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TI's Next-Generation Precision, Low-Voltage Op Amps Target Portable Applications

The manufacturer says . . .

ChipCenter's Paul O'Shea says . . .

Texas Instruments (TI) Incorporated today announced a series of precision 1.8V operational amplifiers from the company's Burr-Brown product line which are optimized for battery-powered applications such as sensor amplification, signal conditioning, data acquisition, handheld instrumentation, portable audio and portable medical. The OPA363 and OPA364 op amps feature low offset (500uV max) and low quiescent current (750uA/ch max). Rail-to-rail input without crossover between input stages reduces distortion and improves CMRR (100dB) over the entire input range. "The OPA363 and OPA364 provide designers with a cost-effective series of precision op amps which extends TI's industry-leading portfolio of 1.8V to 5.5V amplifiers," said Tadija Janjic, strategic marketing engineer of TI's high-performance amplifier products. "Stellar performance at 1.8V makes the OPA363 and OPA364 ideal for 2-cell battery applications. These amplifiers are also excellent front-end devices for driving TI's low-voltage analog-to-digital converters such as the ADS8324 and ADS8325." Other features include micro-size SOT23 packages, rail-to-rail output swing within 5mV of the rails, high bandwidth (7MHz) and high slew rate (5V/us). The OPA363 family includes a shutdown mode which reduces power consumption to 0.9uA. All versions are specified from -40C to +125C, making this series well suited for automotive applications. Texas Instruments Incorporated provides innovative DSP and analog technologies to meet our customers' real world signal processing requirements. In addition to Semiconductor, the company's businesses include Sensors & Controls, and Educational & Productivity Solutions. TI is headquartered in Dallas, Texas, and has manufacturing, design or sales operations in more than 25 countries. Texas Instruments Incorporated Semiconductor Group, SC-02245 Literature Response Center PO Box 954 Santa Clara, CA 91380 Tel: 800-477-8924, ext. 4500 TI Web Site

The release of a 1.8 V rail-to-rail I/O CMOS operational amplifier is no earth shattering news. After all, there are many op amps in the marketplace. What makes this amp different is that it also provides rail-to-rail performance without crossover or distortion. If you have rail-to-rail input, typically there are two input stages that allow the rail-to-rail input. When one input stage turns on and the other turns off there is a change in the offset voltage because each input stage has its own offset. Most amplifier data sheets give you a curve for the bias current versus common mode voltage, allowing you to clearly see where the transition shows up. That transition is a very abrupt jump, so it causes distortion and reduces the performance of the part. You can see that very well if you use the amplifier as a buffer for the higher performance analog-to-digital converters of 14 to 16 bits resolution. The converter will sense the jump and it will show up as a reduction in total harmonic distortion. TI says that until now designers had to deal with input stage crossover as a fact of life. The company says that the OPA363/4 solves this issue because they only have one p-channel input stage, and that allows you to go down to the ground on a common mode voltage and 1.5 V from the supply rail. Additionally, TI provided a low-noise, low-ripple charge pump in the amplifier. The charge-pump increases the power supply voltage on the input stage by only 1.8 V. So the input stage sees Vcc plus about 1.8 volts, preventing it from turning off. Internally the input stage runs on a higher voltage than the Vcc supplied by the customer. This, according to TI, allows the input stage to go from rail to rail. TI has a patent pending for the charge pump topology. The company designed the part to meet the needs of their Data Acquisition Group who had designed a 1.8 V, 14-bit data converter but needed a rail-to-rail I/O amplifier that worked on 1.8 V and didnęt degrade the performance of the converter. In answer to that need, the Burr-Brown group designed a 1.8 V to 5.5 V part that addressed the needs of driving a SAR converter for 100 kHz sampling. The OPA363 amplifier is good on its own but it is also an excellent complement to an analog-to-digital converter. TI says they didnęt emphasize the charge pump in the press release because those words typically make the designer uneasy since charge pumps have a reputation of adding noise to a system. However, since this is an ultra low-power and low-noise design it is very likely that the application will have an RC network as a filter on the output of the amplifier. That RC network will take care of any ripple from the charge pump. The company says that because the charge pump runs at 7 MHz, which is the bandwidth of the amplifier, it wonęt be seen in the signal range unless you use it as a buffer. And even then it is at the end of the spectrum, at a very low amplitude. The press release states that the part can operate with a negative supply, compared to some op amps that go into a latch state when they use a negative supply. The company says that this means you can put 5.5 V between supplies, which means it can be -2.5 V and +2.5 V, and you would use a ground as the true reference point. Another interesting aspect about this part is its 500 micro volts of offset, which is good precision, and the addition of the low noise factor makes for good overall performance. The part also provides shutdown for power sensitive customers. The 363 single and dual supply version offers the shutdown capability, while all the 364 versions and the quad 363 versions donęt provide the shutdown function. TI may not be the first to have this type op amp but they may be the first to have an op amp that goes down to 1.8 V, gives you rail to rail performance and doesnęt penalize you with a crossover region. This is important for some op amps operating on 1.8 V that may have a crossover region of 350 to 400 mV, or about 20 to 25% of the available input range, which limits your performance. This low-voltage amplifier allows you to do processes that used to require a 5-V supply. TI admits that there are a couple amplifiers from other companies that also operate at 1.8 V. However, even though the op amps from competitors have rail-to-rail input, they use two input stages. When used on a 14-bit A to D converter they get some loss - as much as 3-bits. That means you really need to look at where the part will be used, not just its individual specs, before you get the whole picture about the op amp. The reality is that the amplifier can limit the ability of the converter. The OPA363 (single version with shutdown) comes in a SOT23-6 and SO-8. The OPA364 (single without shutdown) is packaged in a SOT23-5 and SO-8. The OPA2363 (dual with shutdown) comes in an MSOP-10 and the OPA2364 (dual without shutdown) is available in an MSOP-8 and SO-8. The OPA363 and OPA364 are priced from $0.55 in 1,000 piece quantities (suggested resale price). The OPA2363 and OPA2364 are priced from $0.88 in 1,000 piece quantities (suggested resale pricing). Quad versions, the OPA4363 and OPA4364, will be available in 1Q03. Both devices will be packaged in a TSSOP-14 and SO-14. OPA363 Data Sheet Product Brochure

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