Datasheet AD7650 (Analog Devices) - 9

AD7650 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 is held low when the acquisition phase (t8) is complete, the internal sample/hold is put into 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 Primary Analog Input with a Range of 0 V to VREF. NOTES AI = Analog Input DI = Digital Input DI/O = Bidirectional Digital DO = Digital Output P = Power
DEFINITION OF SPECIFICATIONS TOTAL HARMONIC DISTORTION (THD)
THD is the ratio of the rms sum of the first five harmonic compo-
INTEGRAL NONLINEARITY ERROR (INL)
Linearity error refers to the deviation of each individual code nents to the rms value of a full-scale input signal and is expressed from a line drawn from “negative full scale” through “positive in decibels. 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
SIGNAL-TO-NOISE RATIO (SNR)
level 1 1/2 LSB beyond the last code transition. The deviation is SNR is the ratio of the rms value of the actual input signal to the measured from the middle of each code to the true straight line. rms sum of all other spectral components below the Nyquist frequency, excluding harmonics and dc. The value for SNR is
DIFFERENTIAL NONLINEARITY ERROR (DNL)
expressed in decibels. In an ideal ADC, code transitions are 1 LSB apart. Differential nonlinearity is the maximum deviation from this ideal value. It is
SIGNAL TO (NOISE + DISTORTION) RATIO (S/[N+D])
often specified in terms of resolution for which no missing codes S/(N+D) is the ratio of the rms value of the actual input signal to are guaranteed. the rms sum of all other spectral components below the Nyquist
FULL-SCALE ERROR
frequency, including harmonics but excluding dc. The value for The last transition (from 011 . 10 to 011 . 11 in two’s comple- S/(N+D) is expressed in decibels. 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).
APERTURE DELAY
The full-scale error is the deviation of the actual level of the last Aperture delay is a measure of the acquisition performance and transition from the ideal level. is measured from the falling edge of the CNVST input to when the input signal is held for a conversion.
UNIPOLAR ZERO ERROR
The first transition should occur at a level 1/2 LSB above analog
TRANSIENT RESPONSE
ground (19.073 µV for the 0 V–2.5 V range). Unipolar zero The time required for the AD7650 to achieve its rated accuracy error is the deviation of the actual transition from that point. after a full-scale step function is applied to its input.
SPURIOUS FREE DYNAMIC RANGE (SFDR)
The difference, in decibels (dB), between the rms amplitude of
OVERVOLTAGE RECOVERY
the input signal and the peak spurious signal. 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
EFFECTIVE NUMBER OF BITS (ENOB)
the full-scale value. 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] − ( 1 7 . )6 /6 0.2 dB and is expressed in bits. REV. 0 –8–