AD7607 datasheet(18/32 Pages) AD

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AD7608

Data Sheet

Rev. A | Page 18 of 32

TERMINOLOGY

Integral Nonlinearity

The maximum deviation from a straight line passing through

the endpoints of the ADC transfer function. The endpoints of

the transfer function are zero scale, at ½ LSB below the first

code transition; and full scale, at ½ LSB above the last code

transition.

Differential Nonlinearity

The difference between the measured and the ideal 1 LSB

change between any two adjacent codes in the ADC.

Bipolar Zero Code Error

The deviation of the midscale transition (all 1s to all 0s) from

the ideal, which is 0 V − ½ LSB.

Bipolar Zero Code Error Match

The absolute difference in bipolar zero code error between any

two input channels.

Positive Full-Scale Error

The deviation of the actual last code transition from the ideal

last code transition (10 V − 1½ LSB (9.99988) and 5 V − 1½ LSB

(4.99994)) after bipolar zero code error is adjusted out. The

positive full-scale error includes the contribution from the

internal reference buffer.

Positive Full-Scale Error Match

The absolute difference in positive full-scale error between any

two input channels.

Negative Full-Scale Error

The deviation of the first code transition from the ideal first

code transition (−10 V + ½ LSB (−9.99996) and −5 V + ½ LSB

(−4.99998)) after the bipolar zero code error is adjusted out.

The negative full-scale error includes the contribution from

the internal reference buffer.

Negative Full-Scale Error Match

The absolute difference in negative full-scale error between any

two input channels.

Signal-to-(Noise + Distortion) Ratio

The measured ratio of signal-to-(noise + distortion) at the

output of the ADC. The signal is the rms amplitude of the

fundamental. Noise is the sum of all nonfundamental signals

up to half the sampling frequency (fS/2, excluding dc).

The ratio depends on the number of quantization levels in

the digitization 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 an 18-bit converter, the signal-to-(noise + distortion)

is 110.12 dB.

Total Harmonic Distortion (THD)

The ratio of the rms sum of the harmonics to the fundamental.

For the AD7608, it is defined as

THD (dB) =

20log

where:

V1 is the rms amplitude of the fundamental.

V2 to V9 are the rms amplitudes of the second through ninth

harmonics.

Peak Harmonic or Spurious Noise

The ratio of the rms value of the next largest component in the

ADC output spectrum (up to fS/2, excluding dc) to the rms value

of the fundamental. Normally, the value of this specification is

determined by the largest harmonic in the spectrum, but for

ADCs where the harmonics are buried in the noise floor, it is

determined by a noise peak.

Intermodulation Distortion (IMD)

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

any active device with nonlinearities creates distortion products

at sum and difference frequencies of mfa ± nfb, where m, n = 0,

1, 2, 3. Intermodulation distortion terms are those for which

neither m nor n is equal to 0. For example, the second-order

terms include (fa + fb) and (fa − fb), and the third-order terms

include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb).

The calculation of the intermodulation distortion is 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 decibels (dB).

Power Supply Rejection Ratio (PSRR)

Variations in power supply affect the full-scale transition but not

the converter’s linearity. PSR is the maximum change in full-

scale transition point due to a change in power supply voltage

from the nominal value. The PSR ratio (PSRR) is defined as the

ratio of the power in the ADC output at full-scale frequency, f,

to the power of a 100 mV p-p sine wave applied to the ADC’s

VDD and VSS supplies of Frequency fS.

PSRR (dB) = 10 log (Pf/PfS)

where:

Pf is equal to the power at Frequency f in the ADC output.

PfS is equal to the power at Frequency fS coupled onto the AVCC

supply.

Channel-to-Channel Isolation

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

between all input channels. It is measured by applying a full-scale

sine wave signal, up to 160 kHz, to all unselected input channels

and then determining the degree to which the signal attenuates

in the selected channel with a 1 kHz sine wave signal applied (see

Figure 25).

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