Datasheet AD7853, AD7853L (Analog Devices) - 8

AD7853/AD7853L 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 AD7853/AD7853L, 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.
Total Unadjusted Error
This is the deviation of the actual code from the ideal code
Peak Harmonic or Spurious Noise
taking all errors into account (Gain, Offset, Integral Nonlinearity, Peak harmonic or spurious noise is defined as the ratio of the and other errors) at any point along the transfer function. 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 Offset Error
fundamental. Normally, the value of this specification is deter- This is the deviation of the first code transition (00 . 000 to mined by the largest harmonic in the spectrum, but for parts 00 . 001) from the ideal AIN(+) voltage (AIN(–) + 1/2 LSB) where the harmonics are buried in the noise floor, it will be a when operating in the unipolar mode. noise peak.
Positive Full-Scale Error Intermodulation Distortion
This applies to the unipolar and bipolar modes and is the devia- With inputs consisting of sine waves at two frequencies, fa and tion of the last code transition from the ideal AIN(+) voltage fb, any active device with nonlinearities will create distortion (AIN(–) + Full Scale – 1.5 LSB) after the offset error has been products at sum and difference frequencies of mfa ± nfb where adjusted out. m, n = 0, 1, 2, 3, etc. Intermodulation distortion terms are
Negative Full-Scale Error
those for which neither m nor n are equal to zero. For example, This applies to the bipolar mode only and is the deviation of the the second order terms include (fa + fb) and (fa – fb), while the first code transition (10 . 000 to 10 . 001) from the ideal third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and AIN(+) voltage (AIN(–) – VREF/2 + 0.5 LSB). (fa – 2fb).
Bipolar Zero Error
Testing is performed using the CCIF standard where two input This is the deviation of the midscale transition (all 1s to all 0s) frequencies near the top end of the input bandwidth are used. In from the ideal AIN(+) voltage (AIN(–) – 1/2 LSB). this case, the second order terms are usually distanced in fre-
Track/Hold Acquisition Time
quency from the original sine waves while the third order terms The track/hold amplifier returns into track mode and the end of are usually at a frequency close to the input frequencies. As a conversion. Track/Hold acquisition time is the time required for result, the second and third order terms are specified separately. the output of the track/hold amplifier to reach its final value, The calculation of the intermodulation distortion is as per the within ± 1/2 LSB, after the end of conversion. THD specification where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the sum
Signal to (Noise + Distortion) Ratio
of the fundamentals expressed in dBs. This is the measured ratio of signal to (noise + distortion) at the 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. –8– REV. B