AnalogTalk

Today’s blog lists common problems associated with using an op amp with a power supply and an input signal on a PC Board. It is divided into four categories: General Suggestions, Input State Problems, Bandwidth Issues, and Single Supply Pitfalls. We would like to hear from you, if you have any other inputs from experience.

In General

1. Be careful of the supply pins. Don’t make them too high per the amplifier specification sheet and don’t make them too low. High supplies will damage the part. In contrast, low supplies won’t bias the internal transistors and the amplifier won’t work or it may not operate properly.

2. Make sure the negative supply (usually ground) is in fact tied to a low impedance potential. Additionally, make sure the positive supply is the voltage you expect when it is referenced to the negative supply pin of the op amp. Placing a volt meter across the negative and positive supply pins will verify that you have the right relationship between the pins.

3. Ground can’t be trusted, especially in digital circuits. Plan your grounding scheme carefully. If the circuit has a lot of digital circuitry, consider separate ground and power planes. It is very difficult, if not impossible, to remove digital switching noise from an analog signal.

4. Decouple the amplifier power supplies with by-pass capacitors as close to the amplifier as possible. For CMOS amplifiers, a 0.1ìF capacitor is usually recommended. Also decouple the power supply with a 10ìF capacitor.

5. Use short lead lengths to the inputs of the amplifier. If you have a tendency to use the white perf. boards for prototyping, be aware that they can cause noise and oscillation. There is a good chance that these problems won’t be a problem with the PCB implementation of the circuit.

6. Amplifiers are static sensitive! If they are damaged, they may fail immediately or exhibit a soft error (like offset voltage or input bias current changes) that will get worse over time.

Input Stage Problems

1. Know what input range is required from your amplifier. If either inputs of the amplifier go beyond the specified input range, the output will typically be driven to one of the power supply rails.

2. If you have a high gain circuit, be aware of the offset voltage of the amplifier. That offset is gained with the rest of your signal and it might dominate the results at the output of the amplifier.

3. Don’t use rail-to-rail input stage amplifiers unless it is necessary. By the way, they are only needed when a buffer amplifier circuit is used or possibly an instrumentation amplifier configuration. Any circuit with gain will drive the output of the amplifier into the rail before the input has a problem.

Do You Have the Bandwidth?

1. Account for the bandwidth of the amplifier when sending signals through the circuit. You may have designed an amplifier for a gain of 10 and find that the AC output signal is much lower than expected. If this is the case, you may have to look for an amplifier with a wider bandwidth.

2. Instability problems can usually be solved by adding a capacitor in parallel with the feedback resistor around the amplifier. This does mean typically and not always. If an amplifier circuit is unstable, a quick stability analysis will show the problem and probably the solution.

Single Supply Rail-to-Rail

1. Operational Amplifier output drivers are capable of driving a limited amount of current to the load.

2. Capacitive loading an amplifier is risky business. Make sure the amplifier is specified to handle any loads that you may have.

3. It is very rare that a single supply amplifier will truly swing rail-to-rail. In reality, the output of most of these amplifiers can only come within 50 to 200mV from each rail. Check the product data sheets of your amplifier.

Related Links

Using Single Supply Operational Amplifiers in Embedded Systems

The Local Interconnect Network (LIN) is being implemented in vehicles around the world to help embedded system designers meet challenges including lower system cost, lower power consumption, weight reduction, and faster time to market for innovative electronic solutions. LIN is a low-cost, serial communication system for distributed electronic systems, such as window controls, seat movement, mirror positioning, LED lighting, and other body-oriented applications.

~20 LIN nodes already populate cars today and that # is growing

Some of the advantages of LIN are:

• Ease of use for the new and experienced designers
• Many components are drop in compatible
• Cheaper than other alternative network communication buses
• Harness reduction
• Provides for a more reliable vehicle
• No protocol license fee required
• Can be used in non-automotive environment similar to CAN

Some frequently asked LIN questions:

Does LIN replace CAN?
Answer:
No, LIN and CAN co exist in the automotive market.

