REV. 0

AD7650

–8–

DEFINITION OF SPECIFICATIONS

INTEGRAL NONLINEARITY ERROR (INL)

Linearity 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 1/2 LSB

before the first code transition. “Positive full scale” is defined as a

level 1 1/2 LSB beyond the last code transition. The deviation is

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

DIFFERENTIAL NONLINEARITY ERROR (DNL)

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

nonlinearity is the maximum deviation from this ideal value. It is

often specified in terms of resolution for which no missing codes

are guaranteed.

FULL-SCALE ERROR

The last transition (from 011 . . . 10 to 011 . . . 11 in two’s comple-

ment coding) should occur for an analog voltage 1 1/2 LSB below

the nominal full scale (2.49994278 V for the 0 V–2.5 V range).

The full-scale error is the deviation of the actual level of the last

transition from the ideal level.

UNIPOLAR ZERO ERROR

The first transition should occur at a level 1/2 LSB above analog

ground (19.073

µV for the 0 V–2.5 V range). Unipolar zero

error is the deviation of the actual transition from that point.

SPURIOUS FREE DYNAMIC RANGE (SFDR)

The difference, in decibels (dB), between the rms amplitude of

the input signal and the peak spurious signal.

EFFECTIVE NUMBER OF BITS (ENOB)

ENOB is a measurement of the resolution with a sine wave input.

It is related to S/(N+D) by the following formula:

ENOB

S N

D

dB

=+

()

176

6 02

./ .

and is expressed in bits.

TOTAL HARMONIC DISTORTION (THD)

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

nents to the rms value of a full-scale input signal and is expressed

in decibels.

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 (S/[N+D])

S/(N+D) 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

S/(N+D) is expressed in decibels.

APERTURE DELAY

Aperture delay is a measure of the acquisition performance and

is measured from the falling edge of the

CNVST input to when

the input signal is held for a conversion.

TRANSIENT RESPONSE

The time required for the AD7650 to achieve its rated accuracy

after a full-scale step function is applied to its input.

OVERVOLTAGE RECOVERY

The time required for the ADC to recover to full accuracy after

an analog input signal 150% of full-scale is reduced to 50% of

the full-scale value.

NOTES

AI = Analog Input

DI = Digital Input

DI/O = Bidirectional Digital

DO = Digital Output

P = Power

PIN FUNCTION DESCRIPTIONS (continued)

Pin No.

Mnemonic

Type

Description

35

CNVST

DI

Start Conversion. A falling edge on

CNVST puts the internal sample/hold into the hold state

and initiates a conversion. In impulse mode (IMPULSE HIGH and WARP LOW), if

CNVST

the hold state and a conversion is immediately started.

36

AGND

P

Must Be Tied to Analog Ground

37

REF

AI

Reference Input Voltage

38

REFGND

AI

Reference Input Analog Ground

39

IN–

AI

Analog Input Ground

43

IN+

AI