Datasheet AD7890 (Analog Devices) - 10

AD7890 TERMINOLOGY Signal to (Noise + Distortion) Ratio Channel-to-Channel Isolation
This is the measured ratio of signal to (noise + distortion) at the Channel-to-channel isolation is a measure of the level of crosstalk output of the A/D converter. The signal is the rms amplitude of between channels. It is measured by applying a full-scale 1 kHz the fundamental. Noise is the rms sum of all nonfundamental signal to any one of the other seven inputs and determining how signals up to half the sampling frequency (fS/2), excluding dc. much that signal is attenuated in the channel of interest. The figure The ratio is dependent upon the number of quantization levels given is the worst case across all eight channels. in the digitization process; the more levels, the smaller the quantization noise. The theoretical signal to (noise + distortion)
Relative Accuracy
ratio for an ideal N-bit converter with a sine wave input is given by: Relative accuracy or endpoint nonlinearity is the maximum deviation from a straight line passing through the endpoints of Signal to (Noise + Distortion) = (6.02N + 1.76) dB the ADC transfer function. Thus, for a 12-bit converter, this is 74 dB.
Differential Nonlinearity Total Harmonic Distortion
This is the difference between the measured and the ideal 1 LSB Total harmonic distortion (THD) is the ratio of the rms sum of change between any two adjacent codes in the ADC. harmonics to the fundamental. For the AD7890, it is defined as
Positive Full-Scale Error (AD7890-10)
2 2 2 2 2 V + V + V + V + V This is the deviation of the last code transition (01 . 110 to 2 3 4 5 6 THD ) dB ( = 20 log V 01 . 111) from the ideal (4 × REF IN − 1 LSB) after the 1 bipolar zero error has been adjusted out. where: V
Positive Full-Scale Error (AD7890-4)
1 is the rms amplitude of the fundamental and V This is the deviation of the last code transition (11 . 110 to 2, V3, V4, V5, and V6 are the rms amplitudes of the second through the sixth harmonics. 11 . 111) from the ideal (1.638 × REF IN − 1 LSB) after the unipolar offset error has been adjusted out.
Peak Harmonic or Spurious Noise
Peak harmonic or spurious noise is defined as the ratio of the
Positive Full-Scale Error (AD7890-2)
rms value of the next largest component in the ADC output This is the deviation of the last code transition (11 . 110 to spectrum (up to f 11 . 111) from the ideal (REF IN − 1 LSB) after the unipolar S/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification is offset error has been adjusted out. determined by the largest harmonic in the spectrum, but for
Bipolar Zero Error (AD7890-10)
parts where the harmonics are buried in the noise floor, it is This is the deviation of the midscale transition (all 0s to all 1s) determined by a noise peak. from the ideal 0 V (AGND).
Intermodulation Distortion Unipolar Offset Error (AD7890-2, AD7890-4)
With inputs consisting of sine waves at two frequencies, fa and This is the deviation of the first code transition (00 . 000 to fb, any active device with nonlinearities creates distortion 00 . 001) from the ideal 0 V (AGND). products at sum and difference frequencies of mfa ± nfb where m, n = 0, 1, 2, 3, and so on. Intermodulation terms are those for
Negative Full-Scale Error (AD7890-10)
which neither m nor n are equal to zero. For example, the This is the deviation of the first code transition (10 . 000 to second-order terms include (fa + fb) and (fa − fb), while the 10 . 001) from the ideal (−4 × REF IN + 1 LSB) after bipolar third-order terms include (2fa + fb), (2fa − fb), (fa + 2fb), and zero error has been adjusted out. (fa − 2fb).
Track/Hold Acquisition Time
The AD7890 is tested using the CCIF standard where two input Track/hold acquisition time is the time required for the output frequencies near the top end of the input bandwidth are used. of the track/hold amplifier to reach its final value, within In this case, the second and third order terms are of different ±1/2 LSB, after the end of conversion (the point at which the significance. The second-order terms are usually distanced in track/hold returns to track mode). It also applies to situations frequency from the original sine waves while the third-order where a change in the selected input channel takes place or terms are usually at a frequency close to the input frequencies. where there is a step input change on the input voltage applied As a result, the second- and third-order terms are specified to the selected VIN input of the AD7890. It means that the user separately. The calculation of the intermodulation distortion is must wait for the duration of the track/hold acquisition time as per the THD specification where it is the ratio of the rms after the end of conversion or after a channel change/step input sum of the individual distortion products to the rms amplitude change to VIN before starting another conversion, to ensure that of the fundamental expressed in dBs. the part operates to specification. Rev. C | Page 9 of 28 Document Outline FEATURES GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS TIMING SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TERMINOLOGY CONTROL REGISTER THEORY OF OPERATION CONVERTER DETAILS CIRCUIT DESCRIPTION AD7890-10 Analog Input AD7890-4 Analog Input AD7890-2 Analog Input TRACK/HOLD AMPLIFIER REFERENCE TIMING AND CONTROL CEXT FUNCTIONING SERIAL INTERFACE SELF-CLOCKING MODE Write Operation EXTERNAL CLOCKING MODE Read Operation Write Operation SIMPLIFYING THE INTERFACE MICROPROCESSOR/MICROCONTROLLER INTERFACE AD7890 TO 8051 INTERFACE AD7890 TO 68HC11 INTERFACE AD7890 TO ADSP-2101 INTERFACE AD7890 TO DSP56000 INTERFACE AD7890 TO TMS320C25/30 INTERFACE ANTIALIASING FILTER PERFORMANCE LINEARITY NOISE DYNAMIC PERFORMANCE EFFECTIVE NUMBER OF BITS OUTLINE DIMENSIONS ORDERING GUIDE