AnalogTalk

One of my colleagues was recently performing some calculations on a signal conditioning circuit that connected to a thermocouple. One of the calculations that he was doing involved the leakage current at the input pins of an auto-zero operational amplifier (in order to calculate the resulting voltage error). This resulted in a conversation that I felt was worthy of sharing with other engineers.

The two specifications that were brought into question were input bias current and input offset current. Let’s take a moment to address the actual definition of these two specifications. Input bias current is defined as the average of the currents into the two input terminals of an amplifier. Recall that convention dictates that for the input leakage, a current into the device is positive, and current out of the device is negative (except for the output pin). Input offset current is defined as the difference between the currents into the two input terminals of an amplifier.

Here it is in equation form. The two physical currents into an op amp’s inputs are:

IB_N = current into the non-inverting input

IB_I = current into the inverting input

From them, we calculate the bias and offset currents respectively:

I_B = (IB_N + IB_I)/2

I_OS = IB_N – IB_I

Rearranging gives:

IB_N = I_B + I_OS/2

IB_I = I_B – I_OS/2

The question originally came up when working with the MCP6V06 auto-zero operational amplifier. The datasheet for this device specifies a typical bias current of +6 pA at room temperature, but a typical  input offset current of -85 pA at room temperature. Without looking at the specific definition of these two specifications, these numbers may seem incorrect, but they are indeed true.

Unlike traditional op amp input stages, these auto-zero operational amplifiers have switches at the input that add a current flow path, through parasitic switched capacitances. It turns out that the current flows through the switches from one input pin to the other. So for the MCP6V06, IB_N is approximately -37 pA and IB_I is approximately +49 pA. This means that the non-inverting input is sourcing current, while the inverting input is sinking current.

Even though the average input current is relatively small (6 pA), the offset current is actually quite large. As noted earlier, this is a function of the self-correcting architecture of the auto-zero amplifier, and is uniquely different from the characteristics of a traditional operational amplifier.

Related Links

MCP6V06

There are few specifications that must be carefully considered, such as, conversion accuracy and noise performance.  Many of the ADC errors can be improved by taking the average of multiple samples to precisely determine temperature.

RA Tolerance and Measurement Accuracy

The variation in RA characteristics introduces temperature accuracy error. A 1% tolerance in RA produces a 20°C error and a 0.1% tolerance produces a 2°C error.

For lower tolerance resistors, RA must be calibrated for precision temperature measurements. In order to precisely calibrate RA, a calibration resistor can be used in place of the RTD, such as 100Ω 0.1% tolerance resistor and the equation for “ADC RESOLUTION and ADC Code Relationship” can be rearranged to determine RA.

RTD Temperature Calculation

RTDs are significantly non-linear. Depending on the RTD type and specification, the resistor to temperature conversion equations have been defined and standardized. The equation for the PT100 RTD can be found at American Society for Testing and Materials (ASTM) [1] specification number E1137E. The figure below shows the error that occurs by ignoring the 2nd and higher power errors from RTD.

FIGURE: RTD to Temperature Conversion Error

Power Supply Noise

Another source of error is the system power supply. Most power supplies for portable systems use switching regulators which generates high frequency glitches at the switching frequency of typically 100 kHz. Other sources of noise include digital switching from system processor or system oscillator. This high frequency noise can couple throughout system and directly influence the measurement accuracy. Therefore, high performance sensor applications require analog filters.

The power supply voltage, VDD, connected to the input of the LDO must be filtered using Resistor Capacitor network (RC network) with low corner frequency, approximately 1 kHz. The filtered voltage can be set to a desired level using a low dropout linear regulator (LDO). Refer to the LDO datasheet for dropout voltage specification when setting the LDO output voltage. The two RC filters provide 40 dB per decade rolloff.

