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First Digitally Programmable, Signal-Conditioning Auto-Zero Amplifier

Analog Devices Inc., a supplier of amplifiers, unveiled the industry's first digitally programmable, signal-conditioning auto-zero amplifier for strain-bridge and other sensors that are widely used in automotive, medical, industrial and communication applications. Within the new chip, ADI has integrated a full range of amplifier, comparator, resistor, trimpot and buffer functionality into a tiny SOIC or 4 mm by 4 mm chipscale package (CSP), providing a complete path from a sensor to analog-to-digital (A/D) converter. This allows systems designers to automatically, under software control, eliminate variations between sensors. Sensor outputs can be digitally programmed for gain and output offset voltage. Prior to the ADI solution engineers relied on devices that had lower accuracy and less stability and involved manual adjustments that were difficult, time consuming and expensive.

With flexible controls enabled by a one-wire serial interface, the AD8555 offers designers many advantages over typical discrete solutions, including zero drift, programmable gain, a very small footprint and lower cost. By using ADI's patented low-noise auto-zero and DigiTrim technologies, the chip accurately converts differential and single-ended sensor outputs to a well-defined output voltage range, effectively rejecting common mode noise in the process. The gain and offset adjustment can be simulated in-circuit before being permanently programmed. The chip also offers extensive fault detection (open and short wire and data integrity) to ensure high reliability. In addition, users can easily add a low-pass filter and clamp the maximum output level to protect low voltage A/D converter inputs.

The AD8555's unique combination of precision performance and functionality in a very compact footprint make it an ideal solution for automotive pressure sensor applications, such as brake, fuel and manifold pressure sensing. It is also suitable for myriad other automotive, industrial and medical applications that rely on high performance process control circuitry, including leak-down pressure detection, resistance temperature detectors, infrared thermopile sensors and ECG/EKG monitors.

“The importance of time-to-market, design flexibility, and system accuracy cannot be stressed enough, especially in mission-critical automotive, industrial and medical applications,” said Steve Sockolov, product line director, Precision Analog Products, Analog Devices. “The AD8555 amplifier allows designers to spend more time on the innovation process and less time on design. ADI is committed to empowering designers with breakthrough products that simplify the design process and allow them to maintain a competitive edge.”

The AD8555 offers integrated low-pass filtering, open and shorted wire fault detection, and output clamp voltage capability — ideal for driving low-voltage analog-to-digital converters. In addition, the AD8555 offers a read-back function to verify correct programming, and a trim lockout function to assure field reliability. By integrating these features, the AD8555 allows users to manage sensor outputs easily and cost effectively.

The AD8555 has 10-uV max input offset voltage over temperature, 50-nV/degrees C max input offset drift, and 96-dB common mode rejection, resulting in high dc accuracy. Gain is digitally programmable with high resolution over a wide range from 70 to 1280 through a single-wire interface. Output offset voltage is digitally programmed and ratiometric to the supply voltage. The AD8555 allows gain and offset adjustment to be simulated in-circuit before being permanently programmed with reliable poly-fuse trim technology. The amplifier can also drive high capacitive loads.

Analog Devices, Inc. Tel: 800-ANAL-OGD (800-262-5643)

The AD8555 op amp from ADI will be a big success in the year ahead. It's based on the company's DigiTrim technology, which allows in-circuit programming by the user. Many users that want this op amp capability are using various types of sensors especially pressure sensors. These customers will be glad to use this op amp because the wide variance in the manufacture of sensors forces users to hand-select resistors to adjust for sensor variations. Customers select the resistors to get an op amp gain that matches the sensor gain they need to achieve the desired system output. You can imagine how tedious and expensive that is in a manufacturing environment.

ADI saw a chance to offer a modified version of its DigiTrim technology for these sensor designs, one that the company used internally to trim its own amplifiers. The company worked with some customers and came up with a new concept to use DigiTrim at the system level. The product allowed ADI to program the gain and output offset, not just the offsets from the part or sensor but any of the other errors in the system can be corrected. ADI also managed to make use of some low-noise auto-zero technology so that the amplifier didn't contribute any error to the system output. Now you only get offsets of approximately 1 uV and drifts less than 50nV/C. That means the amount of error caused by the amp is negligible compared to what you get in other parts of the system.

The basic principles underlying DigiTrim are applicable to adjusting gain, offset and a variety of other parameters as well. The difficult part with DigiTrim was getting the information onto the part and translating that into an analog circuit. That's where ADI has some patents, and they have some active components on the die. While the part is in circuit the company makes some poly fuses that set the internal adjustments. In the case of the DigiTrim amplifiers ADI is using the poly fuses to adjust some D/A converters on an amplifier to adjust the current on the input stages to balance the input stages to eliminate the offset voltage. ADI is able to adjust the gain and offset by adjusting resistors rather than current sources.

It's true that other op amps are programmable but the gain is only programmable in large discrete steps such as 1, 5, 10, 50, or even 100. That may be good for adjusting overall gain but it doesn't allow you adjust for individual sensor variation. For that you need a very fine adjustment in the gain. The 8555 has an adjustable gain from about 80 to approximately 1300 in steps of less than one. That's equivalent to about 10-bits resolution.