LIN addresses an application space that requires less bandwidth and versatility when compared to the CAN application space.

What are the target applications for LIN Bus?
Answer:
Target automotive applications include mirrors, window lift, doors switches, door lock, HVAC motors, control panel, engine sensors, engine cooling fan, seat positioning motors, seat switches, wiper control, light switches, interface switches to radio/navigation/phone, rain sensor, light control, sun roof, RF receivers, body computer/smart junction box, interior lighting and more.

Can LIN be used for non-automotive applications?
Answer:
Although designed for automotive applications, LIN is not limited to automotive applications and has found it’s way already into applications such as appliance and consumer.

Microchip is a provider of some of the best in class Transceivers that have been accepted by the major OEMs.  Meeting automotive requirements allows our products to more easily proliferate across other non-automotive applications since the parts already meet stringent requirements.

Related Links

LIN Product Pages

LIN Design Center

Remember the good old days when one had to manually turn a hand-crank on the inside of the car door to make the window go up or down?  Or how about when a ‘portable’ drill was only as portable as the length of its power cord?  For those of you who don’t remember these two applications, I’m feeling a bit nostalgic.  I still use an old-fashioned ‘portable’ drill like this on occasion.  So what has been happening that makes these two old-school manual processes (and many others too) a thing of the past?

Advancements in DC motor technology, manufacturing capabilities and sophisticated, low-cost digital and analog electronics have enabled small DC motors to be used as a cost-effective way to automate operations that used to be done manually in a variety of applications including medical devices, toys, industrial automation, power tools and automobiles.  Nowhere is this trend more obvious then in the automobile.  Many of the operations that used to be done manually are now automated using motors.  Seat adjustment, window open/close, mirror adjustment, and cabin temperature control are just a few examples of where small DC motors automate processes that used to be done manually.  Market data suggests that there are more than 30 small DC motors per vehicle today with a forecast of close to 40 per vehicle in five years.  At a worldwide manufacturing rate of between 50M and 60M automobiles per year, it is easy to see why the automotive market helps to drive this automation trend.

So what the does future hold?  The trend to automate simple manual processes by the use of small DC motors will continue as the costs of the motors and drive electronics continue to go lower.  I look forward to the day when there is a cost-effective automated solution that allows me to not have to get up from what I am doing to the age-old question, “Honey, can you take out the trash?”

Ceramic capacitors offer low cost, small size, and can offer improved reliability over tantalum and aluminum capacitors. The ultra-low ESR attribute of ceramic capacitors, however, does effect choice of LDO when used at the output as most older LDOs require the ESR of a capacitor on its output for loop stability.  Although almost all newer designs include loop compensation so that they can be used with ceramic capacitors,  there’s still hope for ones favorite older workhorse LDO (or dirt cheap).  Simply add a small resistance in series with desired ceramic capacitor.

The region of stability is shown in the graph above (in this case for a TC1017).  Any decent datasheet will provide this graph.  Selecting the lowest value for the ESR, while keeping in mind resistor tolerance, is best to minimize load transients.

Figure 1 Region of Stability of ESR vs Load Current

Related Links

LDOs

In the past decade, “green” energy and energy efficiency have become one of the most talked about technology trends. To some, it is a concern about impact on the environment, to others it is a way to cut energy costs. One of the darlings of energy saving technologies have been LEDs and for good reasons. They run cooler than incandescent lights, rival or even beat fluorescent lights in efficiency and can be designed into many space-constrained applications with ease. As a result, LEDs can be found in anything from a blinking light on a remote control, to a flash on camera phone, a light source in a flashlight or headlights on a supercar.

With the prices on LEDs continuously going down, manufacturers push them into more and more application at prices competitive with technologies they are replacing. Sometimes this push for “everything LED” goes a bit far and results in products that are not very useful or even inferior to the “old” technologies they are replacing.

