Datasheet AD7894 (Analog Devices) - 6

AD7894 TERMINOLOGY Relative Accuracy Signal to (Noise + Distortion) Ratio
Relative accuracy or endpoint nonlinearity is the maximum This is the measured ratio of signal to (noise + distortion) at the deviation from a straight line passing through the endpoints of output of the A/D converter. The signal is the rms amplitude of the ADC transfer function. the fundamental. Noise is the rms sum of all nonfundamental
Differential Nonlinearity
signals up to half the sampling frequency (fS/2), excluding dc. This is the difference between the measured and the ideal 1␣ LSB The ratio is dependent upon the number of quantization levels change between any two adjacent codes in the ADC. in the digitization process; the more levels, the smaller the quan- tization noise. The theoretical signal to (noise + distortion) ratio
Positive Gain Error (AD7894-10)
for an ideal N-bit converter with a sine wave input is given by: This is the deviation of the last code transition (01 . 110 to 01 . 111) from the ideal (4 × VREF – 1 LSB) after the Signal to (Noise + Distortion) = (6.02␣ N + 1.76) dB Bipolar Zero Error has been adjusted out. Thus for a 14-bit converter, this is 86.04 dB.
Positive Gain Error (AD7894-3) Total Harmonic Distortion
This is the deviation of the last code transition (01 . 110 to Total harmonic distortion (THD) is the ratio of the rms sum of 01 . 111) from the ideal (VREF – 1 LSB) after the Bipolar harmonics to the fundamental. For the AD7894, it is defined as: Zero Error has been adjusted out.
Positive Gain Error (AD7894-2)
2 2 2 2 2 V2 +V3 +V4 +V5 +V This is the deviation of the last code transition (11 . 110 to THD (dB) = 20 log 6 V 11 . 111) from the ideal (VREF – 1 LSB) after the Unipolar 1 Offset Error has been adjusted out. where V1 is the rms amplitude of the fundamental and V2, V3, V
Bipolar Zero Error (AD7894-10, AD7894-3)
4, V5 and V6 are the rms amplitudes of the second through the sixth harmonics. This is the deviation of the midscale transition (all 0s to all 1s) from the ideal 0 V (GND).
Peak Harmonic or Spurious Noise
Peak harmonic or spurious noise is defined as the ratio of the
Unipolar Offset Error (AD7894-2)
rms value of the next largest component in the ADC output This is the deviation of the first code transition (00 . 000 to spectrum (up to f 00 . 001) from the ideal 1 LSB. S/2 and excluding dc) to the rms value of the fundamental. The value of this specification is normally deter-
Negative Gain Error (AD7894-10)
mined by the largest harmonic in the spectrum, but for parts This is the deviation of the first code transition (10 . 000 to where the harmonics are buried in the noise floor, it will be a 10 . 001) from the ideal (–4 × VREF + 1 LSB) after Bipolar noise peak. Zero Error has been adjusted out.
Intermodulation Distortion Negative Gain Error (AD7894-3)
With inputs consisting of sine waves at two frequencies, fa and This is the deviation of the first code transition (10 . 000 to fb, any active device with nonlinearities will create distortion 10 . 001) from the ideal (– VREF + 1 LSB) after Bipolar products at sum and difference frequencies of mfa ± nfb where Zero Error has been adjusted out. m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which
Track/Hold Acquisition Time
neither m nor n is equal to zero. For example, the second order Track/Hold acquisition time is the time required for the output terms include (fa + fb) and (fa – fb), while the third order terms of the track/hold amplifier to reach its final value, within include (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb). ±1/2␣ LSB, after the end of conversion (the point at which the The AD7894 is tested using two input frequencies. In this case, track/hold returns to track mode). It also applies to situations the second and third order terms are of different significance. where there is a step input change on the input voltage applied The second order terms are usually distanced in frequency from to the VIN input of the AD7894. This means that the user must the original sine waves, while the third order terms are usually at wait for the duration of the track/hold acquisition time after the a frequency close to the input frequencies. As a result, the second end of conversion or after a step input change to VIN before and third order terms are specified separately. The calculation starting another conversion, to ensure that the part operates to of the intermodulation distortion is as per the THD specification specification. where it is the ratio of the rms sum of the individual distortion products to the rms amplitude of the fundamental expressed in dBs. REV. 0 –5–