20 log

3

4

5

6

2