Exhibit A: 100(!) LED flashlight versus 1 LED flashlight. Now we all like to have more of stuff: more salary, more helpings at dinner, and more vacation days. So more LEDs should clearly be better than less LEDs, let alone just one, right? Well, it certainly depends on whether you like to shine the light beyond 10 feet or carry it in a pocket. With 100 small LEDs in a flashlight with no optics, what you get is a flood light that lets you see your feet very well and not much beyond that. If your aim at night is to see the rabid dog before its chewing on your leg, a single LED light with good optics will be a considerably better choice as it will easily have a throw of 100 feet or more. The only saving grace for 100 LED flashlight is that it serves as a good weapon to fight off that rabid dog.

Exhibit B: Cheap LED car light bulb versus halogen light bulb. Pictured are 9005 lights bulbs, an LED based one and a regular halogen bulb. Both fit the same socket and are intended for use as Day Light Running (DLR) lamps or fog lights. This is where their similarities end. A regular halogen light bulb can be approximated with a point light source and the reflectors of car headlights are optimized for it. With the LED bulb, there are numerous directional light sources that are not optimized for the existing reflector design and lead to scattered light and glare for oncoming traffic. Beyond that, even 19 surface mounted LEDs with little to no heat sinking cannot compete in light output with a halogen light bulb. The end result of replacing a halogen light with cheap LED light is less light where a good chunk of it is wasted on useless glare.

So, lesson for the day? If you want your LED fix with the most light output, get the Audi R8 with LED headlight option, it’s only $10,000! On top of the actual price of the car.

Related Links

MCP1650 Multiple White LED Demo Board

In the Smoke Detector industrial there is much debate over using a photo sensor or an ion sensor to detect smoke. The debate is being decided country by country and though the reasons for selection may vary consensus is being reached.

Use of an infrared diode and photo sensor is predominant in most of Europe and Asia. Many nations have simply decided that the amount of radioactive material needed to operate the ion chamber is unacceptable and regulated its use. Even places like the U.K. where the use has not been outlawed the requirement for a nuclear license to store more than 30 detectors has made low cost shipping and receiving untenable.

In the U.S the amount of radioactive material required for a smoke detector is considered acceptable and so the debate continues on the merit of system performance. From the standpoint of cost and fast fires the ion chamber has reigned supreme, but photo sensors have been shown to more quickly detect smoldering fires that cause poisoning or suffocation. As a result both photo and ion sensors are used together to provide better coverage while ion chamber devices dominate single sensor smoke detector sales.

Regardless of the type of sensor used Microchip offers detection I.C. ideal for battery operation, horn driving and sensor interfacing as well as the microcontrollers for systems that require the flexibility of programming.
Related Links

Smoke Detector Devices and Horn Drivers

In the previous post we talked about the charging and discharging behavior or a Li-ion battery.   On our way to a full fledged fuel gauge we need to first measure battery voltage.   Figure 1 shows the battery voltage measurement circuit using and ADC, in this case a MCP3421 18-bit ADC (U1).

Figure 1 Battery Voltage Measurement

Since the MCP3421 device has an internal reference voltage, the measurable maximum input voltage range is limited to the internal voltage reference voltage of up to 2.048V.  To measure the input voltage higher than the internal reference, a voltage divider is used, which is formed by R1, R2, and R3.  The R3 is optional and is used to calibrate the R1 and R2 component tolerance.  By choosing the series resistance value of the voltage divider to be very high (> 1 MΩ), the current losses due to the voltage divider is negligible.

In the example circuit as shown in Figure 1, the ADC is configured as single ended by connecting the positive input pin (VIN+) to the battery voltage, while the negative input pin (VIN-) to the VSS.   The ADC output is available to the MCU via the I2C bus line.
Figure 2 shows the discharge curve of a 3.7V Li-Polymer battery (3.7V, 170 mAH).  The curve shows that the battery voltage reduces linearly until it reaches about 80% of its full capacity.

