Datasheet AD7895 (Analog Devices) - 6

AD7895 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 quantization noise. The theoretical signal to (noise + distortion)
Positive Full-Scale Error (AD7895-10)
ratio for an ideal N-bit converter with a sine wave input is given This is the deviation of the last code transition (01 . 110 to by: 01 . 111) from the ideal (4 × VREF – 1 LSB) after the Bipolar Zero Error has been adjusted out. Signal to (Noise + Distortion) = (6.02 N + 1.76) dB
Positive Full-Scale Error (AD7895-3)
Thus for a 12-bit converter, this is 74 dB. This is the deviation of the last code transition (01 . 110 to
Total Harmonic Distortion
01 . 111) from the ideal ( VREF – 1 LSB) after the Total harmonic distortion (THD) is the ratio of the rms sum of Bipolar Zero Error has been adjusted out. harmonics to the fundamental. For the AD7895, it is defined as:
Positive Full-Scale Error (AD7895-2)
This is the deviation of the last code transition (11 . 110 to 2 2 2 2 2 V +V +V +V +V 11 . 111) from the ideal (VREF – 1 LSB) after the Unipolar THD (dB) = 20 log 2 3 4 5 6 Offset Error has been adjusted out. V1 where V
Bipolar Zero Error (AD7895-10, AD7895-3)
1 is the rms amplitude of the fundamental, and V2, V3, V This is the deviation of the midscale transition (all 0s to all 1s) 4, V5 and V6 are the rms amplitudes of the second through the sixth harmonics. from the ideal 0 V (GND).
Peak Harmonic or Spurious Noise Unipolar Offset Error (AD7895-2)
Peak harmonic or spurious noise is defined as the ratio of the This is the deviation of the first code transition (00 . 000 to rms value of the next largest component in the ADC output 00 . 001) from the ideal 1 LSB. spectrum (up to fS/2 and excluding dc) to the rms value of the
Negative Full-Scale Error (AD7895-10)
fundamental. Normally, the value of this specification is This is the deviation of the first code transition (10 . 000 to determined by the largest harmonic in the spectrum, but for 10 . 001) from the ideal (–4 × VREF + 1 LSB) after Bipolar parts where the harmonics are buried in the noise floor, it will Zero Error has been adjusted out. be a noise peak.
Negative Full-Scale Error (AD7895-3) Intermodulation Distortion
This is the deviation of the first code transition (10 . 000 to With inputs consisting of sine waves at two frequencies, fa and 10 . 001) from the ideal (–VREF + 1 LSB) after Bipolar Zero fb, any active device with nonlinearities will create distortion Error has been adjusted out. products at sum and difference frequencies of mfa ± nfb where
Track/Hold Acquisition Time
m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which Track/Hold acquisition time is the time required for the output neither m or n are equal to zero. For example, the second order of the track/hold amplifier to reach its final value, within terms include (fa + fb) and (fa – fb), while the third order terms ±1/2 LSB, after the end of conversion (the point at which the include (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb). track/hold returns to track mode). It also applies to situations The AD7895 is tested using the CCIF standard where two where there is a step input change on the input voltage applied input frequencies near the top end of the input bandwidth are to the VIN input of the AD7895. This means that the user must used. In this case, the second and third order terms are of wait for the duration of the track/hold acquisition time after the different significance. The second order terms are usually end of conversion or after a step input change to VIN before distanced in frequency from the original sine waves, while the starting another conversion to ensure that the part operates to third order terms are usually at a frequency close to the input specification. frequencies. As a result, the second and third order terms are specified separately. The calculation of the intermodulation distortion is as per the THD 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–