–6–

AD7896

TERMINOLOGY

Relative Accuracy

This is the maximum deviation from a straight line passing

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

AGND + 1/2 LSB), a point 1/2 LSB below the first code transi-

transition (11 ... 110 to 11 ... 111).

Differential Nonlinearity

This is the difference between the measured and the ideal 1 LSB

change between any two adjacent codes in the ADC.

Unipolar Offset Error

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

Positive Full-Scale Error

This is the deviation of the last code transition (11 ... 110 to

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 into track mode). It also applies to a

situation where there is a step input change on the input voltage

the user must wait for the duration of the track-and-hold acquisi-

tion time after the end of conversion or after a step input change

operates to specification.

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 fun-

damental. Noise is the sum of all nonfundamental signals up to

dependent on the number of quantization levels in the digitiza-

tion 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) = (6.02N + 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

harmonics to the fundamental. For the AD7896, it is defined as:

THD dB

VV

V

V

V

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 parts

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 include (fa + fb) and (fa – fb), while the

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

(fa – 2fb).

The AD7896 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

fundamental expressed in dB.

Rev. D