Datasheet AD7859, AD7859L (Analog Devices) - 7

AD7859/AD7859L 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 AD7859/AD7859L, 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
Positive Full-Scale Error
fundamental. Normally, the value of this specification is deter- This applies to the unipolar and bipolar modes and is the devia- mined by the largest harmonic in the spectrum, but for parts tion of the last code transition from the ideal AIN(+) voltage where the harmonics are buried in the noise floor, it will be a (AIN(–) + Full Scale – 1.5 LSB) after the offset error has been noise peak. adjusted out.
Intermodulation Distortion Negative Full-Scale Error
With inputs consisting of sine waves at two frequencies, fa and This applies to the bipolar mode only and is the deviation of the fb, any active device with nonlinearities will create distortion first code transition (10 . 000 to 10 . 001) from the ideal products at sum and difference frequencies of mfa ± nfb where AIN(+) voltage (AIN(–) – VREF/2 + 0.5 LSB). m, n = 0, 1, 2, 3, etc. Intermodulation distortion terms are
Bipolar Zero Error
those for which neither m nor n are equal to zero. For example, This is the deviation of the midscale transition (all 0s to all 1s) the second order terms include (fa + fb) and (fa – fb), while the from the ideal AIN(+) voltage (AIN(–) – 1/2 LSB). third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and
Track/Hold Acquisition Time
(fa – 2fb). The track/hold amplifier returns into track mode and the end of Testing is performed using the CCIF standard where two input conversion. Track/Hold acquisition time is the time required for frequencies near the top end of the input bandwidth are used. In the output of the track/hold amplifier to reach its final value, this case, the second order terms are usually distanced in fre- within ± 1/2 LSB, after the end of conversion. quency from the original sine waves while the third order terms
Signal to (Noise + Distortion) Ratio
are usually at a frequency close to the input frequencies. As a This is the measured ratio of signal to (noise + distortion) at the result, the second and third order terms are specified separately. output of the A/D converter. The signal is the rms amplitude of The calculation of the intermodulation distortion is as per the the fundamental. Noise is the sum of all nonfundamental sig- THD specification where it is the ratio of the rms sum of the nals up to half the sampling frequency (f individual distortion products to the rms amplitude of the sum S/2), excluding dc. The ratio is dependent on the number of quantization levels in the of the fundamentals expressed in dBs. 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. –6– REV. A