Datasheet AD7817, AD7818 (Analog Devices) - 10

AD7817/AD7818 Data Sheet TERMINOLOGY Signal-to-(Noise + Distortion) Ratio
specified separately. The calculation of the intermodulation This is the measured ratio of signal-to-(noise + distortion) at distortion is as per the THD specification where it is the ratio the output of the A/D converter. The signal is the rms amplitude of of the rms sum of the individual distortion products to the rms the fundamental. Noise is the rms sum of all nonfundamental amplitude of the fundamental expressed in dBs. signals up to half the sampling frequency (fS/2), excluding dc.
Channel-to-Channel Isolation
The ratio is dependent upon the number of quantization levels Channel-to-channel isolation is a measure of the level of crosstalk in the digitization process; the more levels, the smaller the between channels. It is measured by applying a full-scale 20 kHz quantization noise. The theoretical signal-to-(noise + distortion) sine wave signal to one input channel and determining how much ratio for an ideal N-bit converter with a sine wave input is given by: that signal is attenuated in each of the other channels. The figure Signal-to-(Noise + Distortion) = (6.02N + 1.76) dB given is the worst case across all four channels. Thus, for a 10-bit converter, this is 62 dB.
Relative Accuracy
Relative accuracy or endpoint nonlinearity is the maximum
Total Harmonic Distortion (THD)
deviation from a straight line passing through the endpoints THD is the ratio of the rms sum of harmonics to the fundamental. of the ADC transfer function. For the AD7817/AD7818, it is defined as:
Differential Nonlinearity
2 2 2 2 2     This is the difference between the measured and the ideal 1 LSB THDdB  V V V V V 2 3 4 5 6  20 log V change between any two adjacent codes in the ADC. 1 where:
Gain Error
V This is the deviation of the last code transition (1111 . 110) to 1 is the rms amplitude of the fundamental. (1111 . 111) from the ideal, that is, VREF – 1 LSB, after the V2, V3, V4, V5, and V6 are the rms amplitudes of the second offset error has been adjusted out. through the sixth harmonics.
Gain Error Match Peak Harmonic or Spurious Noise
This is the difference in gain error between any two channels. Peak harmonic or spurious noise is defined as the ratio of the rms value of the next largest component in the ADC output
Offset Error
spectrum (up to fS/2 and excluding dc) to the rms value of This is the deviation of the first code transition (0000 . 000) to the fundamental. Normally, the value of this specification is (0000 . 001) from the ideal, that is, AGND + 1 LSB. determined by the largest harmonic in the spectrum; however,
Offset Error Match
for devices where the harmonics are buried in the noise floor, it This is the difference in offset error between any two channels. is a noise peak.
Track-and-Hold Acquisition Time Intermodulation Distortion
The track-and-hold acquisition time is the time required for the With inputs consisting of sine waves at two frequencies, fa and output of the track-and-hold amplifier to reach its final value, fb, any active device with nonlinearities creates distortion products within ±1/2 LSB, after the end of conversion (the point at which at sum and difference frequencies of mfa ± nfb, where m, n = 0, the track-and-hold returns to track mode). It also applies to 1, 2, 3, etc. Intermodulation terms are those for which neither m situations where a change in the selected input channel takes nor n are equal to zero. For example, the second-order terms place or where there is a step input change on the input voltage include (fa + fb) and (fa − fb), while the third-order terms applied to the selected VIN input of the AD7817 or the AD7818. include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb). It means that the user must wait for the duration of the track- The AD7817/AD7818 are tested using the CCIF standard where and-hold acquisition time after the end of conversion or after a two input frequencies near the top end of the input bandwidth channel change/step input change to VIN before starting another are used. In this case, the second- and third-order terms are of conversion, to ensure that the device operates to specification. different significance. The second-order terms are usually distanced in frequency from the original sine waves, while the third-order terms are usually at a frequency close to the input frequencies. As a result, the second- and third-order terms are Rev. E | Page 10 of 20 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAMS GENERAL DESCRIPTION PRODUCT HIGHLIGHTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS TIMING CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TERMINOLOGY CONTROL BYTE Address Register Overtemperature Register CIRCUIT INFORMATION CONVERTER DETAILS TYPICAL CONNECTION DIAGRAM ANALOG INPUTS Analog Input DC Acquisition Time AC Acquisition Time ON-CHIP REFERENCE ADC TRANSFER FUNCTION TEMPERATURE MEASUREMENT TEMPERATURE MEASUREMENT ERROR DUE TO REFERENCE ERROR SELF-HEATING CONSIDERATIONS OPERATING MODES Mode 1 Mode 2 POWER vs. THROUGHPUT AD7817 SERIAL INTERFACE Read Operation Write Operation Simplifying the Serial Interface AD7818 SERIAL INTERFACE MODE Read Operation Write Operation OUTLINE DIMENSIONS ORDERING GUIDE