AD7939BCPZ datasheet(13/36 Pages) AD

13 / 36 page

Data Sheet

AD7938/AD7939

Rev. C | Page 13 of 36

TERMINOLOGY

Integral Nonlinearity

This is 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, 1 LSB below

the first code transition, and full scale, 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 is the deviation of the first code transition (00…000) to

(00…001) from the ideal (that is, AGND + 1 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

(111…111) from the ideal (that is, VREF − 1 LSB) after the offset

error has been adjusted out.

Gain Error Match

This is the difference in gain error between any two channels.

Zero-Code Error

This applies when using the twos complement output coding

option, in particular to the 2 × VREF input range with −VREF to

+VREF biased about the VREFIN point. It is the deviation of the

midscale transition (all 0s to all 1s) from the ideal VIN voltage

(that is, VREF).

Zero-Code Error Match

This is the difference in zero-code error between any two

channels.

Positive Gain Error

This applies when using the twos complement output coding

option, in particular to the 2 × VREF input range with −VREF to

+VREF biased about the VREFIN point. It is the deviation of the last

code transition (011…110) to (011…111) from the ideal (that

is, VREF − 1 LSB) after the zero-code error has been adjusted out.

Positive Gain Error Match

This is the difference in positive gain error between any two

channels.

Negative Gain Error

This applies when using the twos complement output coding

option, in particular to the 2 × VREF input range with −VREF to

+VREF biased about the VREF point. It is the deviation of the first

code transition (100…000) to (100…001) from the ideal (that is,

−VREFIN + 1 LSB) after the zero-code error has been adjusted out.

Negative Gain Error Match

This is the difference in negative gain error between any two

channels.

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 sine wave signal to all seven nonselected input channels

and applying a 50 kHz signal to the selected channel. The

channel-to-channel isolation is defined as the ratio of the power

of the 50 kHz signal on the selected channel to the power of the

noise signal on the unselected channels that appears in the FFT

of this channel. The noise frequency on the unselected channels

varies from 40 kHz to 740 kHz. The noise amplitude is at 2 × VREF,

while the signal amplitude is at 1 × VREF. See Figure 5.

Power Supply Rejection Ratio (PSRR)

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 VDD supply of frequency, fS. The frequency

of the noise varies from 1 kHz to 1 MHz.

PSRR (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.

Common-Mode Rejection Ratio (CMRR)

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

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.

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