Datasheet AD7899 (Analog Devices) - 8

AD7899 TERMINOLOGY
and third order terms are specified separately. The calculation
Signal to (Noise + Distortion) Ratio
of the intermodulation distortion is as per the THD speci- This is the measured ratio of signal to (noise + distortion) at the fication where it is the ratio of the rms sum of the individual output of the A/D converter. The signal is the rms amplitude of distortion products to the rms amplitude of the fundamental the fundamental. Noise is the rms sum of all nonfundamental expressed in dBs. signals up to half the sampling frequency (fS/2), excluding dc.
Differential Nonlinearity
The ratio is dependent upon the number of quantization levels This is the difference between the measured and the ideal in the digitization process; the more levels, the smaller the quan- 1 LSB change between any two adjacent codes in the ADC. tization noise. The theoretical signal to (noise + distortion) ratio for an ideal N-bit converter with a sine wave input is given by:
Positive Gain Error (AD7899-1, AD7899-3)
This is the deviation of the last code transition (01 . 110 to Signal to (Noise + Distortion) = (6.02N + 1.76) dB 01 . 111) from the ideal 4 × VREF – 3/2 LSB (AD7899 at Thus for a 14-bit converter, this is 86.04 dB. ± 10 V), 2 × VREF – 3/2 LSB (AD7899 at ± 5 V range) or VREF – 3/2 LSB (AD7899 at ± 2.5 V range) after the Bipolar Offset
Total Harmonic Distortion
Total harmonic distortion (THD) is the ratio of the rms sum of Error has been adjusted out. harmonics to the fundamental. For the AD7899 it is defined
Positive Gain Error (AD7899-2)
This is the deviation of the last code transition (11 . 110 to 2 2 2 2 2 V2 V3 V4 V5 V 11 . 111) from the ideal 2 × V as: THD (dB) = + + + + 20 log 6 REF – 3/2 LSB (AD7899 at ± V 10 V), 2 × V 1 REF – 3/2 LSB (AD7899 at 0 V to 5 V range) or VREF – 3/2 LSB (AD7899 at 0 V to 2.5 V range) after the Uni- where V1 is the rms amplitude of the fundamental and V2, V3, polar Offset Error has been adjusted out. V4, and V5 are the rms amplitudes of the second through the fifth harmonics.
Unipolar Offset Error (AD7899-2)
This is the deviation of the first code transition (00 . 00 to
Peak Harmonic or Spurious Noise
00 . 01) from the ideal AGND +1/2 LSB Peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component in the ADC output
Bipolar Zero Error (AD7899-1, AD7899-2)
spectrum (up to f This is the deviation of the midscale transition (all 0s to all 1s) S/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification is deter- from the ideal AGND – 1/2 LSB. mined by the largest harmonic in the spectrum, but for parts
Negative Gain Error (AD7899-1, AD7899-3)
where the harmonics are buried in the noise floor, it will be a This is the deviation of the first code transition (10 . 000 to noise peak. 10 . 001) from the ideal –4 × VREF + 1/2 LSB (AD7899 at
Intermodulation Distortion
±10 V), –2 × VREF + 1/2 LSB (AD7899 at ±5 V range) or –VREF With inputs consisting of sine waves at two frequencies, fa and + 1/2 LSB (AD7899 at ± 2.5 V range) after Bipolar Zero Error fb, any active device with nonlinearities will create distortion 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 nor n are equal to zero. For example, the second order of the track/hold amplifier to reach its final value, within ± 1/2 LSB, terms include (fa + fb) and (fa – fb), while the third order terms after the end of conversion (the point at which the track/hold include (2fa + fb), (2fa – fb), (fa + 2fb) and (fa – 2fb). returns to track mode). It also applies to situations where there The AD7899 is tested using two input frequencies. In this case, the is a step input change on the input voltage applied to the selected second and third order terms are of different significance. The VINA/VINB input of the AD7899. It means that the user must wait second order terms are usually distanced in frequency from the for the duration of the track/hold acquisition time after the end original sine waves while the third order terms are usually at a of conversion or after a step input change to VINA/VINB before frequency close to the input frequencies. As a result, the second starting another conversion, to ensure that the part operates to specification. REV. A –7–