Datasheet AD7865 (Analog Devices) - 9

AD7865 TERMINOLOGY Channel-to-Channel Isolation Signal to (Noise + Distortion) Ratio
Channel-to-channel isolation is a measure of the level of This is the measured ratio of signal to (noise + distortion) at the crosstalk between channels. It is measured by applying a full- output of the A/D converter. The signal is the rms amplitude of scale 10 kHz sine wave signal to one channel and a 50 kHz the fundamental. Noise is the rms sum of all nonfundamental signal to another channel and measuring how much of that signals up to half the sampling frequency (fS/2), excluding dc. signal is coupled onto the first channel. The figure given is the The ratio is dependent upon the number of quantization levels worst case across all four channels of the AD7865. 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) = (6.02 N + 1.76) dB the ADC transfer function. Thus for a 14-bit converter, this is 86.04 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 AD7865 it is defined as:
Positive Gain Error (AD7865-1, AD7865-3)
This is the deviation of the last code transition (01 . 110 to 2 V + V 2 2 2 2 01 . 111) from the ideal 4 × VREF – 3/2 LSB (AD7865 at 3 + V4 + V5 + V THD dB ( ) = 20 2 6 log ± 10 V), 2 × VREF – 3/2 LSB (AD7865 at ± 5 V range) or V1 VREF – 3/2 LSB (AD7865 at ± 2.5 V range), after the Bipolar where V Offset Error has been adjusted out. 1 is the rms amplitude of the fundamental and V2, V3, V4 and V5 are the rms amplitudes of the second through the fifth
Positive Gain Error (AD7865-2)
harmonics. This is the deviation of the last code transition (111 . 110 to 111 . 111) from the ideal 2 × V
Peak Harmonic or Spurious Noise
REF – 3/2 LSB (AD7865 at Peak harmonic or spurious noise is defined as the ratio of the 0 V to 5 V), VREF – 3/2 LSB (AD7865 at 0 V to 2.5 V) after rms value of the next largest component in the ADC output the Unipolar Offset Error has been adjusted out. spectrum (up to fS/2 and excluding dc) to the rms value of the
Unipolar Offset Error (AD7865-2)
fundamental. Normally, the value of this specification is deter- This is the deviation of the first code transition (000 . 000 to mined by the largest harmonic in the spectrum, but for parts 000 . 001) from the ideal AGND + 1/2 LSB. where the harmonics are buried in the noise floor, it will be a
Bipolar Zero Error (AD7865-1, AD7865-3)
noise peak. This is the deviation of the midscale transition (all 0s to 1s)
Intermodulation Distortion
from the ideal AGND – 1/2 LSB. With inputs consisting of sine waves at two frequencies, fa and
Negative Gain Error (AD7865-1, AD7865-3)
fb, any active device with nonlinearities will create distortion This is the deviation of the first code transition (10 . 000 to products at sum and difference frequencies of mfa ± nfb where 10 . 001) from the ideal –4 × VREF + 1/2 LSB (AD7865 at m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for which ± 10 V), –2 × VREF + 1/2 LSB (AD7865 at ± 5 V range) or neither m nor n are equal to zero. For example, the second –VREF + 1/2 LSB (AD7865 at ± 2.5 V range), after Bipolar Zero order terms include (fa + fb) and (fa – fb), while the third order Error has been adjusted out. terms include (2 fa + fb), (2 fa – fb), (fa + 2 fb) and (fa – 2 fb).
Track/Hold Acquisition Time
The AD7865 is tested using two input frequencies. In this case, Track/Hold acquisition time is the time required for the out- the second and third order terms are of different significance. put of the track/hold amplifier to reach its final value, within The second order terms are usually distanced in frequency from ± 1/2 LSB, after the end of conversion (the point at which the the original sine waves, while the third order terms are usually at track/hold returns to track mode). It also applies to situations a frequency close to the input frequencies. As a result, the second where there is a step input change on the input voltage applied and third order terms are specified separately. The calculation of to the selected VINxA/VINxB input of the AD7865. It means that the intermodulation distortion is as per the THD specification the user must wait for the duration of the track/hold acquisition where it is the ratio of the rms sum of the individual distortion time after the end of conversion or after a step input change to products to the rms amplitude of the fundamental expressed in dBs. VINxA/VINxB before starting another conversion, to ensure that the part operates to specification. –8– REV. B