Datasheet AD7856 (Analog Devices) - 9

AD7856 TERMINOLOGY1 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 AD7856, it is defined as: through the endpoints of the ADC transfer function. The end- points of the transfer function are zero scale, a point 1/2 LSB 2 2 2 2 2 V + + + + 2 V 3 V 4 V 5 V6 below the first code transition, and full scale, a point 1/2 LSB THD (dB) = 20 log V above the last code transition. 1 where V1 is the rms amplitude of the fundamental and V2, V3,
Differential Nonlinearity
V This is the difference between the measured and the ideal 1 LSB 4, V5 and V6 are the rms amplitudes of the second through the sixth harmonics. change between any two adjacent codes in the ADC.
Peak Harmonic or Spurious Noise Total Unadjusted Error
Peak harmonic or spurious noise is defined as the ratio of the This is the deviation of the actual code from the ideal code rms value of the next largest component in the ADC output taking all errors into account (Gain, Offset, Integral Nonlinearity spectrum (up to fS/2 and excluding dc) to the rms value of the and other errors) at any point along the transfer function. fundamental. Normally, the value of this specification is deter-
Unipolar Offset Error
mined by the largest harmonic in the spectrum, but for parts This is the deviation of the first code transition (00␣ .␣ .␣ .␣ 000 to where the harmonics are buried in the noise floor, it will be a 00␣ .␣ .␣ .␣ 001) from the ideal AIN(+) voltage (AIN(–) + 1/2 LSB). noise peak.
Positive Full-Scale Error Intermodulation Distortion
This is the deviation of the last code transition from the ideal With inputs consisting of sine waves at two frequencies, fa and AIN(+) voltage (AIN(–) + Full Scale – 1.5 LSB) after the offset fb, any active device with nonlinearities will create distortion error has been adjusted out. products at sum and difference frequencies of mfa ± nfb where
Channel-to-Channel Isolation
m, n = 0, 1, 2, 3, etc. Intermodulation distortion terms are Channel-to-channel isolation is a measure of crosstalk between those for which neither m nor n are equal to zero. For example, the channels. It is measured by applying a full-scale 25 kHz the second order terms include (fa + fb) and (fa – fb), while the signal to the other seven channels and determining how much third order terms include (2fa + fb), (2fa – fb), (fa + 2fb) and that signal is attenuated in the channel of interest. The figure (fa – 2fb). given is the worst case for all channels. Testing is performed using the CCIF standard where two input
Track/Hold Acquisition Time
frequencies near the top end of the input bandwidth are used. In The track/hold amplifier returns into track mode and the end of this case, the second order terms are usually distanced in fre- conversion. Track/Hold acquisition time is the time required for quency from the original sine waves, while the third order terms the output of the track/hold amplifier to reach its final value, are usually at a frequency close to the input frequencies. As a within ± 1/2 LSB, after the end of conversion. result, the second and third order terms are specified separately. The calculation of the intermodulation distortion is as per the
Signal to (Noise + Distortion) Ratio
THD specification where it is the ratio of the rms sum of the This is the measured ratio of signal to (noise + distortion) at the individual distortion products to the rms amplitude of the sum output of the A/D converter. The signal is the rms amplitude of of the fundamentals expressed in dBs. the fundamental. Noise is the sum of all nonfundamental signals up to half the sampling frequency (f
Full Power Bandwidth
S/2), excluding dc. The ratio is dependent on the number of quantization levels in the digiti- The Full Power Bandwidth (FPBW) of the AD7856 is that zation process; the more levels, the smaller the quantization frequency at which the amplitude of the reconstructed (using noise. The theoretical signal to (noise + distortion) ratio for an FFTs) fundamental (neglecting harmonics and SNR) is reduced ideal N-bit converter with a sine wave input is given by: by 3 dB for a full-scale input. Signal to (Noise + Distortion) = (6.02 N + 1.76) dB Thus for a 14-bit converter, this is 86 dB. NOTE 1AIN(+) refers to the positive input of the pseudo-differential pair, and AIN(–) refers to the negative analog input of the pseudo-differential pair or to AGND depending on the channel configuration. –8– REV. A