Datasheet AD7813 (Analog Devices) - 6

AD7813 TERMINOLOGY
in frequency from the original sine waves, while the third order
Signal to (Noise + Distortion) Ratio
terms are usually at a frequency close to the input frequencies. This is the measured ratio of signal to (noise + distortion) at the As a result, the second and third order terms are specified sepa- output of the A/D converter. The signal is the rms amplitude of rately. The calculation of the intermodulation distortion is as the fundamental. Noise is the rms sum of all nonfundamental per the THD specification where it is the ratio of the rms sum signals up to half the sampling frequency (f of the individual distortion products to the rms amplitude of the S/2), excluding dc. The ratio is dependent upon the number of quantization levels fundamental expressed in dBs. in the digitization process; the more levels, the smaller the quan-
Relative Accuracy
tization noise. The theoretical signal to (noise + distortion) ratio Relative accuracy or endpoint nonlinearity is the maximum for an ideal N-bit converter with a sine wave input is given by: deviation from a straight line passing through the endpoints of Signal to (Noise + Distortion) = (6.02N + 1.76) dB the ADC transfer function. Thus for an 10-bit converter, this is 62 dB.
Differential Nonlinearity
This is the difference between the measured and the ideal
Total Harmonic Distortion
1 LSB change between any two adjacent codes in the ADC. Total harmonic distortion (THD) is the ratio of the rms sum of harmonics to the fundamental. For the AD7813 it is defined as:
Offset Error
This is the deviation of the first code transition (0000 . 000) to (0000 . 001) from the ideal, i.e., AGND + 1 LSB. 2 2 2 2 2 V +V +V +V +V THD (dB) = 20 log 2 3 4 5 6
Offset Error Match
V1 This is the difference in Offset Error between any two channels. where V1 is the rms amplitude of the fundamental and V2, V3,
Gain Error
V4, V5 and V6 are the rms amplitudes of the second through the This is the deviation of the last code transition (1111 . 110) sixth harmonics. to (1111 . 111) from the ideal, i.e., VREF – 1 LSB, after the
Peak Harmonic or Spurious Noise
offset error has been adjusted out. Peak harmonic or spurious noise is defined as the ratio of the
Gain Error Match
rms value of the next largest component in the ADC output This is the difference in Gain Error between any two channels. 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 where the harmonics are buried in the noise floor, it will be a of the track/hold amplifier to reach its final value, within noise peak. ±1/2 LSB, after the end of conversion (the point at which the track/hold returns to track mode). It also applies to situations
Intermodulation Distortion
where a change in the selected input channel takes place or With inputs consisting of sine waves at two frequencies, fa and where there is a step input change on the input voltage applied fb, any active device with nonlinearities will create distortion to the selected V products at sum and difference frequencies of mfa ± nfb where IN input of the AD7813. It means that the user must wait for the duration of the track/hold acquisition time m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which after the end of conversion, or after a step input change to V neither m nor n are equal to zero. For example, the second IN, before starting another conversion, to ensure that the part order terms include (fa + fb) and (fa – fb), while the third order operates to specification. terms include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). The AD7813 is tested using the CCIF standard, where two input frequencies near the top end of the input bandwidth are used. In this case, the second and third order terms are of differ- ent significance. The second order terms are usually distanced REV. C –5–