The ever-improving balance of bandwidth, resolution, and power consumption are making modern analog-to-digital converters an essential component in high-frequency systems.
Analog signals once dominated high-frequency designs, such as receivers. But with the growing availability of high-speed analog-to-digital converters (ADCs), with impressive capabilities in translating analog signals to the digital realm, an increasing amount of signal processing is being done by digital hardware. ADCs are currently available from a wide range of suppliers, in numerous formats include as chips, as plug-in circuit cards, and as printed-circuit-board (PCB) assemblies.
ADCs can be readily differentiated by a handful of basic performance specifications, including bit resolution, input bandwidth, sampling rate, bit linearity, power consumption, noise performance, and output types. In operation, ADCs generate a digital code for a discrete value of input voltage. The number of codes is simply 2N where N = the bit resolution of the ADC. An 8-b device, for example, uses 28 or 256 different digital codes to represent an analog waveform. Although high-performance digital-audio applications have standardized on the use of 24-b converters at sampling rates as high as 192 kSamples/s (kHz), RF and microwave applications generally rely on ADCs with anywhere from 6 to 14 b resolution. (For those wishing a thorough introduction to ADCs, the 69-page "ABCs of ADCs," written by Nicholas Gray, Staff Applications Engineer for the Data Conversion Systems of National Semiconductor, is available for free download from National's website at www.national.com.)
The number, N, of digital bits is directly related to the ADC's signal-to-noise ratio (SNR) and spurious-free dynamic range (SFDR). For an ADC, the dynamic range is the ratio of the largest resolvable signal to the smallest resolvable signal. For an ideal ADC it is 6.02N, or about 48 dB for a 8-b ADC. Thus, while it is desirable to have as many bits of resolution for a given application, a general trade-off for ADCs is decreasing bits for increasing bandwidth. Several suppliers, for example, offer ADCs with 1 GHz or greater input bandwidths, although the bit resolution is generally 8 b or less. Nyquist theory is a reliable guide when specifying an ADC for an application. According to Nyquist's criteria, an analog signal with a given bandwidth must be sampled with a sampling rate of at least two times that bandwidth to avoid loss of information. If sampling occurs at a rate of less than two times the bandwidth, a phenomenon known as aliasing of the analog signal will occur resulting in distortion of the analog waveforms. Even when the bandwidth of an ADC is properly matched to an application, aliasing can occur from an excessively wide bandwidth filter preceding the ADC that allows broadband noise to pass into the ADC and/or signal harmonics which the ADC attempts to digitize.
It should be noted that, due to noise and bandwidth limitations, the effective number of bits (ENOB) is a specification often quoted by ADC manufacturers to represent a practical limit for the bit resolution of their devices. Ideally, an 8-b ADC would deliver all 8 b of resolution. Practical considerations, however, typically reduce an ideal 8-b ADC to an ENOB of 7.5 b.
For an integrated-circuit (IC) ADC, the analog input bandwidth is generally set by an on-board track-and-hold (T/H) amplifier preceding the quantizer circuitry. The MAX108 single-channel ADC from Maxim Integrated Products (Sunnyvale, CA), for example, is a 8-b device with sampling rate of 1.5 GSamples/s. It features an integrated T/H amplifier with 2.2-GHz full-power bandwidth and an on-chip +2.5-V precision bandgap voltage reference. The device achieves a near-ideal SNR of 46.8 dB and SFDR of −54 dBc. Designed for supply voltage of ±5 VDC, the ADC handles input signals over a range of ±250 mV. The accuracy of the component can be summarized in terms of its integral-nonlinearity (INL) and differential-nonlinearity (DNL) performance levels, which generally refer to the accuracy of an ADC's overall analog-to-digital transfer function and the accuracy of the step sizes in the ADC, respectively. Measured in terms of a range of an ADC's least-significant bit (LSB)—the smallest digital "slice" of the analog signal—the INL and DNL specifications are generally less than ±1 LSB to avoid ambiguities in the ADC's output data. In some cases, manufacturers will note "no missing codes" on their ADC data sheets to assure engineers that all digital codes are used. For the MAX108, the INL is ±0.25 LSB while the DNL is also ±0.25 LSB. The MAX108 is supplied in a 192-contact enhanced super ball-grid-array (ESBGA) housing. Lower-speed, pin-compatible versions of the device are also available, including the 1-GSamples/s model MAX104 and the 600-MSamples/s model MAX106.
