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AD7927

Data Sheet

Rev. D | Page 8 of 28

TERMINOLOGY

Integral Nonlinearity (INL)

INL 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. Figure 9 shows a typical INL

plot for the AD7927.

Differential Nonlinearity (DNL)

DNL is the difference between the measured and the ideal

1 LSB change between any two adjacent codes in the ADC.

Figure 10 shows a typical DNL plot for the AD7927.

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, REFIN − 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 × REFIN input range with −REFIN

to +REFIN biased about the REFIN point. It is the deviation of

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

voltage, that is, REFIN − 1 LSB.

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 × REFIN input range with −REFIN

to +REFIN biased about the REFIN point. It is the deviation of the

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

(that is, +REFIN − 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 × REFIN input range with −REFIN

to +REFIN biased about the REFIN point. It is the deviation of

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

ideal (that is, −REFIN + 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 400 kHz sine wave signal to all seven nonselected input

channels and determining how much that signal is attenuated

in the selected channel with a 50 kHz signal. The figure given is

the worst case across all eight channels for the AD7927.

Power Supply Rejection (PSR)

Variations in power supply affect the full-scale transition,

but not the converter’s linearity. Power supply rejection is the

maximum change in full-scale transition point due to a change

in power supply voltage from the nominal value (see the Typical

Performance Characteristics section).

Power Supply Rejection Ration (PSRR)

The power supply rejection ratio is defined as the ratio of the

power in the ADC output at full-scale frequency (f) to the

power of a 200 mV p-p sine wave applied to the ADC AVDD

supply of frequency (fS):

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

where:

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

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

ADC AVDD.

Here a 200 mV p-p sine wave is coupled onto the AVDD supply.

Figure 6 shows the power supply rejection ratio vs. supply ripple

frequency for the AD7927 with no decoupling.

Track-and-Hold Acquisition Time

The track-and-hold amplifier returns into track mode at the

end of conversion. Track-and-hold acquisition time is the time

required for the output of the track-and-hold amplifier to reach

its final value, within ±1 LSB, after the end of conversion.

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