Datasheet AD7810 (Analog Devices) - 6

AD7810 TERMINOLOGY
The AD7810 is tested using the CCIF standard where two
Signal to (Noise + Distortion) Ratio
input frequencies near the top end of the input bandwidth are This is the measured ratio of signal to (noise + distortion) at the used. In this case, the second and third order terms are of differ- output of the A/D converter. The signal is the rms amplitude of ent significance. The second order terms are usually distanced the fundamental. Noise is the rms sum of all nonfundamental in frequency from the original sine waves while the third order signals up to half the sampling frequency (f /2), excluding dc. terms are usually at a frequency close to the input frequencies. S The ratio is dependent upon the number of quantization levels in As a result, the second and third order terms are specified sepa- the digitization process; the more levels, the smaller the quantiza- rately. The calculation of the intermodulation distortion is as tion noise. The theoretical signal to (noise + distortion) ratio for per the THD specification where it is the ratio of the rms sum of an ideal N-bit converter with a sine wave input is given by: the individual distortion products to the rms amplitude of the Signal to (Noise + Distortion) = (6.02N + 1.76) dB fundamental expressed in dBs. Thus for a 10-bit converter, this is 62 dB.
Relative Accuracy
Relative accuracy or endpoint nonlinearity is the maximum
Total Harmonic Distortion
deviation from a straight line passing through the endpoints of Total harmonic distortion (THD) is the ratio of the rms sum of the ADC transfer function. harmonics to the fundamental. For the AD7810 it is defined as:
Differential Nonlinearity
This is the difference between the measured and the ideal V 2 + V 2 + V 2 + V 2 THD dB ( ) = 20 log 2 3 4 5 1 LSB change between any two adjacent codes in the ADC. V1
Offset Error
where V1 is the rms amplitude of the fundamental and V2, V3, This is the deviation of the first code transition (0000 . 000) V 2 4, V5 and V6 are the rms amplitudes of the second through to (0000 . 001) from the ideal, i.e., AGND + 1 LSB. the sixth harmonics.
Gain Error Peak Harmonic or Spurious Noise
This is the deviation of the last code transition (1111 . 110) Peak harmonic or spurious noise is defined as the ratio of the to (1111 . 111) from the ideal (i.e., VREF – 1 LSB) after the rms values of the next largest component in the ADC output offset error has been adjusted out. spectrum (up to fS/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification is deter-
Track/Hold Acquisition Time
mined by the largest harmonic in the spectrum, but for parts Track/hold acquisition time is the time required for the output of where the harmonics are buried in the noise floor, it will be a the track/hold amplifier to reach its final value, within ±1/2 LSB, noise peak. after the end of conversion (the point at which the track/hold returns to track mode). It also applies to situations where there
Intermodulation Distortion
is a step input change on the input voltage applied to the VIN+ With inputs consisting of sine waves at two frequencies, fa and input of the AD7810. It means that the user must wait for the fb, any active device with nonlinearities will create distortion duration of the track/hold acquisition time, after the end of conver- products at sum and difference frequencies of mfa ± nfb where sion or after a step input change to VIN+, before starting another m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which conversion to ensure that the part operates to 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). REV. B –5–