Linear Technology (Milpitas, CA) also offers a family of high-speed ADCs in their LTC1740 series. The 12- and 14-b converters deliver SNR performance of better than 72 dB and SFDR performance of better than −85 dBc across wide input bandwidths. The model LTC1748, for example, is a 14-b ADC with 80 MSamples/s sampling rate. It achieves a 76.3-dB SNR at 5 MHz with a SFDR of −90 dBc at 5 MHz. As with the integrated devices from Maxim, the on-board T/H amplifiers dictate the maximum input bandwidths for these devices, and the LTC1748 offers among the widest bandwidth with a 240-MHz full-power T/H amplifier bandwidth.
Analog Devices (Norwood, MA) also supplies a wide range of high-speed ADCs, including the 14-b model AD6645 with 105 MSamples/s sampling rate. The monolithic converter includes a 200-MHz T/H amplifier and on-board voltage reference and maintains an SNR of 72 dB through the full bandwidth. The SFDR is 89 dBc for input signals to 70 MHz at sampling rates to 105 MSamples/s. The fourth-generation device is designed for multichannel, multimode receivers in which high multitone dynamic range is essential. The company's model AD9245 is a 14-b, 80-MSamples/s ADC that delivers 72 dB SNR and 85 dBc SFDR while consuming less than 500 mW power from a +3-VDC supply.
At lower-bit resolution, Analog Devices' model AD9410 is a 10-b, 210-MSamples/s ADC with 500-MHz analog bandwidth, on board voltage reference, integrated T/H amplifier, and demultiplexed outputs. Designed for +3.3 and +5 VDC supplies, it typically dissipates 2.1 W power and achieves an SNR of 54 dB with a 99-MHz input signal. The SFDR is 62 dBc for two-tone analog inputs at 80 and 81 MHz.
The company recently announced its AD92x9 family of quad ADCs: four converters fabricated on a single chip. Designed for space-constrained systems, such as medical imaging systems and multicarrier cellular base stations, and well suited for digitizing signals in multicarrier cellular base stations, the 12-b, 65-MSamples/s model AD9229 quad converter achieves a 70-dB SNR and 85-dBc SFDR with ±0.3 LSB DNL and ±0.6 LSB INL. It is designed for +3-VDC supplies and is supplied in a 48-pin LFCSP package. In addition, model AD9289 is an 8-b version in a ball-grid-array (BGA) package, with 47 dB SNR and 60 dBc SFDR. The ADCs feature serial low-voltage differential-signaling (LVDS) data outputs to simplify PCB layouts.
National Semiconductor (Santa Clara, CA) offers a 1-GHz ADC with their new ADC081000, an 8-b model with integral voltage reference and S/H amplifier. With DNL of ±0.25 LSB and power consumption of 1.4 W typical from a single +1.9-V supply, the converter employs an internal 1:2 demultiplexer to feed two LVDS buses and reduce differential output data on each bus to one-half the sampling rate. The ADC, which is ideal for direct downconversion in RF receivers, WLAN systems, and instrumentation and supplied in a 128-lead exposed-pad LQFP housing, features a power-down mode to reduce power consumption to typically 5 mW.
Texas Instruments (which acquired Burr-Brown) offers a 14-b ADC, model ADS5500, capable of sample rates to 125 MSamples/s. The single-channel device features an input bandwidth of 750 MHz, SNR of 70 dB, and SFDR of 82 dBc. The pipelined converter, which achieves INL of ±0.5 LSB, is designed for a +3-VDC supply and typically consumes 750 mW power.
