Datasheet AD7822, AD7825, AD7829 (Analog Devices) - 8

AD7822/AD7825/AD7829 TERMINOLOGY Signal-to-(Noise + Distortion) Ratio
The AD7822/AD7825/AD7829 are tested using the CCIF The measured ratio of signal-to-(noise + distortion) at the standard, where two input frequencies near the top end of the output of the analog-to-digital converter. The signal is the rms input bandwidth are used. In this case, the second- and third- amplitude of the fundamental. Noise is the rms sum of all order terms are of different significance. The second-order terms nonfundamental signals up to half the sampling frequency are usually distanced in frequency from the original sine waves, (f and the third-order terms are usually at a frequency close to the S/2), excluding dc. The ratio is dependent upon the number of quantization levels in the digitization process: the more levels, input frequencies. As a result, the second- and third-order terms the smaller the quantization noise. The theoretical signal-to-(noise are specified separately. The calculation of the intermodulation + distortion) ratio for an ideal N-bit converter with a sine wave distortion is as per the THD specification, where it is the ratio input is given by of the rms sum of the individual distortion products to the rms amplitude of the fundamental expressed in decibels (dB). Signal-to-(Noise + Distortion) = (6.02 N + 1.76) dB Thus, for an 8-bit converter, this is 50 dB.
Channel-to-Channel Isolation
A measure of the level of crosstalk between channels. It is
Total Harmonic Distortion (THD)
measured by applying a full-scale 20 kHz sine wave signal to The ratio of the rms sum of harmonics to the fundamental. For one input channel and determining how much that signal is the AD7822/AD7825/AD7829, it is defined as attenuated in each of the other channels. The figure given is the worst case across all four or eight channels of the AD7825 and V 2 + V 2 + V 2 + V 2 + V 2 2 3 4 5 6 AD7829, respectively. THD (dB) = 20 log V1
Relative Accuracy or Endpoint Nonlinearity
where V1 is the rms amplitude of the fundamental and V2, V3, The maximum deviation from a straight line passing through V4, V5, and V6 are the rms amplitudes of the second through the the endpoints of the ADC transfer function. sixth harmonics.
Differential Nonlinearity Peak Harmonic or Spurious Noise
The difference between the measured and the ideal one LSB The ratio of the rms value of the next largest component in the change between any two adjacent codes in the ADC. ADC output spectrum (up to fS/2 and excluding dc) to the rms value of the fundamental. Normally, the value of this specification
Offset Error
is determined by the largest harmonic in the spectrum, but for The deviation of the 128th code transition (01111111) to parts where the harmonics are buried in the noise floor, it is a (10000000) from the ideal, that is, VMID. noise peak.
Offset Error Match Intermodulation Distortion
The difference in offset error between any two channels. With inputs consisting of sine waves at two frequencies, fa and
Zero-Scale Error
fb, any active device with nonlinearities creates distortion The deviation of the first code transition (00000000) to products at sum and difference frequencies of mfa ± nfb, where (00000001) from the ideal; that is, VMID − 1.25 V + 1 LSB (VDD = m, n = 0, 1, 2, 3, … . Intermodulation terms are those for which 5 V ± 10%), or VMID − 1.0 V + 1 LSB (VDD = 3 V ± 10%). neither m nor n is equal to zero. For example, the second-order terms include (fa + fb) and (fa − fb), and the third-order terms
Full-Scale Error
include (2fa + fb), (2fa − fb), (fa + 2fb), and (fa − 2fb). The deviation of the last code transition (11111110) to (11111111) from the ideal; that is, VMID + 1.25 V − 1 LSB (VDD = 5 V ± 10%), or VMID + 1.0 V − 1 LSB (VDD = 3 V ± 10%). Rev. C | Page 8 of 28 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS TIMING CHARACTERISTICS TIMING DIAGRAM ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TERMINOLOGY CIRCUIT INFORMATION CIRCUIT DESCRIPTION TYPICAL CONNECTION DIAGRAM ADC TRANSFER FUNCTION ANALOG INPUT POWER-UP TIMES POWER VS. THROUGHPUT OPERATING MODES PARALLEL INTERFACE MICROPROCESSOR INTERFACING AD7822/AD7825/AD7829 TO 8051 AD7822/AD7825/AD7829 TO PIC16C6x/PIC16C7x AD7822/AD7825/AD7829 TO ADSP-21xx INTERFACING MULTIPLEXER ADDRESS INPUTS AD7822 STANDALONE OPERATION OUTLINE DIMENSIONS ORDERING GUIDE