REV. A

AD7899

–7–

TERMINOLOGY

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 rms sum of all nonfundamental

The ratio is dependent upon the number of quantization levels

in the digitization process; the more levels, the smaller the quan-

tization 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 14-bit converter, this is 86.04 dB.

Total Harmonic Distortion

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

harmonics to the fundamental. For the AD7899 it is defined

as: THD dB

VVV

V

V

V

(

)

log

=

++

+

+

20

2

2

3

2

4

2

5

2

6

2

1

fifth 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 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 AD7899 is tested using two input frequencies. In this case, the

second and third order terms are of different significance. 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 speci-

fication where it is the ratio of the rms sum of the individual

distortion products to the rms amplitude of the fundamental

expressed in dBs.

Differential Nonlinearity

This is the difference between the measured and the ideal

1 LSB change between any two adjacent codes in the ADC.

Positive Gain Error (AD7899-1, AD7899-3)

This is the deviation of the last code transition (01 . . . 110 to

01 . . . 111) from the ideal 4

± 10 V), 2 × V

± 5 V range) or V

REF

– 3/2 LSB (AD7899 at

± 2.5 V range) after the Bipolar Offset

Error has been adjusted out.

Positive Gain Error (AD7899-2)

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

11 . . . 111) from the ideal 2

polar Offset Error has been adjusted out.

Unipolar Offset Error (AD7899-2)

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

00 . . . 01) from the ideal AGND +1/2 LSB

Bipolar Zero Error (AD7899-1, AD7899-2)

This is the deviation of the midscale transition (all 0s to all 1s)

from the ideal AGND – 1/2 LSB.

Negative Gain Error (AD7899-1, AD7899-3)

This is the deviation of the first code transition (10 . . . 000 to

10 . . . 001) from the ideal –4

+ 1/2 LSB (AD7899 at

±2.5 V range) after Bipolar Zero 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). It also applies to situations where there

is a step input change on the input voltage applied to the selected

for the duration of the track/hold acquisition time after the end

starting another conversion, to ensure that the part operates to

specification.