Datasheet AD7470, AD7472 (Analog Devices) - 9

AD7470/AD7472 TERMINOLOGY Peak Harmonic or Spurious Noise Integral Nonlinearity
Peak harmonic or spurious noise is defined as the ratio of the This is the maximum deviation from a straight line passing rms value of the next largest component in the ADC output through the endpoints of the ADC transfer function. The end- spectrum (up to fS/2 and excluding dc) to the rms value of the points of the transfer function are zero scale, a point 1/2 LSB fundamental. Normally, the value of this specification is deter- below the first code transition, and full scale, a point 1/2 LSB mined by the largest harmonic in the spectrum, but for ADCs above the last code transition. where the harmonics are buried in the noise floor, it will be a
Differential Nonlinearity
noise peak. This is the difference between the measured and the ideal 1 LSB
Intermodulation Distortion
change between any two adjacent codes in the ADC. With inputs consisting of sine waves at two frequencies, fa and
Offset Error
fb, any active device with nonlinearities will create distortion This is the deviation of the first code transition (00 . 000) to products at sum and difference frequencies of mfa ± nfb where (00 . 001) from the ideal, i.e., AGND + 0.5 LSB. m, n = 0, 1, 2, 3, etc. Intermodulation distortion terms are those for which neither m nor n is equal to zero. For example,
Gain Error
the second-order terms include (fa + fb) and (fa – fb), while the The last transition should occur at the analog value 1.5 LSB third-order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and below the nominal full scale. The first transition is a 0.5 LSB (fa – 2fb). above the low end of the scale (zero in the case of AD7470/ AD7472). The gain error is the deviation of the actual difference The AD7470/AD7472 are tested using the CCIF standard between the first and last code transitions from the ideal differ- where two input frequencies near the top end of the input band- ence between the first and last code transitions with offset errors width are used. In this case, the second-order terms are usually removed. distanced in frequency from the original sine waves while the third-order terms are usually at a frequency close to the input
Track-and-Hold Acquisition Time
frequencies. As a result, the second- and third-order terms are The track-and-hold amplifier returns into track mode after the specified separately. The calculation of the intermodulation end of conversion. Track-and-Hold acquisition time is the time distortion is as per the THD specification where it is the ratio required for the output of the track-and-hold amplifier to reach of the rms sum of the individual distortion products to the rms its final value, within ± 1 LSB, after the end of conversion. amplitude of the sum of the fundamentals expressed in dBs.
Signal to (Noise + Distortion) Ratio Aperture Delay
This is the measured ratio of signal to (noise + distortion) at the In a sample-and-hold, the time required after the hold command output of the A/D converter. The signal is the rms amplitude of for the switch to open fully is the aperture delay. The sample is, the fundamental. Noise is the sum of all nonfundamental sig- in effect, delayed by this interval, and the hold command would nals up to half the sampling frequency (fS/2), excluding dc. The have to be advanced by this amount for precise timing. ratio is dependent on the number of quantization levels in the digitization process; the more levels, the smaller the quantization
Aperture Jitter
noise. The theoretical signal to (noise + distortion) ratio for an Aperture jitter is the range of variation in the aperture delay. ideal N-bit converter with a sine wave input is given by In other words, it is the uncertainty about when the sample is taken. Jitter is the result of noise which modulates the phase Signal to (Noise + Distortion) = (6.02 N + 1.76) dB of the hold command. This specification establishes the ulti- Thus for a 12-bit converter, this is 74 dB and for a 10-bit con- mate timing error, hence the maximum sampling frequency verter is 62 dB. for a given resolution. This error will increase as the input
Total Harmonic Distortion (THD)
dV/dt increases. Total harmonic distortion is the ratio of the rms sum of har- monics to the fundamental. For the AD7470/AD7472 it is defined as V ( 2 2 2 2 2 2 + V3 + V4 + V5 + V6 ) THD (dB) = 20 log V1 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 sixth harmonics. REV. B –9– Document Outline FEATURES FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION PRODUCT HIGHLIGHTS SPECIFICATIONS TIMING SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ORDERING GUIDE PIN CONFIGURATIONS PIN FUNCTION DESCRIPTIONS TERMINOLOGY Integral Nonlinearity Differential Nonlinearity Offset Error Gain Error Track-and-Hold Acquisition Time Signal to (Noise + Distortion) Ratio Total Harmonic Distortion (THD) Peak Harmonic or Spurious Noise Intermodulation Distortion Aperture Delay Aperture Jitter CIRCUIT DESCRIPTION CONVERTER OPERATION TYPICAL CONNECTION DIAGRAM ADC TRANSFER FUNCTION AC ACQUISITION TIME Reference Input DC ACQUISITION TIME ANALOG INPUT CLOCK SOURCES PARALLEL INTERFACE OPERATING MODES Mode 1 (High Speed Sampling) Mode 2 (Sleep Mode) Burst Mode VDRIVE POWER-UP Power vs. Throughput Mode 1 Mode 2 Typical Performance Characteristics GROUNDING AND LAYOUT POWER SUPPLIES MICROPROCESSOR INTERFACING AD7470/AD7472 to ADSP-2185 Interface AD7470/AD7472 to ADSP-21065 Interface AD7470/AD7472 to TMS320C25 Interface AD7470/AD7472 to PIC17C4x Interface AD7470/AD7472 to 80C186 Interface OUTLINE DIMENSIONS Revision History