FIGURE: RTD Biasing Circuit

Note that the RC filter is applied before the LDO. Typically, the Power Supply Rejection Ratio (PRSS) of an LDO is ~0 dB at higher frequencies. Therefore, It is necessary to filter the input voltage to prevent the noise coupling through the LDO to the ADC and RTD. In addition, when designing PCB layout, avoid placing digital signal traces in close proximity to the RTD biasing circuit.

Effect of RTD Self-heat Due to Power Dissipation

When biasing RTD, self heat due to power dissipation can compromise system accuracy. The effect of Self heat can be reduced by reducing the biasing current magnitude. The current magnitude needs to be sufficiently low to reduce self-heat while providing adequate voltage range and measurement resolution. Ideally, the added temperature due to self-heat must be lower than the temperature measurement resolution, TRES

To determine error due to self-heat, refer to the RTD datasheet for Self-heat coefficient specification in degree Celsius per milli-watt (°C/mW). This coefficient is used to convert heat due to power dissipation to temperature. For example, a small surface mount PT100 RTD with 0.2°C/mW self-heat coefficient would dissipate 0.002°C with 300 μA bias current at 0°C (100Ω), and 0.006°C at high temperature (350Ω). In this case, the maximum heat dissipated due to selfheat is less then 0.008°C TRES. Therefore, error due to self heat is not measurable.

EQUATION: RTD POWER

This approach was validated using Microchip’s MCP3551 ADC device. The ratiomatric solution was used with a calibrated RTD simulator to generate the data as shown in the table below.

TABLE: RATIOMETRIC TEST RESULTS USING AN RTD SIMULATOR

CONCLUSION

This discussed an RTD application which uses a ratiometric relation between the ADC LSB quanta and the RTD temperature coefficient. This was achieved using low tolerance resistor and a reference voltage to bias the RTD and ADC and measure temperature ratiometrically with 0.01°C temperature resolution from -200°C to 800°C temperature range. A 0.1°C accuracy can be achieved using a single point calibration. This approach eliminates the need for a high-performance RTD systems that require constant current source and complex instrumentation systems. This technique provides a low cost, high performance, plug and play solution for all RTDs.

Today RTDs (Resistive Temperature Detector) are the benchmark for high accuracy temperature measurement and are a key component in many high performance thermal management applications. Today we will discuss how to use a high resolution Delta-Sigma Analog-to- Digital converter, and two resistors to measure RTD resistance ratiometrically.  We will demonstrate how a ±0.1°C accuracy and ±0.01°C measurement resolution can be achieved across the RTD temperature range of -200°C to +800°C with a single point calibration.

A high resolution Delta-Sigma ADC can serve well for high performance thermal management applications. Traditionally RTDs are biased with a constant current source. The voltage drop across the RTD is conditioned using an Instrumentation Amplifier circuit.  The instrumentation amplifier can be built using discrete operational amplifiers with multiple resistors/capacitors or there are many stand-alone instrumentation amplifier offered in the industry today. Using the analog instrumentation amplifier technique requires a low noise and stable system to calibrate and accurately measure temperature. It also requires an operator for optimization on the production floor.

Utilizing a Delta-Sigma ADC solution, the RTD is directly connected to the ADC (Microchip’s MCP3551 family of 22 bit Delta-Sigma ADCs) and a single low tolerance resistor is used to bias the RTD from the ADC reference voltage (see figure below) and accurately measure temperature ratiometrically. A low drop out linear regulator (LDO) is used to provide a reference voltage.

SOLUTION

This solution uses a common reference voltage to bias the RTD and the ADC which provides a ratio-metric relation between the ADC resolution and the RTD temperature resolution. Only one biasing resistor, RA, is needed to set the measurement resolution ratio.

RTD RESISTANCE

For  example, a 2V ADC reference voltage (VREF) results in a 1 μV/LSb (Least Significant Bit) resolution.

