Datasheet AD7854, AD7854L (Analog Devices) - 7

AD7854/AD7854L TERMINOLOGY Total Harmonic Distortion Integral Nonlinearity
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 AD7854/AD7854L, it is through the endpoints of the ADC transfer function. The end- defined as: points of the transfer function are zero scale, a point 1/2 LSB 2 2 2 2 2 below the first code transition, and full scale, a point 1/2 LSB (V +V +V +V +V ) above the last code transition. THD (dB) = 20 log 2 3 4 5 6 V1
Differential Nonlinearity
This is the difference between the measured and the ideal 1 LSB where V1 is the rms amplitude of the fundamental and V2, V3, change between any two adjacent codes in the ADC. V4, V5 and V6 are the rms amplitudes of the second through the sixth harmonics.
Unipolar Offset Error
This is the deviation of the first code transition (00 . 000 to
Peak Harmonic or Spurious Noise
00 . 001) from the ideal AIN(+) voltage (AIN(–) + 1/2 LSB) Peak harmonic or spurious noise is defined as the ratio of the when operating in the unipolar mode. rms value of the next largest component in the ADC output spectrum (up to fS/2 and excluding dc) to the rms value of the
Unipolar Gain Error
fundamental. Normally, the value of this specification is deter- This is the deviation of the last code transition (111 . 110 to mined by the largest harmonic in the spectrum, but for ADCs 111 . 111) from the ideal, i.e., AIN(–) +VREF/2 – 1.5 LSB, where the harmonics are buried in the noise floor, it will be a after the unipolar offset error has been adjusted out. noise peak.
Bipolar Positive Full-Scale Error Intermodulation Distortion
This applies to the bipolar modes only and is the deviation of the With inputs consisting of sine waves at two frequencies, fa and last code transition from the ideal AIN(+) voltage. For bipolar fb, any active device with nonlinearities will create distortion mode, the ideal AIN(+) voltage is (AIN(–) +VREF/2 – 1.5 LSB). products at sum and difference frequencies of mfa ± nfb where
Negative Full-Scale Error
m, n = 0, 1, 2, 3, etc. Intermodulation distortion terms are This applies to the bipolar mode only and is the deviation of the those for which neither m nor n are equal to zero. For example, first code transition (10 . 000 to 10 . 001) from the ideal the second order terms include (fa + fb) and (fa – fb), while the AIN(+) voltage (AIN(–) – VREF/2 + 0.5 LSB). third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and
Bipolar Zero Error
(fa – 2fb). This is the deviation of the midscale transition (all 0s to all 1s) Testing is performed using the CCIF standard where two input from the ideal AIN(+) voltage (AIN(–) – 1/2 LSB). frequencies near the top end of the input bandwidth are used. In
Track/Hold Acquisition Time
this case, the second order terms are usually distanced in fre- The track/hold amplifier returns into track mode and the end of quency from the original sine waves while the third order terms conversion. Track/Hold acquisition time is the time required for are usually at a frequency close to the input frequencies. As a the output of the track/hold amplifier to reach its final value, result, the second and third order terms are specified separately. within The calculation of the intermodulation distortion is as per the ±1/2 LSB, after the end of conversion. THD specification where it is the ratio of the rms sum of the
Signal to (Noise + Distortion) Ratio
individual distortion products to the rms amplitude of the sum This is the measured ratio of signal to (noise + distortion) at the of the fundamentals expressed in dBs. output of the A/D converter. The signal is the rms amplitude of the fundamental. Noise is the sum of all nonfundamental sig- nals up to half the sampling frequency (fS/2), excluding dc. The ratio is dependent on the number of quantization levels in the digitization process; the more levels, the smaller the quantiza- tion noise. 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) = (6.02 N + 1.76) dB Thus for a 12-bit converter, this is 74 dB. REV. B –7–