REV.

–6–

AD7485

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

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

offset error has been adjusted out.

Track/Hold Acquisition Time

Track/hold acquisition time is the time required for the output

of the track/hold amplifier to reach its final value, within

±1/2 LSB,

after the end of conversion (the point at which the track/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

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

N

dB

()

.

.

+=

+

()

602

176

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

Total Harmonic Distortion

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

of the harmonics to the fundamental. For the AD7485, it is

defined as:

THD dB

log

VVV

V

V

V

++

+

+

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, and so on. 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 AD7485 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.

A