Setting RA = RB = 6.8 kΩ provides 111.6 μV/°C temperature coefficient (PT100 RTD with 0.385Ω/°C temperature coefficient). This provides 0.008°C/LSb temperature measurement resolution for the entire range of 20Ω to 320Ω or -200°C to +800°C. A single point calibration with a 0.1% 100Ω resistor provides ±0.1°C accuracy (as shown in the above figure). This approach provides a plug-and-play solution with minimum adjustment. However, the system accuracy depends on several factors such as the RTD type, biasing circuit tolerance and stability, error due to power dissipation or self-heat, and RTD non-linearity.

Ratiometric Measurement

The key feature of a ratiometric measurement technique is that the temperature accuracy does not depend on an accurate reference voltage. The ADC reference voltage varies with respect to change in RTD resistance due to the voltage divider relation. This measurement maintains constant resolution. It eliminates the need for a constant biasing current source or a voltage source, which can be costly, while providing a highly accurate temperature measurement solution. The figure below shows circuit block diagram with the ADC reference.

EQUATION: REFERENCE VOLTAGE

Figure: RTD Biasing Circuit

EQUATION: VOLTAGE ACROSS RTD

EQUATION: ADC RESOLUTION and ADC Code Relationship

When RA = RB = 6800Ω, the bias current is ~290 μA. This provides < 0.01°C/LSb temperature resolution. As the RTD resistance varies due to temperature, the IBIAS

(biasing current) varies and temperature resolution remains below 0.01°C/LSb as shown in the figure below.

FIGURE: TRES vs. RTD Resistance

There are few specifications that must be carefully considered, such as, conversion accuracy and noise performance which will discuss in Part II.

Related Links

MCP3551

Circuits such as RF oscillators, RF amplifiers, and GFI circuit breaker are sensitive to noise on the supply line. LC filters and large electrolytic capacitors are commonly used to filter out noise.  What can be helpful in controlling the amount of noise or ripple in a given power rail is with the use of an LDO with a high Power Supply Rejection Ratio (PSRR). Many application use DC-DC switching converters (for efficiency) to convert a higher voltage to a lower voltage required the circuit.  Noise or a ripple results due the switching transients and the addition of an LDO will suppress this while providing the secondary voltage needed.  Another example would be applications in noisy environments such as automotive applications where noise results due to the alternator, motors, solenoids, relays running and turning on and off.

Today high PSRR LDOs are available that can be used to help reduce this noise so that sensitive circuits can operate properly. PSRR is a measure of how well the LDO rejects noise coming from the input power supply at various frequencies. The formula below shows PSRR ratio as a function of frequency.

The diagram above shows a typical application for the MCP1754S. Ripple and noise at the input to the MCP1754S. The plot below shows the ripple/noise rejection as a function of frequency.  This performance does come at a cost of higher quiescent current.

The  MCP1754/MCP1754S delivers 70 dB of ripple rejection at 1 kHz with 56 uA of quiescent current, making the device ideal for the situations described above. As shown below, the PSRR varies with the output current. With an output current of 10ma the PSRR is -90 db from 10Hz to almost 300 hz. The PSRR at 150 ma output current has a sweet spot at 1Khz with a PSRR of -80db.

Cascading High PSRR LDOs can also be used to further reduce the input Ripple/Noise to a certain limit.

High PSRR LDOs are very important building blocks in applications the have circuits that require low noise input voltage.

Related Links

MCP1754

I recently volunteered for a Girl Scout First Lego League as a mentor for ten promising  nine and ten year olds.  The challenge is composed of two main parts; identify a problemàcreate an innovative solutionàshare/present your solution with others AND a ROBOT GAME.  While the project side of this challenge definitely helps the girls improve and develop their problem solving skill and focus on positive elements of teamwork, it’s the nerd inside me that really wants to focus on the robot challenge.

Those of you that are not familiar with Lego® Mindstorms®, welcome to an amazing robotics tool set helping to spark an interest in technology at a young age.  The flexibility of the lego set allows our future young engineers and scientists an easy way to “Dream it, built it, program and see results”!

