REV. 0

AD7866

–7–

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 LSB

below the first code transition, and full scale, a point 1 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 applies when using Straight Binary output coding. It is the

deviation of the first code transition (00 . . . 000) to (00 . . . 001)

from the ideal, i.e., AGND + 1 LSB.

Offset Error Match

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

Gain Error

This applies when using Straight Binary output coding. It is the

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

adjusted out.

Gain Error Match

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

Zero Code Error

This applies when using the two’s complement output coding option,

in particular with the 2

Zero Code Error Match

This is the difference in Zero Code Error between the two

channels.

Positive Gain Error

This applies when using the two’s complement output coding

option, in particular with the 2

last code transition (011 . . . 110) to (011 . . . 111) from the

been adjusted out.

Negative Gain Error

This applies when using the two’s complement output coding

option, in particular with the 2

code transition (100 . . . 000) to (100 . . . 001) from the ideal

adjusted out.

Track/Hold Acquisition Time

The track/hold amplifier returns into track mode after 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 signals

is dependent on the number of quantization levels in the digiti-

zation 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) = (6.02 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 AD7866, it is defined as:

THD dB

VV

V

V

V

()

=

++

+

+

20

3

2

4

2

5

2

6

2

1

log

V

2

2

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 AD7866 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 dB.

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 (2

455 kHz sine wave signal to all non selected input channels and

determining how much that signal is attenuated in the selected

the worst-case across all four channels for the AD7866.

PSR (Power Supply Rejection)

See Performance Curves section.