REV. B

AD7887

–6–

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.

Offset Error Match

This is the difference in Offset Error between any two channels.

Gain Error

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

offset error has been adjusted out.

Gain Error Match

This is the difference in Gain Error between any two channels.

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

±1/2 LSB, after the end of conversion.

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 sig-

ratio is dependent on 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) = (6.02 N + 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 AD7887, it is defined

as:

THD dB

VVV

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

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

(fa – 2fb).

The AD7887 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

sum of the fundamentals expressed in dBs.

Channel-to-Channel Isolation

Channel-to-channel isolation is a measure of the level of

crosstalk between channels. It is measured by applying a full-

scale 25 kHz sine wave signal to the nonselected input channel

and determining how much that signal is attenuated in the se-

lected channel. The figure given is the worst case across both

channels for the AD7887.

PSR (Power Supply Rejection)

Variations in power supply will affect the full-scale transition,

but not the converter’s linearity. Power Supply Rejection is the

maximum change in the full-scale transition point due to a

change in power-supply voltage from the nominal value.