AD7779 datasheet(30/97 Pages) AD

30 / 97 page

AD7779

Data Sheet

Rev. 0 | Page 30 of 97

TERMINOLOGY

Common-Mode Rejection Ratio (CMRR)

CMRR is 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 common-mode voltage of VIN+ and VIN− at

frequency, fS.

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

where:

Pf is the power at frequency, f, in the ADC output.

PfS is the power at frequency, fS, in the ADC output.

Differential Nonlinearity (DNL) Error

In an ideal ADC, code transitions are 1 LSB apart. Differential

nonlinearity is the maximum deviation from this ideal value.

DNL error is often specified in terms of resolution for which

no missing codes are guaranteed.

Integral Nonlinearity (INL) Error

Integral noninearity error refers to the deviation of each

individual code from a line drawn from negative full scale

through positive full scale. The point used as negative full scale

occurs ½ LSB before the first code transition. Positive full scale is

a level 1½ LSB beyond the last code transition. The deviation is

measured from the middle of each code to the true straight line.

Dynamic Range

Dynamic range is the ratio of the rms value of the full-scale

input signal to the rms noise measured for an input. The value

for dynamic range is expressed in decibels.

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 frequency sweep sine wave signal to all seven nonselected

input channels and determining how much that signal is

attenuated in the selected channel. The figure is given for

worst case scenarios across all eight channels of the AD7779.

Intermodulation Distortion

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 and nfb,

where m, n = 0,1, 2, 3, and so on. Intermodulation distortion

terms are those for which neither m nor n are 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 AD7779 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, and 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 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.

Gain Error

The first transition (from 100 … 000 to 100 … 001) occurs at a

level ½ LSB above nominal negative full scale (−2.49999 V for

the ±2.5 V range). The last transition (from 011 … 110 to

011 … 111) occurs for an analog voltage 1½ LSB below the

nominal full scale (2.49999 V for the ±2.5V range). The gain

error is the deviation of the difference between the actual

level of the last transition and the actual level of the first

transition from the difference between the ideal levels.

Gain Error Drift

Gain error drift is the ratio of the gain error change due to a

temperature change of 1°C and the full-scale range (2N). It is

expressed in parts per million.

Least Significant Bit (LSB)

The least significant bit, or LSB, is the smallest increment that

can be represented by a converter. For a fully differential input

ADC with N bits of resolution, the LSB expressed in volts is

LSB (V) =

REF

2

The LSB referred to the input is

LSB (VIN) =

GAIN

REF

PGA

2 

Power Supply Rejection Ratio (PSRR)

Variations in power supply affect the full-scale transition but

not the linearity of the converter. PSRR is the maximum change

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

supply voltage from the nominal value.

Signal-to-Noise Ratio (SNR)

SNR is the ratio of the rms value of the actual input signal to

the rms sum of all other spectral components below the Nyquist

frequency, excluding harmonics and dc. The value for SNR is

expressed in decibels.

Signal-to-(Noise + Distortion) Ratio (SINAD)

SINAD is the ratio of the rms value of the actual input signal

to the rms sum of all other spectral components below the

Nyquist frequency, including harmonics but excluding dc.

The value for SINAD is expressed in decibels.

Spurious-Free Dynamic Range (SFDR)

SFDR is the difference, in decibels, between the rms amplitude of

the input signal and the peak spurious signal (including

harmonics).

Total Harmonic Distortion (THD)

THD is the ratio of the rms sum of the first five harmonic

components to the rms value of a full-scale input signal and

is expressed in decibels.

Offset Error

Offset error is the difference between the ideal midscale input

voltage (0 V) and the actual voltage producing the midscale

output code.

Offset Error Drift

Offset error drift is the ratio of the offset error change due to

a temperature change of 1°C and the full-scale code range

(2N). It is expressed in μV/°C.

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