Datasheet AD7862 (Analog Devices) - 7

AD7862 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 100 kHz sine wave signal to each of the four inputs the fundamental. Noise is the rms sum of all nonfundamental individually. These, in turn, are individually referenced to the signals up to half the sampling frequency (f /2), excluding dc. other three channels whose inputs are grounded, and the ADC S The ratio is dependent upon the number of quantization levels output is measured to determine the level of crosstalk from the in the digitization process; the more levels, the smaller the other channel. The figure given is the worst case across all four quantization noise. The theoretical signal to (noise + distortion) channels. ratio for an ideal N-bit converter with a sine wave input is given
Relative Accuracy
by: Relative accuracy or endpoint nonlinearity is the maximum Signal to (Noise + Distortion) = (6.02 N + 1.76) dB deviation from a straight line passing through the endpoints of Thus for a 12-bit converter, this is 74 dB. the ADC transfer function.
Total Harmonic Distortion Differential Nonlinearity
Total harmonic distortion (THD) is the ratio of the rms sum of This is the difference between the measured and the ideal 1 LSB harmonics to the fundamental. For the AD7862 it is defined as: change between any two adjacent codes in the ADC.
Positive Full-Scale Error
V 2 + V 2 + V 2 + V 2 This is the deviation of the last code transition (01 . 110 to THD (dB) = 20 log 2 3 4 5 01 . 111) from the ideal 4 × VREF – 3/2 LSB (AD7862-10 V1 ±10 V range) or VREF – 3/2 LSB (AD7862-3, ±2.5 V range) where V is the rms amplitude of the fundamental and V , V , V 1 2 3 4 after the Bipolar Offset Error has been adjusted out. and V are the rms amplitudes of the second through the fifth 5
Positive Full-Scale Error (AD7862-2, 0 V to 2.5 V)
harmonics. This is the deviation of the last code transition (01 . 110 to
Peak Harmonic or Spurious Noise
01 . 111) from the ideal VREF – 3/2 LSB after the unipolar Peak harmonic or spurious noise is defined as the ratio of the offset error has been adjusted out. rms value of the next largest component in the ADC output
Bipolar Zero Error (AD7862-10,
6
10 V, AD7862-3,
6
2.5 V)
spectrum (up to fS/2 and excluding dc) to the rms value of the This is the deviation of the midscale transition (all 1s to all 0s) 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 where the harmonics are buried in the noise floor, it will be a
Unipolar Offset Error (AD7862-2, 0 V to 2.5 V)
noise peak. This is the deviation of the first code transition (00 . 000 to 00 . 001) from the ideal AGND + 1/2 LSB.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and
Negative Full-Scale Error (AD7862-1,
6
10 V; AD7862-3,
fb, any active device with nonlinearities will create distortion 6
2.5 V)
products at sum and difference frequencies of mfa ± nfb where This is the deviation of the first code transition (10 . 000 to m, n = 0, 1, 2, 3, etc. Intermodulation terms are those for 10 . 001) from the ideal –4 × VREF + 1/2 LSB (AD7862-10 ± which neither m nor n are equal to zero. For example, the 10 V range) or –VREF + 1/2 LSB (AD7862-3, ± 2.5 V range) second order terms include (fa + fb) and (fa – fb), while the after Bipolar Zero Error has been adjusted out. third order terms include (2 fa + fb), (2 fa – fb), (fa + 2 fb) and
Track/Hold Acquisition Time
(fa – 2 fb). Track/Hold acquisition time is the time required for the output The AD7862 is tested using the CCIF standard where two input of the track/hold amplifier to reach its final value, within ± frequencies near the top end of the input bandwidth are used. 1/2 LSB, after the end of conversion (the point at which the In this case, the second and third order terms are of different track/hold returns to track mode). It also applies to situations significance. The second order terms are usually distanced in where a change in the selected input channel takes place or frequency from the original sine waves, while the third order where there is a step input change on the input voltage applied terms are usually at a frequency close to the input frequencies. to the selected VAX/BX input of the AD7862. It means that the As a result, the second and third order terms are specified user must wait for the duration of the track/hold acquisition separately. The calculation of the intermodulation distortion is time, after the end of conversion or after a channel change/step as per the THD specification where it is the ratio of the rms sum input change to VAX/BX, before starting another conversion to of the individual distortion products to the rms amplitude of the ensure that the part operates to specification. fundamental expressed in dBs. –6– REV. 0