Datasheet AD7940 (Analog Devices) - 9

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AD7940 TERMINOLOGY Integral Nonlinearity Total Harmonic Distortion (THD)
This is the maximum deviation from a straight line passing THD is the ratio of the rms sum of harmonics to the through the endpoints of the ADC transfer function. The fundamental. For the AD7940, it is defined as endpoints of the transfer function are zero scale, a point 1/2 2 2 2 2 2 V + V + V + V + V LSB below the first code transition, and full scale, a point 1/2 2 3 4 5 6 THD(dB) = 20log V LSB above the last code transition. 1
Differential Nonlinearity
where V1 is the rms amplitude of the fundamental and V2, V3, This is the difference between the measured and the ideal 1 LSB V4, V5, and V6 are the rms amplitudes of the second through the change between any two adjacent codes in the ADC. sixth harmonics.
Offset Error Peak Harmonic or Spurious Noise
This is the deviation of the first code transition (00 . 000) to Peak harmonic or spurious noise is defined as the ratio of the (00 . 001) from the ideal, i.e., AGND + 1 LSB. rms value of the next largest component in the ADC output spectrum (up to f
Gain Error
S/2, excluding dc) to the rms value of the fundamental. Normally, the value of this specification is This is the deviation of the last code transition (111 . 110) to determined by the largest harmonic in the spectrum, but for (111 . 111) from the ideal (i.e., VREF − 1 LSB) after the offset ADCs where the harmonics are buried in the noise floor, it will error has been adjusted out. be a noise peak.
Track-and-Hold Acquisition Time Intermodulation Distortion
The track-and-hold amplifier returns to track mode at the end With inputs consisting of sine waves at two frequencies, fa and fb, of conversion. The track-and-hold acquisition time is the time any active device with nonlinearities wil create distortion required for the output of the track-and-hold amplifier to reach products at sum and difference frequencies of mfa ± nfb where m, its final value, within ±1 LSB, after the end of the conversion. n = 0, 1, 2, 3. Intermodulation distortion terms are those for See the Serial Interface section for more details. which neither m nor n are equal to zero. For example, the second-
Signal-to-(Noise + Distortion) Ratio
order terms include (fa + fb) and (fa − fb), while the third-order This is the measured ratio of signal-to-(noise + distortion) at terms include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa −2fb). the output of the ADC. The signal is the rms amplitude of the The AD7940 is tested using the CCIF standard where two input fundamental. Noise is the sum of all nonfundamental signals up frequencies near the top end of the input bandwidth are used. to half the sampling frequency (fS/2, excluding dc). The ratio In this case, the second-order terms are usually distanced in depends on the number of quantization levels in the digitization frequency from the original sine waves, while the third-order process; the more levels, the smaller the quantization noise. The terms are usual y at a frequency close to the input frequencies. theoretical signal-to-(noise + distortion) ratio for an ideal N-bit As a result, the second- and third-order terms are specified converter with a sine wave input is given by separately. The calculation of the intermodulation distortion is Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB as per the THD specification where it is the ratio of the rms Thus, for a 14-bit converter, this is 86.04 dB. sum of the individual distortion products to the rms amplitude of the sum of the fundamentals expressed in dBs. Rev. A | Page 8 of 20 Document Outline Table of Contents Specifications1F Timing Specifications Absolute Maximum Ratings ESD Caution Pin Configurations and Function Descriptions Terminology Typical Performance Characteristics Circuit Information Converter Operation Analog Input ADC Transfer Function Typical Connection Diagram Digital Inputs Modes of Operation Normal Mode Power-Down Mode Power vs. Throughput Rate Serial Interface Microprocessor Interfacing AD7940 to TMS320C541 AD7940 to ADSP-218x AD7940 to DSP563xx Application Hints Grounding and Layout Evaluating the AD7940 Performance Outline Dimensions Ordering Guide