Datasheet AD7823 (Analog Devices) - 6

AD7823 TERMINOLOGY
The AD7823 is tested using the CCIF standard where two input
Signal to (Noise + Distortion) Ratio
frequencies near the top end of the input bandwidth are used. This is the measured ratio of signal to (noise + distortion) at the In this case, the second and third order terms are of different output of the A/D converter. The signal is the rms amplitude of significance. The second order terms are usually distanced in the fundamental. Noise is the rms sum of all nonfundamental frequency from the original sine waves while the third order signals up to half the sampling frequency (f /2), excluding dc. S terms are usually at a frequency close to the input frequencies. The ratio is dependent upon the number of quantization levels As a result, the second and third order terms are specified sepa- in the digitization process; the more levels, the smaller the rately. The calculation of the intermodulation distortion is as quantization noise. The theoretical signal to (noise + distortion) per the THD specification where it is the ratio of the rms sum of ratio for an ideal N-bit converter with a sine wave input is the individual distortion products to the rms amplitude of the given by: fundamental expressed in dBs. Signal to (Noise + Distortion) = (6.02N + 1.76) dB
Relative Accuracy
Relative accuracy or endpoint nonlinearity is the maximum Thus for an 8-bit converter, this is 50 dB. deviation from a straight line passing through the endpoints of
Total Harmonic Distortion
the ADC transfer function. Total harmonic distortion (THD) is the ratio of the rms sum of
Differential Nonlinearity
harmonics to the fundamental. For the AD7823 it is defined as: This is the difference between the measured and the ideal 2 2 2 2 2 1 LSB change between any two adjacent codes in the ADC. V +V +V +V +V THD (dB) = 20 log 2 3 4 5 6
Offset Error
V1 This is the deviation of the first code transition (0000 . 000) where V1 is the rms amplitude of the fundamental and V2, V3, to (0000 . 001) from the ideal, i.e., AGND + 1 LSB. V4, V5 and V6 are the rms amplitudes of the second through the
Gain Error
sixth harmonics. This is the deviation of the last code transition (1111 . 110)
Peak Harmonic or Spurious Noise
to (1111 . 111) from the ideal (i.e., VREF – 1 LSB) after the Peak harmonic or spurious noise is defined as the ratio of the offset error has been adjusted out. rms value of the next largest component in the ADC output
Track/Hold Acquisition Time
spectrum (up to fS/2 and excluding dc) to the rms value of Track/hold acquisition time is the time required for the output the fundamental. Normally, the value of this specification is of the track/hold amplifier to reach its final value, within determined by the largest harmonic in the spectrum, but for ± 1/2 LSB, after the end of conversion (the point at which the parts where the harmonics are buried in the noise floor, it track/hold returns to track mode). It also applies to situations will be a noise peak. where there is a step input change on the input voltage applied
Intermodulation Distortion
to the VIN+ input of the AD7823. It means that the user must With inputs consisting of sine waves at two frequencies, fa and wait for the duration of the track/hold acquisition time, after the fb, any active device with nonlinearities will create distortion end of conversion or after a step input change to VIN, before products at sum and difference frequencies of mfa ± nfb where starting another conversion to ensure that the part operates to m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which specification. neither m nor n are equal to zero. For example, the second order terms include (fa + fb) and (fa – fb), while the third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb).
Typical Performance Characteristics 10 0 AD7823 –10 2048 POINT FFT SAMPLING 136.054 –20 fIN 29.961 0 –30 mW –40 – –50 dBs POWER –60 0.1 –70 –80 –90 0.01 0 10 20 30 40 50 –100 1 23 45 67 89 THROUGHPUT – kSPS 111 133 155 177 199 221 243 265 287 309 331 353 375 397 419 441 463 485 507 529 551 573 595 617 639 661 683 705 727 749 771 793 815 837 859 881 903 925 947 969 991 1013 FREQUENCY BINS
Figure 2. Power vs. Throughput Figure 3. AD7823 SNR REV. C –5–