Some of the highest-speed ADCs currently on the market are GaAs monolithic devices from Rockwell Scientific Company, including the 6-b model RAD006, a 6-GSamples/s ADC. With an analog input bandwidth of 10 GHz, the device offers ENOB of 5.5 dB from DC to 3 GHz with SNR of better than 34 dB and SFDR or better than 49 dBc. With an INL of less than 1 LSB and DNL of less than 0.5 LSB, the high-speed ADC features a differential full scale range of 2 V peak to peak (pp). For those requiring slightly less speed but higher resolution, the company also offers the RAD008, with 8 b resolution and sampling rates to 3 GSamples/s (also a 10-GHz input bandwidth) and the 10-b model RAD0010, usable at sampling rates to 1 GSamples/s over a differential full-scale range of 1 V pp. The 8-b device offers SNR of better than 46 dB and SFDR of better than 67 dBc while the 10-b device has SNR of better than 55 dB. In addition to the companies mentioned, high-performance ADC ICs are also available from DATEL (Mansfield, MA), Fairchild Semiconductor (South Portland, ME), and Philips Semiconductors (Sunnyvale, CA).
When the digitizing function is needed along with supporting circuitry, such as memory and digital signal processing (DSP), it may make more sense to specify a board-level ADC rather than a chip. Maxtek, a subsidiary of Tektronix (Beaverton, OR), offers a variety of design and manufacturing services, including the use of thick-film ceramic and cofired multilayer-ceramic circuit boards, to create custom ADC-based signal-processing solutions.
In some cases, ADC cards and boards (also known as digitizers) resemble full-function measurement systems. The DBS908 digitizer from Analogic Corp. (Peabody, MA), for example, is a modular mezzanine board designed for use with the company's model DBS9905 C-size VXI carrier module. The digitizer features a sampling rate of 2 GSamples/s with 8-b resolution, 500-MHz analog input bandwidth, and 4-MSamples on-board memory. The single-channel DBS908 has an oscilloscope-type front end that can be programmed for AC or DC coupling and 50-(omega) or 1 M(omega) input impedance for a variety of measurement options. Signal acquisition can be initiated by triggering on the signal of interest (using an internal trigger), with an external trigger, or by VXI TTLTRG lines. By mounting several modules on the DBS9905 carrier, multiple signal-processing functions are possible from a single C-size VXI card. The DBS908 digitizer is well suited for automatic-test-equipment (ATE), mass spectrometry, and telecommunications testing applications.
Acqiris (Monroe, NY) is a Swiss company long associated with digitizer-on-board products. The company's model DC440 digitizer, for example, has been developed specifically for frequency-domain applications. It offers a 100-MHz DC-coupled standard input and a 300-MHz AC-coupled high-frequency input. Signals are captured on two channels at sample rates to 400 MSamples/s (per channel) and 12-b resolution. The board's ADC's achieve signal-to-noise ratios of better than 65 dB and spurious-free dynamic range (SFDR) of better than 80 dB. The digitizer is supplied on a PXI-compatible, 6U CompactPCI card for personal computers. The high-speed PCI bus can transfer data from the card to the computer at sustained rates to 100 Mb/s. The digitizer, which is supplied with device drivers for Windows 95/98/NT/2000/XP, VxWorks, and Linux, includes application code examples for C/C+, Visual Basic, and LabVIEW from National Instruments. The card consumes less than 25 W power.
LeCroy Corp. (Chestnut Ridge, NY) supplies the PXD series of PXI-based digitizers with bandwidths from 150 to 1000 MHz. Operating at sample rates to 2 GSamples/s, the cards are available with generous 256k acquisition memory (which can be increased as an option) and from 1 to 4 measurement channels. The PXD1022, for example, is a two-channel digitizer that fits into three PXI slots. It has a 1-GHz bandwidth, maximum single-shot sampling rate of 2 GSamples/s, and maximum repetitive sampling rate of 50 GSamples/s. The digitizer is ideal for commercial communications and military/aerospace test applications.
Additional board-level digitizers are listed in the table, including Echotek (Huntsville, AL), which offers the ECAD-1-081000 single-channel VMEbus ADC. This 8-b digitizer features a 1-GHz input bandwidth and 1 GSamples/s sampling rate with better than 42-dB SNR and 52 dBc SFDR. With trigger accuracy within two clock cycles, the digitizer board includes external analog, clock, and trigger input ports and as much as 16 MSamples of on-board memory storage, contained on a single-slot 6U VME module.