Datasheet AD7887 (Analog Devices) - 10

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Data Sheet AD7887 TERMINOLOGY Integral Nonlinearity Peak Harmonic or Spurious Noise
This is the maximum deviation from a straight line passing Peak harmonic or spurious noise is defined as the ratio of the through the endpoints of the ADC transfer function. The end- rms value of the next largest component in the ADC output points of the transfer function are zero scale, a point ½ LSB spectrum (up to fS/2 and excluding dc) to the rms value of the below the first code transition, and full scale, a point ½ LSB fundamental. Normally, the value of this specification is above the last code transition. determined by the largest harmonic in the spectrum, but for ADCs where the harmonics are buried in the noise floor, the
Differential Nonlinearity
largest harmonic could be a noise peak. This is the difference between the measured and the ideal 1 LSB change between any two adjacent codes in the ADC.
Intermodulation Distortion
With inputs consisting of sine waves at two frequencies, fa and fb,
Offset Error
any active device with nonlinearities creates distortion products This is the deviation of the first code transition (00 . 000) to at sum and difference frequencies of mfa ± nfb where m, n = 0, (00 . 001) from the ideal, that is, AGND + 0.5 LSB. 1, 2, 3, and so on. Intermodulation distortion terms are those for
Offset Error Match
which neither m nor n are equal to 0. For example, the second- This is the difference in offset error between any two channels. order terms include (fa + fb) and (fa − fb), and the third order
Gain Error
terms include (2fa + fb), (2fa − fb), (fa + 2fb) and (fa − 2fb). This is the deviation of the last code transition (111 . 110) to The AD7887 is tested using the CCIF standard in which two (111 . 111) from the ideal (that is, VREF − 1.5 LSB) after the input frequencies near the top end of the input bandwidth are offset error has been adjusted out. used. In this case, the second-order terms are usually distanced
Gain Error Match
in frequency from the original sine waves, and the third-order This is the difference in gain error between any two channels. terms are usually at a frequency close to the input frequencies. As a result, the second- and third-order terms are specified
Track/Hold Acquisition Time
separately. The calculation of the intermodulation distortion is The track/hold amplifier returns to track mode at the end of as per the THD specification, where it is the ratio of the rms conversion. Track/hold acquisition time is the time required for sum of the individual distortion products to the rms amplitude the output of the track/hold amplifier to reach its final value, of the sum of the fundamentals expressed in decibels. within ±1/2 LSB, after the end of a conversion.
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 ADC. The signal is the rms amplitude of the fun- scale 25 kHz sine wave signal to the nonselected input channel damental. Noise is the sum of all nonfundamental signals up to half and determining how much that signal is attenuated in the the sampling frequency (fS/2), excluding dc. The ratio is dependent selected channel. The figure given is the worst case across both on the number of quantization levels in the digitization process: the channels for the AD7887. more levels, the smal er the quantization noise. The theoretical signal to (noise + distortion) ratio for an ideal N-bit converter
Power Supply Rejection (PSR)
with a sine wave input is given by Variations in power supply affect the full-scale transition, but not the converter’s linearity. PSR is the maximum change in the Signal to (Noise + Distortion) = (6.02 N + 1.76) dB full-scale transition point due to a change in power supply voltage Thus for a 12-bit converter, this is 74 dB. from the nominal value. See Figure 7.
Total Harmonic Distortion
PSRR is defined as the ratio of the power in the ADC output at Total harmonic distortion (THD) is the ratio of the rms sum of frequency f to the power of a full-scale sine wave applied to the harmonics to the fundamental. For the AD7887, it is defined as ADC of frequency fS: 2 2 2 2 2 V + + + + PSRR (dB) = 10 log(Pf/Pfs) 2 V3 V4 V5 V6 THD ) dB ( = 20 log where Pf is the power at frequency f in ADC output and Pfs is 1 V the power at frequency fS in ADC ful -scale input. where V1 is the rms amplitude of the fundamental and V2, V3, V4, V5, and V6 are the rms amplitudes of the second through the sixth harmonics. Rev. E | Page 9 of 24 Document Outline Features Applications Functional Block Diagram General Description Product Highlights Table of Contents Revision History Specifications Timing Specifications6F Absolute Maximum Ratings ESD Caution Pin Configurations and Function Descriptions Typical Performance Characteristics Terminology Control Register Theory of Operation Circuit Information Converter Operation ADC Transfer Function Typical Connection Diagram Analog Input On-Chip Reference Power-Down Options Power-Up Times Power vs. Throughput Rate Modes of Operation Mode 1 (PM1 = 0, PM0 = 0) Mode 2 (PM1 = 0, PM0 = 1) Mode 3 (PM1 = 1, PM0 = 0) Mode 4 (PM1 = 1, PM0 = 1) Serial Interface Microprocessor Interfacing AD7887 to TMS320C5x AD7887 to ADSP-21xx AD7887 to DSP56xxx AD7887 to MC68HC11 AD7887 to 8051 AD7887 to PIC16C6x/PIC16C7x Application Hints Grounding and Layout Outline Dimensions Ordering Guide Automotive Products