The Mindstorm® features a 32-bit programmable MCU, easy to use programming software GUI, multiple types of smart sensors and interactive servo motors.  In addition, it comes with Bluetooth and USB connectivity abilities.

I don’t recall having these types of “toys” available when I was a kid but it’s great to see tools like this that really can help increase our youth’s creativity, innovativeness and learning while seen amazing results.

I was walking through downtown Boston a few weeks ago and came across the following right outside a well-known bakery (the name is particular appropriate since it’s outside a bakery):

This was the first time I’d seen one of these.  After a little research… very little research actually as one only need type ‘BigBelly Solar’ into Google, which brings up bigbellysolar.com.

Essentially this thing 1) reduces the number of collection cans, and/or 2) increases the amount of time between collections, meaning that it “lowers the operating costs, fuel consumption, and greenhouse gas emissions associated with the waste and recycling collection process” (from bigbellysolar.com website), and they claim up to an 80% reduction in the collection frequency.

That sounds neat…  I don’t know what this thing costs, which is crucial to figuring out if it’s worth it, but a neat idea nonetheless.  I started thinking about, appropriate enough standing outside a bakery, BigBelly ‘upgrades’.  (I’m an engineer… so I’m normally this dorky).  I know -  the “ultimate” upgrade is for us all to generate less trash… and there’s probably another interesting discussion to be had about convert waste into something that’s usable (energy) – but those are different discussions (maybe next time!).  So, here’s the list that I came up with:

1^st – Add Wifi…  I was all set to discuss this… but then I noticed Bigbelly already offers this service.  Not only does it compact trash and increase its usable capacity, but it can contact the collections department and relay how ‘full’ it is. (presumably so they can decide when it’s economical to send a truck out)

2^nd –  Add Wifi.  I know I know,  I just said this, but this is different.  How about “Hotspot”!  With these located all around a city you’d have a pretty good network going, and it would address the typical challenges of coverage within the city such as obstructions and ownership. (and it would have been helpful looking up BigBelly on my smartphone outside of the bakery!).

3^rd – Integration.  How about combining services?  How about those dreaded parking pay stations?  Or locate these at train, bus, or taxi stops and add a screen  to update the waiting passengers on schedules and ETA’s , or pre-purchase the fare ticket (and keep the trains/buses/taxi’s clean of trash at the same time)?

4^th – Automatic Door.  The only downside of my experience was having to touch the handle to open the door.  I understand there needs to be a level of safety so that nothing gets compacted that isn’t supposed to be compacted… and automatic doors add their own challenges…  but I didn’t really want to touch that handle (alternatively, a disinfectant would have addressed this concern, AND assisted me in cleaning up after having my pastry!).

There are more ‘upgrades’ (feel free to post your ideas), but overall I think this is a pretty neat concept, and I think it’s scalable to similar-footprint machines… such as a soda machine!  That would have been convenient while having my pastry!

Back when I was a young product marketing engineer (many years ago now), this question was actually asked of us when we were interviewing a candidate for a non-engineering support position.  At the time we all thought it was pretty funny; however it can be just as relevant of a question today.  In the media, we are continually bombarded with images of intelligence and ‘coolness’ being built into everything.  Examples include cars that parallel-park themselves, appliances/homes/buildings that can manage/optimize power requirements and minimize usage costs, and tablet computers/mobile phones with touch screens for easy user interfacing just to name a few.  All of these example applications embed intelligence into their functionality by integrating some type of microcontroller or microprocessor.  As many electronics users can attest to, the pace at which next generation products and the chips that drive them is incredibly fast and seems to be getting faster.

