What we’re trying to say is, sometimes the best solution is the simplest solution, and an analog solution maybe one that should be looked at over an overly intelligent one.  This is particularly true when it comes to price.

Consider the elegant OneDown mousetrap, a Gold Award Winner at the International Design Excellence Awards.  It swings upright from a rodent’s own weight, providing a visual indication it’s been set off without risking your fingers nor creating any mess or smell.

A local family that has been feeling drained by life gets an unexpected boost from Microchip Technology.

“When I was younger, I was at my fullest and felt that nothing could stop me”, said D. Alkaline, a father of three. “But as time went on, I found that I could no longer do the same things as fast as I used to and it drained me more and more just to do them!”

C. Alkaline, D.’s wife, continued, “I couldn’t watch his spirits spiraling down any longer and decided to do something about it.”

Fortunately for the whole family, the newly released MCP1640 boost regulator from Microchip Technology provided the lift everybody needed. “I feel like I can last forever,” beamed D. “Now with the help of MCP1640 even my little darlings, AA., AAA and AAAA can easily reach beyond their potential and give their uncle Li a run for his money!” C. added, “And we will make sure to let my cousins Nim. H and Nic D. know all about the uplifting MCP1640 and then even they could compete with their cousin L. Ion!”

Related Links

MCP1640

Antenna placement these days is affected on FCC rules on how much energy a device can send into a user’s head.  That’s the reason most antennas are around the bottom of phones nowadays. Similarly, the antennas in the new iPhone are on the sides of the phone, which means that when you go to grasp the phone to make a call or play on its touch screen a lot of the transmit energy is being absorbed.

Solution?  Use a blue tooth headset and walk around like you have a roach creeping up your ear.

Nonetheless if you are reading a site like this you are just geeky enough to want to know more. You’re a step beyond the mere mortals that go through life blissfully oblivious. You enjoy being blissfully aware. Knowledge is power, so you cannot resist more. And more you shall have.

What is this device? This unheralded guardian faithfully communicating with its fellow guardians? It is the smoke detector. And much of its operation is likely not understood by those protected by it. Sure we all know if it goes off get out! Or at least determine why it went off and get out if a real fire is life threatening. And still more of you likely know in a simple home set up all these detectors are connected by a loop, so if one detects danger they can all sound together. This makes it possible for a remote fire to be reported locally. In others, you are in bed and find out about the fire in the basement. That’s good.

But that is where things get tricky. They all alarm together. Like the old Christmas lights that all went out together if one had a problem, the all for one approach in a system makes it really difficult to find the culprit. And finding the culprit can be very important if the alarm is false.

What can we do? It turns out we can look and learn. If you have a loop system that false trips during the next event you can go from unit to unit and see which one is flashing. The flashing unit is the one that initiated the alarm. Once you know which unit is the culprit you can more easily determine if the device is just poorly placed and have it move, or needs replacing, that is getting a new one.

Now at Microchip, if someone has a good idea far be it from us not to make it better. It did not escape our attention that while the above flashing is a good idea in practice it has its downfalls. First, your alarm is sounding and lives possibly threaten, not the best time for cool headed flashing light checking. And again in the arena of bad timing, the light only flashes when the alarm sounds; an alarm typically capable of 85dB at ten feet. If you are like me you need to be closer than ten feet to see the light flash.

So there you are calmly covering you ears walking from room to room checking for little flashing lights and maybe miss your house mates call that they found the fire and to get out. There has to be a better way, and now there is.

With the new Alarm Memory product from Microchip the smoke detector will flash that light for 24 hours after the event and if you press the test button after the 24 hour period the device will give a special flash and then reset itself. Don’t panic: with the RE46C162 & 3 for Ion and RE46C165/6/7/8 for photo the culprit can be located under the calmest of conditions.

Related Links

RE46C162/3

RE46C165/6/7/8

Let’s look at some other parameters that could help us.  One of these is distortion, which is typically characterized by the parameter THD or total harmonic distortion as introduced by the ADC that was not present in the original signal.  Harmonic distortion results from spurious signals created from multiples of the fundamental frequency.  It is specified as a ratio in dB of the highest harmonic spurious signal relative to the carrier frequency.  The MCP3901 shows a specification of -104dBc THD.  It would be smaller with a smaller input voltage.  Similar devices from other suppliers typically have a THD of -90dBc or smaller.  Naturally a larger value for THD is better, indicating a smaller value for the spurious harmonic signal.

