Datasheet AD6640 (Analog Devices) - 20

AD6640 +5V (A) +3.3V (D) CMOS AD6620 ADSP-2181 PRESELECT 5MHz–15MHz BUFFER (REF. FIG 27) FILTER LNA PASS BAND 348
⍀
D11 AIN LO I & Q AIN DRIVE 12 DATA NETWORK AD6640 CONTROLLER 1900MHz INTERFACE ENCODE M/N PLL SYNTHESIZER REF ENCODE IN D0 CLK 65MHz REFERENCE CLOCK
Figure 28. Simplified Wideband PCS Receiver band-pass filter will remove harmonics generated within the
Table II.
amplifier, but intermods should be better than the performance of the A/D converter. In the case of the AD6640, amplifier ENCODE Rate 60 MSPS intermods must be better than –80 dBFS when driving full- Fundamental 7.5 MHz–15 MHz scale power. As mentioned earlier, there are several amplifiers Second Harmonic 15 MHz–30 MHz to choose from and the specifications depend on the end Third Harmonic 22.5 MHz–30 MHz, 30 MHz–15 MHz application. Figure 29 shows a typical multitone test. Another option can be found through band-pass sampling. If the
0
analog input signal range is from dc to fS/2, then the amplifier and filter combination must perform to the specification required. However, if the signal is placed in the third Nyquist zone (f
–20
S to
ENCODE = 65MSPS
3 fS/2), the amplifier is no longer required to meet the harmonic performance required by the system specifications since all
–40
harmonics would fall outside the pass-band filter. For example, the pass-band filter would range from fS to 3 fS/2. The second
–60
harmonic would span from 2 fS to 3 fS, well outside the pass- band filter’s range. The burden then has been passed off to the
–80
filter design, provided that the ADC meets the basic specifications at the frequency of interest. In many applications, this is a worth-
–100
while trade-off since many complex filters can easily be realized using SAW and LCR techniques at these relatively high IF fre-
POWER RELATIVE TO ADC FULL SCALE – dB –120
quencies. Although harmonic performance of the drive amplifier
dc 6.5 13.0 19.5 26.0 32.5 FREQUENCY – MHz
is relaxed by this technique, intermodulation performance cannot be sacrificed since intermods must be assumed to fall in-band for Figure 29. Multitone Performance both amplifiers and converters. Two other key considerations for the digital wideband receiver
Noise Floor and SNR
are converter sample rate and IF frequency range. Since perfor- Oversampling is sampling at a rate that is greater than twice the mance of the AD6640 converter is largely independent of both bandwidth of the signal desired. Oversampling does not have sample rate and analog input frequency (TPCs 4, 5, and 10), the anything to do with the actual frequency of the sampled signal; designer has greater flexibility in the selection of these parameters. it is the bandwidth of the signal that is key. Band-pass or IF Also, since the AD6640 is a bipolar device, power dissipation is sampling refers to sampling a frequency that is higher than Nyquist not a function of sample rate. Thus there is no penalty paid in and often provides additional benefits such as down conversion power by operating at faster sample rates. All of this is good using the ADC and replacing a mixer with a track-and-hold. Over- because, by carefully selecting the input frequency range and sampling leads to processing gains because the faster the signal is sample rate, some of the drive amplifier and ADC harmonics digitized, the wider the distribution of noise. Since the integrated can actually be placed out-of-band. noise must remain constant, the actual noise floor is lowered by For example, if the system has second and third harmonics that 3 dB each time the sample rate is doubled. The effective noise are unacceptably high, by carefully selecting the ENCODE rate density for an ADC may be calculated by the equation and signal bandwidth, these second and third harmonics can be placed out-of-band. For the case of an ENCODE rate equal to V / Hz = 10−SNR/20 60 MSPS and a signal bandwidth of 7.5 MHz, placing the fun- NOISE rms 4 FS damental at 7.5 MHz places the second and third harmonics out of band as shown in the Table II. For a typical SNR of 68 dB and a sample rate of 65 MSPS, this is equivalent to 25 nV/√Hz. This equation shows the relationship between the SNR of the converter and the sample rate fS. This equation may be used for computational purposes to determine overall receiver noise. REV. A –19– Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM PRODUCT HIGHLIGHTS SPECIFICATIONS DC SPECIFICATIONS DIGITAL SPECIFICATIONS SWITCHING SPECIFICATIONS AC SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS EXPLANATION OF TEST LEVELS ORDERING GUIDE PIN FUNCTION DESCRIPTIONS PIN CONFIGURATION DEFINITION OF SPECIFICATIONS Analog Bandwidth (Small Signal) Aperture Delay Aperture Uncertainty (Jitter) Differential Nonlinearity Encode Pulsewidth/Duty Cycle Integral Nonlinearity Minimum Conversion Rate Maximum Conversion Rate Output Propagation Delay Power Supply Rejection Ratio Signal-to-Noise-and-Distortion (SINAD) Signal-to-Noise Ratio (SNR) Spurious-Free Dynamic Range (SFDR) Two-Tone Intermodulation Distortion Rejection Two-Tone SFDR Worst Harmonic Equivalent Circuits Typical Performance Characteristics THEORY OF OPERATION APPLYING THE AD6640 Encoding the AD6640 Driving the Analog Input Power Supplies Output Loading Layout Information Evaluation Boards DIGITAL WIDEBAND RECEIVERS Introduction System Description System Requirements Noise Floor and SNR Processing Gain Overcoming Static Nonlinearities with Dither Receiver Example IF Sampling Using the AD6640 as a Mix-Down Stage RECEIVE CHAIN FOR A PHASED-ARRAY CELLULAR BASE STATION OUTLINE DIMENSIONS Revision History