The question is as valid today as it was long ago.  Will the need for analog eventually go away?  The simple fact is that natural phenomena are analog (sound, pressure, temperature, velocity, acceleration, position, flow, speed, vibration, etc.).  Sensors are used for measuring the state of these phenomena at any giving time.  This signal must be converted from analog to digital so the MCU or MPU can process it.  Once processed, the MCU or MPU tells some type of actuator to modulate an output (motor, generator, valve, LED light, power supply, etc.).  Prior to the actuation process, the signal must be converted from a digital signal (from the MCU or MPU) to an analog signal (to the ‘actuator’).  This basic process of sensing or measuring inputs, processing this data and generating new output requirements, and then driving outputs to a new position is used most if not all of today’s intelligent applications.  This means that in virtually every application, analog components will be required.

This is good news for analog chip or systems designers.  Their services will be needed for the foreseeable future…that until natural phenomena miraculously change from analog-based to digital-based, or MCUs/MPUs can interface directly to all types of analog inputs and drive all types of analog outputs.

China’s State Grid Corp has a potential market of 300 million smart meter users with a meter market value of 50 billion yuan ($7.7 billion), according to Fei Yuhang, a member of the National Standardization Committee for Electrical Meters, quoted in the China Daily.

Further details

Due to a break through in the understanding of the Laws of Physics, all Microchip Analog products have zero power consumption.

It is very easy to find news articles, blogs, application notes, etc. that speak about power consumption and current consumption. Yet in the strictest sense power and current consumption is always zero. They are never consumed. Ostensibly engineers know flows are not consumed and they know power and current are flows, but the dots are not being connected.
Power is the measure of the flow of energy over time and current the flow of electrons over time. Kirchhoff’s current law simply stated is: what comes in goes out. If everything that went in, went out, how does anything get consumed? Doesn’t anyone believe Gustav Kirchhoff anymore? Sure he had an interesting first name and looked funny by today’s standards, but I bet he was happening in 1840’s Prussia.
In the 1840’s the word consumption referred to the disease we now call tuberculosis, as tuberculosis appears to consume the body. It appears now the mental disease of miss using the word consumption has consumed our industry. Why is it that consumption has become preferred to words like “operation” or “performance” or more accurately “flow” as the Laws of Physics would dictate? In all honesty the trade magazines and many of our own new releases claim low power consumption and low current consumption.
Ironically this miss use has some truth to it. As a flow cannot be consumed the consumption of a flow is zero. And zero is low. So claiming a device has low current consumption is in some ways accurate. But as no device has any current consumption, we cannot claim one device has better current consumption than another. They are all zero.

The industry is now incorrectly sure in general what is meant by power consumption. So much so that I could claim in a news release that my device has zero power consumption (according to the laws of Physics) and yet the fall out to this claim in a news release would be such that our Mar-Com department would block my attempt. No sense of humor. Still, I think the next time someone asks me what the power or current consumption of a device is I will let them know, ZERO!

Past several years saw a great advance in low power digital and analog devices, allowing for longer lasting portable applications. Yet, selecting the optimal battery chemistry remains a daunting task. Primary (non-rechargeable) or secondary (rechargeable)? Coin cell, cylindrical or form fitted lithium-polymer? Each has its own benefits and drawbacks. Let’s take a quick look at them.

Cylindrical (AA, AAA) Primary Batteries

These are the batteries everyone recognizes and can pick up at just about any store with very little confusion. These are best if the end user needs to have the ability to easily and cheaply replace the power source. Typically these are alkaline batteries with a nominal voltage of ~1.5V, wherein lies the challenge: very few devices can run directly off of one alkaline battery, especially since it drains to ~0.9V. To enable single-cell use, a boost regulator would be necessary to provide the 3V or 5V rail. Even two batteries in series pose a challenge as they drain to a combined 1.8V, below the capability of many microcontrollers. So for a typical 3.3V to 5V application, three (2.7V-4.5V) or even four cells (3.6V-6V) in series might be required. Depending on the analog and digital components used, that might in turn require a buck regulator or an LDO for stable voltage or an even more complicated buck-boost regulator. So a consumer friendly power solution can become quite a quagmire of component selection, all along increasing the size of the device to a point where the battery cavity is probably dwarfing the rest of the components. Using AAA or even AAAA batteries instead of AA can yield significant weight and size gains at this level.