This part has such a small input offset adjustment that you don't need to adjust for it. However, the output offset must be adjusted. For pressure sensors for example, the amount of the offset on the sensor is often the same order of magnitude as the full scale output of the sensor. So if the full scale pressure output is 5 or 10 mV then the offset may also be 5 or 10 mV. It is hard to adjust for that much offset when you're still looking for a full scale output that's the same order of magnitude. The AD8555 allows you to do that. You also could have a part that has a nominal full scale output of 10 mV but the range of output may only be between 5mV and 15mV. So you would need a very wide range of adjustment and you would need a fine scale of adjustment to adjust it to an individual sensor. This is important to some customers because it could solve some manufacturability problems and also provide better overall accuracy than a manual process could offer.


Schematic
AD8555 Schematic

ADI also looked for common elements in a sensor environment. In almost any environment that uses sensors you typically are concerned about broken or shorted wires that give an invalid output from the system. Feedback control loops especially need to have accurate output from sensors because it could cause incorrect control, which is especially important in safety applications. ADI was able to build-in open- and shorted-wire fault detection into the circuit. It does, however, require additional comparators and other circuits.

Another common attribute of sensors is that they frequently are low frequency devices and typically use a low-pass filter. So ADI built-in some hooks with an appropriate value resistor between a couple buffer stages on the amplifier. All you need to do is connect one external capacitor to get a low-pass filter implementation.

ADI also wanted this 8555 to be easy to use with data converters. So it has a signal chain with a front end that can have converters following it. It's also possible to use the 8555 with a converter that is built into a microprocessor, which is common in automotive applications. Some competitive parts have some of the capability but nothing has the fine adjustment capability of the 8555.

Although the major application is in the automotive industry there are applications for control and measurement applications in the industrial space, as well as the medical market. Other areas like photo diode sensors or current sensing that might be used in broad band controls or network control applications are other possibilities.

The 8555 has a digital interface and an internal state machine with internal controller used to interpret the incoming serial data word to control the function of the adjustment mechanisms on the part. ADI is able simulate all the trimming before making it permanent. So you can use a simulation mode and change something like the gain to see what would happen. For example, you can change the gain to 500 and look at your system measurements to verify that you are getting a gain of 500 or equivalently you are getting the system gain to be the right value to provide the desired outputs. You can also do the same thing to program the output offset voltage or system offset. Once you have the desired values for the offset and gain you go through the program sequence where you permanently blow the fuses that set the values. These are one-time programmable events so those values are then fixed. ADI has been using this poly fuse technology on a number of converters and other parts for many years so it is very reliable. It also works well over extreme temperatures which is a big advantage for the automotive industry. Additionally, ADI says it's very easy to do this programming.

What's the learning curve for programming the part? The company admits that there is a small investment up front for the software development. The algorithm is fairly simple to understand on how to do the adjustments and it doesn't take much time to determine how to do it. All the adjustments after that are mostly automatic. ADI also provides algorithm support with good documentation support in the data sheet, for how the programming works. A demonstration is provided offering some example software of how you can program all the bits in the system. It also demonstrates the use of the part so you can use different gain and offset values and observe what happens at the system level.

The part also has a clamp function. There's an output clamp function that allows you to program the maximum output voltage. It may not be immediately obvious why you would need this but you generally want to run the AD8555 from as high a supply voltage as you can for dynamic range purposes. Although the part will operate down to 3V most customers will want to operate it near 5V to get the best dynamic range from the bridge or whatever sensor type they are using. However, converter technology is going down rapidly to lower line widths to get higher accuracy and lower costs. So, many converters are using full scale 3V input. By using the clamp function you can run the AD8555 at 5V and still clamp the output so you don't over-range the A/D converter that follows in the circuit. Many of those converters can be damaged if you apply to much voltage to the input.

The clamp function is basically a buffered input that drives the supply on the output amplifier stage. Using the buffered input allows you to use a very simple resistor divider to set the clamp voltage. This came about from one of ADI's customers wanted to be able to drive either 5V or 3V converters. This customer was building a clamp circuit externally and it cost several components and didn't provide the accuracy of the clamp voltage that they wanted. That's when ADI built it into the functionality of the op amp device. This is great move by ADI because many customers will want this capability.

You may wonder what you need to get up and running. You will need some typical test equipment to test the sensor systems. Possible additions would include something to enable you to feed a digital serial word into the part, which is fairly simple. The only other requirement is that you need to be able to adjust the supplies to about 5.5V to ensure proper programming. That's usually very simple with programmable supplies that are used in a test system. Finally, the supplies also have to be able to supply enough current, about 100 to 200 mA, to blow the fuses.

The AD8555 is currently sampling in 8-lead SOIC or 16-lead LFCSP packaging. The device is priced at $2.80 and $2.90, respectively, per unit in 1,000-piece quantities.

For more information, please visit:AD8555

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