hf/rf ZONE Products for the week of December 1, 2003
Texas Instruments Says . . .
ADS5500: 14-Bit, 125-Msample/s ADC for Advanced Signal
Processing Applications
Combines Performance (SNR/SFDR) at Highest Speed with Only
750-mW Total Power
Utilizing its leading-edge high-performance analog design expertise, Texas Instruments Incorporated (TI) announced a technology advancement with a 14-bit, 125-MSPS (mega samples per second) analog-to-digital converter (ADC). The unmatched performance at 125 MSPS allows increased system capabilities in advanced communications, test and measurement, medical, video and imaging applications, while the low total power dissipation (750 mW) improves system reliability and allows higher channel density.
The ADS5500, operating at 125 MSPS, features 70-dB SNR (signal-to-noise ratio) and 82-dB SFDR (spurious-free dynamic range) at 100-MHz input frequency. Equipment manufacturers can capitalize on the performance of the ADS5500 to achieve a wide range of benefits, such as improved receiver performance in wireless communications, higher quality imaging in video systems, extended signal analysis capabilities for test equipment, and more precise scans and lower magnetic field energy in medical Magnetic Resonance Imaging (MRI) equipment.
The low power dissipation reduces power supply requirements and the need for thermal management (cooling), resulting in smaller footprint and equipment size, and higher circuit density. The power savings also enables next-generation, high-performance portable wireless, test and measurement and video imaging systems.
"The ADS5500 combines speed, performance and power attributes unmatched
in the data converter market and extends TI's technology leadership in the
high-performance analog industry," said Gregg Lowe, senior vice president
of TI's high-performance analog business. "The device builds upon many
recent data converter breakthroughs and is further evidence of the collaborative
technology advancements TI has achieved through the Burr-Brown acquisition.
The ADS5500 is one of several significant high-performance data converter
announcements that TI will make over the next year."
analogZONE Says . . .
When you bandy around terms like "highest" you have to be very careful what exactly it is that you mean. Another vendor got a parry from me about this subject a couple of weeks ago, and the initial company riposte was "best is in the eye of the beholder." In an industry dominated by measurement rather than impression, I hope that can never be true. First then, let's make it clear that the ADS5500 is not the fastest clocking 14-bit ADC out there. That accolade belongs to TelASIC's TC1410, released in July 2003, which can be clocked up to 240 Msample/s and which has an input bandwidth in excess of 1 GHz. It is very difficult to compare apples-to-apples in the high-speed ADC arena but the ADS5500 seems to offer -- at frequencies it can input -- better second-harmonic distortion numbers (at 220 MHz) by about 10 dB, and very similar third-harmonic distortion numbers. SNRs seem very similar but whereas the TelASIC part uses an input span of 4 V with a 5-V rail here TI has a common-mode voltage of 1.5 V with a 3.3-V rail (there is a separate supply pin for the same voltage to the digital parts of the circuit.)
The other most significant specification difference is that the SiGe TC1410 dissipates a typical 13 W in a BGA-256, while the CMOS ADS5500 dissipates a typical 750 mW in a TQFP-64. The fact is that the two products are going to go into different sockets and the focus of the companies is rather different.
No, the ADS5500 is set dead-to-rights against ADI's AD6645 whose latest claim was that it could be clocked at better than 92 Msample/s although there was supposed to be a 110-Msample/s version when it was released in October 2001. Apart from the extra speed of the ADS5500 it also halves the power dissipation of the AD6645, although the SFDR numbers still seem to be better on the ADI part -- but if you cannot even use a part at the frequency you want, then it doesn't really matter if some lower operating frequency numbers are better or not.
The input bandwidth of the sample-and-hold of the ADS5500 switched-capacitor pipeline ADC is a typical 750 MHz (50-Ohm source impedance, with an input impedance of 10 MOhm and 6 pF) and the data sheet specifies performance up to 220 MHz inputs. Data latency is 16 clock cycles (both rising and falling edges are used to propagate the sample through the pipeline every half clock cycle) while the voltage overload recovery delay is 4 clock cycles. Aperture delay is a typical 1.5 ns.
With no missing codes guaranteed DNL is a typical ±0.5 LSB (2.4-MHz input) while the INL is not yet characterized, nor are offset and gain errors. At 220 MHz input, the SNR is 69 dBFS, SFDR is 75 dBc, 2nd harmonic is -75 dBc, 3rd harmonic is -81 dBc, and the worst other spur is at -86 dBc.
TI/Burr-Brown made the decision here not to go to LVDS on the output and to stick to standard logic. (If they produce a next-generation, improved version of the part that is a decision which may well need to be looked at again.) But a very nice touch is that the output format can be selected by the voltage on a Data Format Select (DFS) pin to give either straight binary or two's complement and with the data valid on either the rising edge or the falling edge.
The voltage reference is internal but the clock is external and conversion is initiated on the clock's rising edge. Differential operation, of course, is advised for maximum performance and the data sheet offers some circuits to convert, particularly RF, single-ended inputs into differential. Clock drive can be either differential or single-ended with virtually no difference in performance, but from a layout and common-mode noise position differential is preferable. Because propagation is every half clock cycle the clock should have a 50% duty cycle.
This part targets, of course, undersampling of IF signals -- which it will be extremely good at, probably up to about 190 MHz -- high-performance instrumentation, imaging -- especially MRI -- and high-performance video. With the speed, performance, low power consumption and packaging it is going to receive an attentive audience. I think the premium being charged over the ADI part (which is listed at $54 in 1-k lots) will be accepted by early adopters but I would expect that to be the <500 piece price within 6 months of production quantities being available. Will it be a winner? Without any doubt whatever.
The ADS5500 is sampling with a reset required at power-up, Production volumes (with the reset expected to be eliminated) will be in Q1, 2004, in a thermally-enhanced TQFP-64 and priced at $95.00 in 1000-piece lots. An evaluation module is available.