Another challenge in using alkaline batteries is the variation in capacity depending on the current draw. Going from 25 mA to 250 mA diminishes battery capacity by about 25% and having the batteries in series does not decrease the effect in anyway. One way to address that from end-user perspective is to use 1.5V Lithium primary batteries that provide virtually the same capacity regardless of the current draw. The downside to that solution is that the cost of the lithium batteries is 5 to 20 times more than alkaline batteries, pretty much eliminating the main benefit of having cylindrical primary batteries in the application from end user perspective. As the result, the consumers are more likely to use cheap alternative and complain about the application not lasting very long on battery power. Lithium batteries’ fast voltage drop also shortens the period of “almost discharged” time that consumers typically associate with AA/AAA batteries and observe through diminished brightness, slower motor, etc.

Finally, using more than one cylindrical battery in an application increases the chances of a consumer putting in cells of different chemistries or different discharge state, thus increasing the chances of malfunction or battery leakage.

Coin Cells (2032, etc.) Lithium Primary Batteries

The de facto batteries for low power and small volume applications, lithium coin cells come in a wide variety of sizes. Their voltage curve of 3V down to 2V makes them well suited for use with many electronic components and the small, flat size allows form factors that cylindrical batteries can only approximate with AAAA size. ‘2032’ coin cells are some of the most popular for consumer applications, but still fall in the category of “unusual” batteries as they are less common then AA/AAA batteries and considerably more expensive in general stores.

From application perspective, lithium coin cells have the same downside as alkaline in that their capacity greatly diminishes with increased current, but with even lower continuous current capability. In fact, using more than a few mA will have these batteries at a disadvantage to even a single alkaline with a boost regulator.

Photo Battery (primary), typically CR123

Based on the same chemistry as the coin batteries, CR123 photo batteries have the same electronics friendly discharge curve with increased capacity, continuous and peak current capabilities. While a single battery is a bit smaller than two AA or AAA batteries, the price to average consumer is considerably higher than alkaline batteries and on par with 1.5V Lithium batteries making it less consumer friendly.

Cylindrical (AA, AAA) Secondary Batteries

NiMh (Nickel Metal Hydride) and NiCd (Nickel Cadmium) are the main chemistries for these form factors with Low Self Discharge (LSD) NiMh reigning supreme for the past several years. With the nominal voltage of ~1.2V, these batteries have similar pros and cons as the primary alkaline batteries, somewhat higher continuous discharge currents and of course, the added benefit of recharging the batteries. The prices have been dropping in the past years and long term cost is extremely low

The problems of mixing different chemistries and charge conditions in multiple-cell applications remains and is complicated by the nominal voltage and internal resistance being different than those of alkaline and lithium batteries.

Li-ion Secondary Batteries

Lithium-ion rechargeable batteries have the distinction of coming in the widest variety of sizes as they can be created to fit just about any mold. This makes them popular for many space constrained applications like Bluetooth headsets. It also means that they are either non-user replaceable by design or expensive to replace.

By nature of being rechargeable, Li-ion battery will most likely need to include some sort of charging circuit in the application, complicating overall electronics design. Li-ion batteries have a 4.2V to ~2.7V discharge curve. This necessitates some sort of power conversion, be it an LDO, boost or buck-boost, for any application requiring a stable 3V/3.3V or 5V rail, adding yet another degree of complexity.

At last, shipping Li-ion batteries around the world is subject to many regulations, a potential additional cost that might not be taken into account when designing the product.

Mr. Fusion

Few improvements have been done to this technology since the ‘80s and the devices remain rather large, requiring special high power electronics, but providing nearly unlimited amount of power for DeLorean sized portable applications.