Harmonic distortion is not the only parameter to look at to determining the accuracy of the ADCs.  In order to include noise introduced by the ADCs, THD+Noise is used.  A more common view of this parameter is the inverse of THD+Noise, called SINAD or signal in noise and distortion.  The MCP3901 shows a SINAD of 91 dB.  With a full scale input of 1V, the smallest detectable signal would be 28µV.  Similar 16-bit resolution devices from other suppliers typically have a SINAD specification of 60-70 dB, indicating much higher noise.  On the higher end of 70 dB, this would yield a smallest detectable signal of 316µV.  This is a significant difference in accuracy between devices with the same resolution.

Looking at specifications other than resolution, it appears than specifications such as THD and SINAD can be very revealing with respect to the actual accuracy of the ADCs being used to make measurements.  Looking at the specifications of the MCP3901 the 28µV detectable signal threshold with a 1V input signal is more than adequate for the 40µV application requirement.  This is much better than what might be obtained from similar 16-bit resolution ADCs from other suppliers, which could not even meet the basic application requirements.

Related Links

MCP3901

To answer this question, let’s start with the most basic assertion: Resolution does not equate to accuracy.  You can have a 16-bit resolution ADC but only have 13-bit accurate measurements.  There are many specifications that can be viewed, but when it comes to actually designing your application, what you need to know is the smallest signal level you need to measure to obtain your accurate measurements.

Let’s start with the current measurement as that is where the biggest dynamic range of the application will occur.  To use a power measurement example, you know from the application requirements that the maximum current input will be 50A.  You also learn that you need at least 2% accuracy at 5A input as the minimum requirement

To determine the smallest signal that we need to measure, let’s look at the smallest current input error.  We know that at 5A we can only have 2% error.  In this case it is equal to 100mA.  This would be the smallest detectable current to be measured in the application.  For practical purposes you would select a value less than this to give some extra head room.

The maximum current input to your application will determine the size of the current shunt.  If the current values are too large, often a shunt cannot be used due to heating.  For 50A maximum current input, a small shunt of 400 µohms can be used.  That would give a voltage reading of 20 mV.  Remember that the smallest current we need to detect is 100mA.  Using our 400 µohm shunt, we would read a value of 40 µV.  That is a very small value.

So how do I determine if my ADC will be able to meet these requirements?  Stay tuned for part 2 when we discuss ADC specifications and how they relate to your measurements.

CHANDLER, Ariz., May 3, 2010 [NASDAQ: MCHP] — Microchip Technology Inc., a leading provider of microcontroller and analog semiconductors, today announced the MCP4801/2, MCP4811/2 (MCP48XX) and MCP4901/2, MCP4911/2 (MCP49XX) Digital-to-Analog Converters (DACs). The new products expand the Company’s portfolio of DACs with single- and dual-channel 8- and 10-bit devices featuring an SPI communication interface, as well as internal and external voltage-reference options. The MCP48XX family has an internal voltage reference, while the MCP49XX family accepts an external voltage reference.

Related Links

Digital-to-Analog Converters

Press Release

I recall back in the day when Guitar Hero first hit the market, there was quite a buzz of excitement, and rightfully so in my opinion, as the game is rather addicting. However, one of the comments that I heard repeatedly was “I wish this game could actually teach you to play a guitar”. If you are not familiar with the game, the video game controller sits inside of a plastic “guitar” and the user strums a small plastic switch. The frets of the guitar are actually five colored buttons that the user presses in accordance with the music. While fun and entertaining, skill at playing Guitar Hero does not translate to actual ability to play a real guitar.

However, a new generation of video games that implement musical instruments is underway. One such game is Power Gig: Rise of the Sixstring. The interesting thing about this game is that the guitar used to interact with the game is actually a fully functioning guitar. So while this game may not teach the gamer to read music or how to surf a mosh pit, it does get the user a step closer to actually playing a guitar. As the spokesman for the studio that created the game stated, “You are actually working your finger dexterity and you are getting used to strumming and where the strings are aligned”.

Of course quality analog components will continue to be critical in making these sorts of games a reality. Microchip has been very successful in this market and will continue to capitalize with its low power analog and microcontroller offerings.

