Datasheet AD7896 (Analog Devices) - 7

AD7896 TERMINOLOGY Total Harmonic Distortion Relative Accuracy
Total harmonic distortion (THD) is the ratio of the rms sum of This is the maximum deviation from a straight line passing harmonics to the fundamental. For the AD7896, it is defined as: through the endpoints of the ADC transfer function. The end- points of the transfer function are zero scale (which is VIN = V 2 2 2 2 2 2 + V3 + V4 + V5 + V6 AGND + 1/2 LSB), a point 1/2 LSB below the first code transi- THD (dB) = 20 log tion (00 . 000 to 00 . 001), and full scale (which is V V IN = 1 AGND + VDD – 1/2 LSB), a point 1/2 LSB above the last code transition (11 . 110 to 11 . 111). where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5, and V6 are the rms amplitudes of the second through the
Differential Nonlinearity
sixth harmonics. This is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC.
Peak Harmonic or Spurious Noise
Peak harmonic or spurious noise is defined as the ratio of the
Unipolar Offset Error
rms value of the next largest component in the ADC output This is the deviation of the first code transition (00 . 000 to spectrum (up to fS/2 and excluding dc) to the rms value of the 00 . 001) from the ideal VIN voltage (AGND + 1 LSB). fundamental. Normally, the value of this specification is deter-
Positive Full-Scale Error
mined by the largest harmonic in the spectrum, but for parts This is the deviation of the last code transition (11 . 110 to where the harmonics are buried in the noise floor, it will be a 11 . 111) from the ideal (VIN = AGND + VDD – 1 LSB) noise peak. after the offset error has been adjusted out.
Intermodulation Distortion Track-and-Hold Acquisition Time
With inputs consisting of sine waves at two frequencies, fa and Track-and-hold acquisition time is the time required for the fb, any active device with nonlinearities will create distortion output of the track-and-hold amplifier to reach its final value, products at sum and difference frequencies of mfa ± nfb where within ± 1/2 LSB, after the end of conversion (the point at which m, n = 0, 1, 2, 3, etc. Intermodulation distortion terms are the track-and-hold returns into track mode). It also applies to a those for which neither m nor n are equal to zero. For example, situation where there is a step input change on the input voltage the second order terms include (fa + fb) and (fa – fb), while the applied to the selected VIN input of the AD7896. It means that third order terms include (2fa + fb), (2fa – fb), (fa + 2fb), and the user must wait for the duration of the track-and-hold acquisi- (fa – 2fb). tion time after the end of conversion or after a step input change The AD7896 is tested using the CCIF standard where two to VIN before starting another conversion, to ensure the part input frequencies near the top end of the input bandwidth are operates to specification. used. In this case, the second order terms are usually distanced
Signal-to-(Noise + Distortion) Ratio
in frequency from the original sine waves while the third order This is the measured ratio of signal-to-(noise + distortion) at the terms are usually at a frequency close to the input frequencies. output of the ADC. The signal is the rms amplitude of the fun- As a result, the second and third order terms are specified sepa- damental. Noise is the sum of all nonfundamental signals up to rately. The calculation of the intermodulation distortion is as half the sampling frequency (fS/2), excluding dc. The ratio is per the THD specification where it is the ratio of the rms sum of dependent on the number of quantization levels in the digitiza- the individual distortion products to the rms amplitude of the tion process; the more levels, the smaller the quantization noise. fundamental expressed in dB. The theoretical signal-to-(noise + distortion) ratio for an ideal N-bit converter with a sine wave input is given by: Signal-to-(Noise + Distortion) = (6.02N + 1.76) dB Thus, for a 12-bit converter, this is 74 dB. –6– Rev. D Document Outline FEATURES GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS SPECIFICATIONS TIMING CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION PIN FUNCTION DESCRIPTIONS TERMINOLOGY Relative Accuracy Differential Nonlinearity Unipolar Offset Error Positive Full-Scale Error Track-and-Hold Acquisition Time Signal-to-(Noise + Distortion) Ratio Total Harmonic Distortion Peak Harmonic or Spurious Noise Intermodulation Distortion CONVERTER DETAILS CIRCUIT DESCRIPTION Analog Input Section Track-and-Hold Section Timing and Control OPERATING MODES Mode 1 Operation (High Sampling Performance) Mode 2 Operation (Auto Sleep after Conversion) Serial Interface MICROPROCESSOR/MICROCONTROLLER INTERFACE AD7896–8051 Interface AD7896–68HC11/L11 Interface AD7896–ADSP-2103/ADSP-2105 Interface AD7896–DSP56002/L002 Interface AD7896 PERFORMANCE Linearity Noise Dynamic Performance (Mode 1 Only) Effective Number of Bits Power Considerations OUTLINE DIMENSIONS ORDERING GUIDE Revision History