AD7827

–5–

REV. 0

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 quantiza-

tion 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 an 8-bit converter, this is 50 dB.

Total Harmonic Distortion

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

harmonics to the fundamental. For the AD7827 it is defined as:

THD (dB)

= 20 log

3

4

5

6

2

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 sec-

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

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

The AD7827 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 and third order terms are of dif-

ferent significance. The second order terms are usually dis-

tanced in frequency from the original sine waves while the third

order terms are usually at a frequency close to the input fre-

quencies. 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 fundamental expressed in dBs.

Relative Accuracy

Relative accuracy or endpoint nonlinearity is the maximum

deviation from a straight line passing through the endpoints of

the ADC transfer function.

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 128th code transition (01111111) to

Zero Scale Error

This is the deviation of the first code transition (00000000) to

(00000001) from the ideal, i.e., V

5 V

± 10%), or 0.8 V

± 10%).

Full-Scale Error

This is the deviation of the last code transition (11111110) to

(11111111) from the ideal, i.e., V

5 V

± 10%), or V

± 10%).

Gain Error

This is the deviation of the last code transition (1111 . . . 110) to

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 point at which the track/hold returns to

track mode. This happens approximately 120 ns after the falling

edge of

CONVST.

It also applies when there is a step input change on the input

the user must wait for the duration of the track/hold acquisition

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

operates to specification.