You can read more about this new game in USA Today at:

http://www.usatoday.com/life/lifestyle/2010-03-09-powergig09_ST_N.htm

Welcome back!  Continuing our discussion of PWM DACs we will overview RC filtering design considerations and the importance of minimizing power consumption.

RC Filter Design Considerations for PWM DAC

One key design consideration when determining the PWM’s resolution is output-voltage ripple. Ripple occurs due to overshoot and undershoot as the PWM charges and discharges the RC circuit. One way to approximate the charge characteristics is to modify the equations to charge and discharge an RC-filter circuit. As the effects of this are cumulative, the following equations can be used as approximations.

(Equation 4)

(Equation 5)

(Equation 6)

VLH is the voltage increase for a specific PWM period, and VHL is the voltage decrease for a specific PWM period. The values of VLH and VHL are dependant upon not only the RC filter values, but also upon the PWM frequency and duty cycle. The PWM frequency and duty cycle determine the time available for the PWM to charge (tcharge) and discharge (tdischarge) the capacitor. Vripple is the difference between VLH and VHL for the same PWM period.

Figure 4 illustrates the magnitude of the voltage ripple across the output capacitor vs. time. The vertical axis displays the magnitude of the ripple voltage, while the horizontal axis provides the corresponding time. The plot shows how the ripple voltage settles at approximately 125 mV in a time interval of approximately 40 ms for R = 10K ohms, and 250 mV at 20 ms for R = 1 K ohm. In the previous example, an LSB size of less than 1.2mV for a 12-bit system was needed. This ripple is greater than 100 LSB for 12-bit DAC with a 5V reference, meaning that the resulting PWM DAC solution has an effective resolution of less than 6 bits due to the ripples.

Figure 4: Voltage Ripple Comparison for VOH = 5V, C = 1μF, PWM frequency = 1 kHz, Duty Cycle = 50%

The ripple can be reduced by increasing the capacitor and the resistor values or increasing the PWM frequency. All of these will decrease the ripple’s magnitude.

As shown in Figure 4, the ripple decreases as the resistance value increases. However, nothing comes without a price-the settling time doubles as the ripple decreases by 50 %.

For applications that require faster settling time (increased bandwidth) and higher resolution, one could add a second RC filter. The obvious tradeoffs include the cost of additional components and the increased board space occupied. Figure 5 shows a model for a two-pole RC low-pass filter. Figure 6 shows the analog output voltage of this model.

Figure 5: PSPICE Model for Two-Pole RC Low-Pass Filter

Figure 6: Analog Voltage Output vs. Settling Time of the Two Pole RC Low-Pass Filtere

There are a couple of things to keep in mind when designing the filter. First, make sure that the pole of the RC is set at a much greater frequency than that of the signal being generated. Secondly, if you are designing a two-pole filter, make sure that R2 > R1.

The 3dB corner frequency of the RC filter is given by:

(Equation 7)

There are a couple of additional things to consider. Increasing the PWM frequency will also decrease the ripple, but the tradeoff is increased settling time. Figure 7 shows the cases for 10 kHz and 5 kHz

Figure 7: Voltage Ripple at the PWM DAC when PWM Frequency Changes. Where VOH = 5V, C = 1μF, R = 10 kΩ, Duty Cycle = 50 %, PWM Frequency = 10 kHz and 5 kHz

The worst-case ripple occurs at a 50% duty cycle. The ripple will decrease as the duty cycle moves closer to 0% or 100%. Figure 8 shows the peak-to-peak magnitude of ripples on the PWM DAC output. The ripple decreases almost two times as the duty cycle changes from 50 % to 85 %.

Figure 8: Voltage Ripple at the PWM DAC when PWM Duty Cycle Changes. VOH = 5V, C = 10 μF, R = 1 kΩ, PWM Frequency = 1 kHz, Duty Cycle = 50 % (Solid Curve) and 85 % (Dotted Curve)

PWM DACs and Power Consumption

Many electronic products today are portable or handheld devices. These devices are battery powered, and many have strict constraints with regard to power consumption. Therefore, it is a good idea to minimize the PWM DAC’s power consumption. The current consumed in the PWM solution is simple to approximate, using the following equation:

(Equation 8 )

Figure 9: Current and Voltage Plots of PWM DAC with R =1000 Ω, C = 10μF, Duty Cycle = 50 %, VOH = 5V

Figure 9 shows the current and voltage plots. As sown in the plot, the PWM DAC with a lower resistor draws a significant amount of current (in the range of a few mA). This high level of current consumption is unacceptable for many battery-powered applications. Current can be decreased by increasing the resistor value.

