Fastest Fixed-Gain Amplifier for Video Signal Processing

Texas Instruments Incorporated (TI) announced an ultra-wideband, fixed-gain video buffer amplifier from the company's Burr-Brown product line providing gains of +1, -1 or +2 using internal resistors. The combination of high bandwidth (700 MHz), high slew rate (2500 V/us), and low supply current (13 mA) makes the OPA693 an ideal video line driver or analog-to-digital converter input driver.

“The OPA693 eclipses previous video line driver solutions, providing an unsurpassed pulse response for emerging high resolution RGB applications. In addition to being the fastest +/-5 V capable fixed-gain amplifier on the market, the OPA693 offers considerably improved DC and AC precision at lower pricing than competitive devices,” said Michael Steffes, strategic marketing manager for TI's high-speed signal processing group. “Video applications will benefit from the exceptional pulse fidelity at high pixel rates offered by the 700 MHz bandwidth and 2500 V/us slew rate. The OPA693 is also the only fixed gain of +2 video line driver in the >250 MHz speed range that is capable of operating on a single supply from +5 V to +12 V.”

The OPA693 features an optional disable function for further system power reduction. Under normal operating conditions, the OPA693 uses only 13 mA supply current. The supply current drops to less than 170 uA when the disable control pin is pulled low.

Block Diagram
Example Application Block Diagram

Depending on the external connections, the internal resistor network may be used to provide either a fixed gain of +2 video buffer or a +/-1 voltage buffer with better than 1% gain accuracy. A new output stage architecture delivers high output current with a minimal headroom and crossover distortion, allowing exceptional single-supply operation. Using a single +5 V supply, the OPA693 can deliver a 2 Vpp output swing with over 90 mA drive current and 400 MHz bandwidth at a gain of +2.

Texas Instruments Incorporated Semiconductor Group, SC-03251 Literature Response Center P.O. Box 954 Santa Clarita, CA 91380 Tel: 1-800-477-8924

The OPA693 took shape from requests that TI had from some high-end customers who had been using an earlier version op amp, the OPA692. The 692 is a 240 MHz part that also has internal resistors that is simple to use only requiring you to connect it and voila!, you get a gain of two. It is a very popular part but customers said that since the pixel rates are increasing there is a need for a 400 MHz part. This new part at 700 MHz ensures that over a process and temperature the customer can get a solid gain of 2, 400 MHz bandwidth op amp. The 693 is a fairly large step going from 240 to 700 MHz. This is a +/-5V part so the outputs don't swing to the rails. But for AC coupled applications, at 5V it can do 3V swings.

However, this is no slam-dunk winning part in a market crowded with big competitors, including companies like Elantec/Intersil. TI says it is not yet doing single supply rail-to-rail I/O with these parts because when you go to a rail-to-rail output stage you run into problems with slew rate, and stability issues with load. “If you look at the pieces of amplifiers and do a loop analysis, what really changes going rail-to-rail inside the loop is that you go from a low output impedance Class AB emitter-follower type design to a collector output, which is a high impedance device that is sensitive to load,” says Mike Steffes, TI's Strategic Marketing Manager for High-Speed Amps. “It is very difficult to get the performance with rail-to-rail output stages. If you say you are getting it with a reasonable power level then you need to look closely at the specs to determine what spec game is being played, because it's physically hard to do,” continued Steffes.

This part is a current feedback device, and that provides nearly unlimited slew rate performance. However, the slew rate number is a difficult number because it was derived from the voltage feedback architecture, so you need to know how it meets your other requirements related to slew rate, such as settling time.

The settling time is another interesting but slippery term used to spec op amps. “You can write equations for settling time to within a certain error band, depending on the second order of response that you impose on the device,” says Mr. Steffes. “Every device is usually very complex in its frequency response characteristics. If you can map it into second order response then you can write equations for the settling time. With a part like this you can expect about 8-10 bits of resolution for video D/A converters,” he said. TI does spec a settling time and it's always non-slew limited and in this case it uses a 2-V step. At 0.1%, which is about a 10-bit level, and it has a 12ns settling time.

One of the critical parts a designer chooses with this device is the feedback resistor. The resistor is a compensation element in a current feedback device. It sets your flatness, and your pulse response characteristics such as overshoot and settling time. TI decided that the best internal resistor for this amp is a 300 ohms part.

How did TI decide that a 300 ohm resistor was the best for this part? “If you use the complete loop gain analysis for current feedback, what you care about from the output pin to the inverting node is the transimpedance (voltage to current into the inverting node) number,” explained Mr. Steffes. “The inverting node has its own frequency response and it's typically inductive with some parasitic capacitance, so it has a self-resonance at really high frequencies. As you put different feedback resistor values they can have an effect, especially when you get up to GHz frequencies. So it helps to use lower value resistors. The drawback to using lower values is that the network becomes part of the load to the output stage. There are parts that only provide the specified frequency response when you use less than 100 ohms feedback resistor because of the parasitic input capacitance looking to the inverting node. So, to get the frequency response, you need less than a 100 ohm feedback resistor. However, at a gain of 2 you would have a 200-ohm load in parallel with your actual load. That forces you into a higher output stage capability to get that,” said Mr. Steffes. TI looked at it from all those different angles to determine the 300 ohm resistor was best. It's a value that makes a lot of sense and typically it's used in parallel with a 150 ohm load that is a typical video load. Additionally, it's important to remember that in anything analog you don't improve on one spec without lessening another spec value. The game is to determine how much you've lost, where you lost it and if it is acceptable.

The OPA693 current is trimmed in production ranging from 13.3 to 12.5 mA at room temperature and varies from 11 to 14.1mA at -40C to +85C. The part is nominally in the 700 MHz region with approximately 2500 V/us slew rate. At full temperature range (-40C to +85C) it has a range from 480 to 2,000 V/us. This means that over process and temperature variances it can provide these specs for the targeted 400 MHz region.

The part is intended to be an easy to use, drop-in, line driver. It's hard to get a smaller footprint than the SOT package for that function. TI sees this op amp as an easy to use gain of 2 pulse amplifier. That means it will pass whatever you put into the output as well as any competitor's part available, especially when you consider the very aggressive price of $1.30.

The OPA693 is packaged in a SOT23-6 and an SO-8, and is available now from TI and its authorized distributors. The device is priced from $1.30 in 1,000 piece quantities.

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