INTERMODULATION DISTORTION (IMD)

With inputs consisting of sine waves at two frequencies, fa and

fb, any device with nonlinearities will create distortion products,

of order (m+n), at sum and difference frequencies of mfa

± nfb,

where m, n = 0, 1, 2, 3 . . . . Intermodulation terms are those

for which m or n is not equal to zero. For example, the second

order terms are (fa + fb) and (fa – fb), and the third order terms

are (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb). The IMD

products are expressed as the decibel ratio of the rms sum of the

measured input signals to the rms sum of the distortion terms.

The two signals applied to the converter are of equal amplitude,

and the peak value of their sum is –0.5 dB from full scale. The

IMD products are normalized to a 0 dB input signal.

APERTURE DELAY

Aperture delay is the time required after SAMPLE pin is taken

LOW for the internal sample-hold of the AD677 to open, thus

APERTURE JITTER

Aperture jitter is the variation in the aperture delay from sample

to sample.

POWER SUPPLY REJECTION

DC variations in the power supply voltage will affect the overall

transfer function of the ADC, resulting in zero error and full-

scale error changes. Power supply rejection is the maximum

change in either the bipolar zero error or full-scale error value.

Additionally, there is another power supply variation to con-

sider. AC ripple on the power supplies can couple noise into the

ADC, resulting in degradation of dynamic performance. This is

displayed in Figure 15.

INPUT SETTLING TIME

Settling time is a function of the SHA’s ability to track fast

slewing signals. This is specified as the maximum time required

in track mode after a full-scale step input to guarantee rated

conversion accuracy.

NOISE/DC CODE UNCERTAINTY

Ideally, a fixed dc input should result in the same output code

for repetitive conversions. However, as a consequence of un-

avoidable circuit noise within the wideband circuits in the ADC,

there is a range of output codes which may occur for a given in-

put voltage. If you apply a dc signal to the ADC and record a

large number of conversions, the result will be a distribution of

codes. If you fit a Gaussian probability distribution to the histo-

gram, the standard deviation is approximately equivalent to the

rms input noise of the ADC.

REV. A

–7–

Definition of Specifications–AD677

NYQUIST FREQUENCY

An implication of the Nyquist sampling theorem, the “Nyquist

frequency’’ of a converter is that input frequency which is one

half the sampling frequency of the converter.

TOTAL HARMONIC DISTORTION

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

the harmonic components to the rms value of a full-scale input

signal and is expressed in percent (%) or decibels (dB). For in-

put signals or harmonics that are above the Nyquist frequency,

the aliased components are used.

SIGNAL-TO-NOISE PLUS DISTORTION RATIO

Signal-to-noise plus distortion is defined to be the ratio of the

rms value of the measured input signal to the rms sum of all

other spectral components below the Nyquist frequency, includ-

ing harmonics but excluding dc.

+/– FULL-SCALE ERROR

The last + transition (from 011 . . . 10 to 011 . . . 11) should

occur for an analog voltage 1.5 LSB below the nominal full

scale (4.99977 volts for a

±5 V range). The full-scale error is

the deviation of the actual level of the last transition from the

ideal level.

BIPOLAR ZERO ERROR

Bipolar zero error is the difference between the ideal midscale

input voltage (0 V) and the actual voltage producing the mid-

scale output code.

DIFFERENTIAL NONLINEARITY (DNL)

In an ideal ADC, code transitions are one LSB apart. Differen-

tial nonlinearity is the maximum deviation from this ideal value.

It is often specified in terms of resolution for which no missing

codes are guaranteed.

INTEGRAL NONLINEARITY (INL)

The ideal transfer function for an ADC is a straight line bisect-

ing the center of each code drawn between “zero” and “full

scale.” The point used as “zero” occurs 1/2 LSB before the

most negative code transition. “Full scale” is defined as a level

1.5 LSB beyond the most positive code transition. Integral non-

linearity is the worst-case deviation of a code center average

from the straight line.

BANDWIDTH

The full-power bandwidth is that input frequency at which the

amplitude of the reconstructed fundamental is reduced by 3 dB

for a full-scale input.