In Figure 10, the resistor value has been increased by a factor of ten, which has likewise decreased the current consumption by a factor of ten.

Figure 10:  Current and Voltage Plots of PWM DAC with R =10,000 Ω, C = 10μF, Duty Cycle = 50 %, VOH = 5V

As the resistor limits the current available to charge/discharge the capacitor, decreasing the amount of current available (increasing resistance) to the circuit will increase the settling time.

Another factor to consider is the filter’s pole. As the resistor value increases, the 3 dB frequency decreases by the same magnitude. This can be compensated for by reducing the capacitor value by the same magnitude, which offsets the increased settling time and maintains the original pole of the filter. Figure 11 demonstrates this. Note that, as the capacitor value reduces, the circuit becomes more susceptible to loading. This is another important design consideration.

Figure 11: Current and Voltage Plots of PWM DAC with R =10,000 Ω, C = 1μF, Duty Cycle = 50 %, VOH = 5Vircuit

The generation of an analog voltage using a digital Pulse-Width-Modulated signal is known as a PWM DAC.  As most designers’ PCB boards have a microcontroller with a built-in PWM-output feature onboard, a simple digital-to-analog data conversion can be easily realized by adding a few passive components at the MCU’s PWM-output pin.  This is an alternative to using a standalone DAC.  In the MCU application environment, system designers can have DAC functionality nearly free of charge.  PWM DACs are widely used in very low-cost applications, where accuracy is not a primary concern.   Standalone DACs, however, are used for applications requiring higher accuracy.

Although the PWM DAC can be realized with the simple addition of a few passive components, implementing a PWM DAC for system applications is not a simple task.  There are many limitations associated with this. Understanding the complexities of using the PWM DAC and its effects can save significant development time and effort.   This article presents a technique for converting a PWM pulse to an analog voltage using a simple, RC low-pass filter. This entry also reviews the PWM DAC’s limitations and its key design constraints on resolution, frequency, ripple, settling time and current consumption are discussed, which are very important design parameters that are largely affected by the R and C values, as well as the PWM duty cycle and frequency.

Standalone DACs

Figure 1 shows an example of a standalone DAC.  Its analog output voltage is given by:

Where Dn is the digital code.  For example, with a 12-bit DAC, the user can get Vout = 2.5V with Vref = 5V and Dn = 1000-0000-0000.  Typical standalone DAC devices provide good linearity and a short settling time, which is the time required to update each output voltage.

Figure 1 Configuration of Standalone DAC

How PWM DACs Work

Figure 2 shows a basic configuration of the PWM DAC. The MCU outputs a PWM signal to an RC low pass-filter. The PWM pulse train’s digital value becomes an analog voltage, when it passes through the RC filter.  At a given period of time, the analog output is proportional to the PWM pulse’s high durations.

Figure 2 Block Diagram of a PWM DAC

A PWM signal is defined as a digital signal with a fixed frequency, but a varying duty cycle.  Figure 3 illustrates a PWM signal.  The PWM period (T) is the time interval required to complete one full PWM cycle.  The duty cycle is the ratio of the high duration (t) to the total period (T).

Figure 3 PWM Signal

The PWM signal and RC-filter circuit parameters affect the analog output’s resolution, amplitude, settling time and ripple.  The PWM DAC’s limitations are clearly demonstrated by analyzing the interaction of the PWM parameters and the RC filter.  A better understanding of the relationship between these parameters enables designers to optimize the PWM to best suit their application’s requirements, while minimizing design time.

PWM DAC Bit Resolution

The PWM counter length (L) and the smallest duty-cycle change in the PWM counter (C) determine the PWM DAC’s bit resolution.  The following equation expresses the maximum bit resolution of the PWM DAC:

For example, if the system generates an analog output voltage from a PWM DAC with a counter of 4096 (L) and a minimum count step of one (C), the PWM DAC’s bit resolution is 12-bits.

When the PWM resolution is determined, it is possible to calculate the Least Significant Bit (LSB) size.  The LSB size is dependant upon the PWM resolution and the PWM’s output-high level voltage (VOH), and can be calculated using the following equation.

For example, a 12-bit PWM DAC with a VOH of 5V has an LSB size of 1.2mV.