Figure 2 Li-Polymer Battery Voltage Discharging Curve
Since the battery discharging characteristics are very linear until the point where the curve falls off sharply, measuring only the battery voltage is an alternative low-cost method to estimate the current status of the battery. In this case, the measured battery voltage can be compared with the fuel values in the lookup table in the MCU firmware.
The circuit shown can be used for measuring the battery voltage of any battery type. When the circuit is used, the voltage divider (R1, R2, R3) must be properly adjusted in order to keep the maximum input voltage (or the voltage at VIN+ pin when the battery is fully charged) to the ADC device is less than the ADC internal reference voltage (2.048V).
Although using the voltage alone is not sufficient to represent the battery fuel status, this method is widely used for simple and cost-sensitive applications because of its straightforward implementation.

Related Links

Battery Management

Delta-Sigma Analog-to-Digital Converters

The battery discharging behavior changes with various parameters, such as battery chemical type, load current, temperature, and aging. The figure below shows the general behavior of battery discharging curves of several battery chemical types.  The battery discharging curve of most batteries is almost flat until it reaches about 80% of its full range, and then falls off sharply after that. Intelligent battery chargers, or delta V chargers look for this drop off.
Since the battery’s internal chemical reaction is largely governed by voltage and temperature, the battery discharging behavior is greatly affected by the temperature.  The low temperature limit is determined by the freezing temperature of the electrolyte.  Most batteries do not work well below -40°C.  The battery performs better at higher temperatures because the chemical reaction processing is accelerated.  However, the rate of undesirable chemical reactions increases and results in a decrease of battery life.  At extremely high temperatures, the active chemicals become unstable and can destroy the battery and your laptop.

Related Links

Battery Management

Modern low voltage DC/DC converters have changed where the advantages of multiphase power conversion and point of load (POL) architectures can not be ignored. Point of load (POL) distributes the power via a high-voltage bus ( typ 12V) , and convert down to a lower voltage ( .6-3.3V) close to the point of use. This is similar to how power is delivered to your house. The voltage is sent over long distances at very high voltage and then converted down to 115v by a transformer close to your house. This reduces the power dissipation in the wire due to the wire resistance. High peak current demands also drives the move towards POL systems. Conventional POL solutions typical require two-stage conversion. The first conversion typically converts 12V input down to the rail voltage (typically 3.3V or 5V). The second POL converter produces the required output voltage (.6-2V). This approach reduces the system efficiency because each converter has power loss. Future POL converters will operate from any input voltage from 4.5V to 32V and will be more efficient since only one conversion will be required.

There are tons of gadgets and goodies for the engineer out there but what about ones for an Analog lover?

Steampunk Anything

Steampunk denotes works set in an era or world where steam power is still widely used and can include alternate history-style presentations of “the path not taken” of such technology as dirigibles, analog computers, or digital mechanical computers.

More Steampunk creations

Tunable Glasses

Josh Silver delivers his brilliantly simple solution for correcting vision at the lowest cost possible — adjustable, liquid-filled lenses. Although generally not available to the public, one could donate to his cause to make it available to impoverished areas.

Golden Slinky

Yes, that’s 14-carat gold .  The original was invented by a Unites States Navy engineer and machined from high grade blue-black Swedish steel.  This version is 14-carat gold plated brass core, great for spoiled children, conversation pieces, and snazzy physic experiments.

Best of all, it requires no batteries!

Multi-Tool Hammer

If you’re looking for something a little more practical then check this twist on the Swiss army knife.

The multi tool features 14 built-in utility tools including: Hammer Head, Nail Claw, Linesman Pliers, Regular Pliers, 1/4″ Slotted Screwdriver, 2-1/2″ Drop Point Blade, 1/8″ Slotted Screwdriver, #2 Phillips Screwdriver, 2″ Serrated Blade, Can Opener, Wire Cutters, Wire Stripper, Metal File and Key Ring. High quality precision built durable stainless steel construction with elegant polished hardwood handle insures comfort and sure positive grip.

Sheffield 14-in-1 Hammer Tool