AnalogTalk

In recent years more and more focus has been placed on “alternative fuel” vehicles. As manufacturers scramble to address the growing concerns about gas prices and minimizing our environmental footprint, many automotive manufacturers are investing in the development of hybrid electric vehicles (HEVs). The proliferation of electric motors and batteries in this type of application has increased the need for effectively monitoring current flow, either for battery management purposes or for real-time control of electric motors.

There are several ways in which sensing current flow has been accomplished, including the use of a simple shunt resistor, a transformer, or via magnetic current sensing. Each of these techniques has advantages and disadvantages, but magnetic sensing has the advantage of providing electrical isolation and can sense both ac as well as dc currents, which is critical in applications such as HEVs.

Magnetic current sensing works by placing a current sensing unit (either a hall effect sensor or a magnetoresistive sensor) in a gap within a magnetic core that is placed around an insulated wire that carries the measured current (see the diagram below).

A hall effect sensor will output a voltage based on the amount of magnetic flux flowing through the sensor. A magnetoresistive sensor will vary resistance based on the amount of magnetic flux flowing through the sensor. Either way, a small output signal is generated that is proportional to the monitored current, and additional circuitry is needed to properly condition this small output signal.

An amplifier is used to perform this conditioning and has a lot of the same requirements as in other signal conditioning applications. However, the environment in which magnetic current sensors are typically located (in automobiles, power supplies, etc.) can be very electrically noisy, and hence immunity to electromagnetic interference (EMI) becomes a critical issue. The use of ASICs, which combine the sensor, signal conditioning and digitization (and A/D converter and possibly some post filtering) are also alternatives.

Since the magnetic current sensing solution is electrically isolated from the current being monitored, supporting a high common mode voltage is typically not required. Therefore, Microchip’s portfolio of low power CMOS amplifiers is well positioned to play within this marketplace. The Microchip amplifier portfolio offers a wide variety of devices for this market that includes a range of price versus performance. The MCP62xx series of general purpose amplifiers offer a very cost effective solution, while the MCP60xx series of trimmed devices offer greater performance in terms of voltage offset. The MCP6V0x families of auto-zero amplifiers offer another alternative for the best in DC performance. Finally, the MCP621 and MCP651 devices which feature mCal, offer a high speed, high precision solution that can also counter the adverse effects of drift over time and temperature via the use of the on-board calibration circuitry.

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Op Amps

In a previous post we briefly discussed how sensors are proliferating into numerous different markets and into numerous different type of products.  With these new markets and products also comes design challenges.

In general, typical sensor applications requirements are the following:

• Suitable Accuracy - Product quality and reliability
• Low Power - Longer battery life
• Small Form Factor - Product packaging / size requirements
• Low Cost - Cost competitiveness in the marketplace

One good example of a sensor application that I’d like to introduce is an altimeter watch.  The sensor of choice for this example is an absolute pressure sensor in a Wheatstone bridge configuration and we also introduce a high resolution ADC.

Sensors for temperature, pressure, load or other physical excitation  quantities are most often configured in a Wheatstone bridge configuration.  The bridge can have anywhere from one to all four elements reacting to the physical excitation, and should be used in a ratiometeric configuration when possible, with the system reference driving both the sensor and the ADC voltage reference.

One example sensor from GE NovaSensor is an absolute pressure sensor, shown below, a four element varying bridge.

The MCP355X are a family of high resolution delta sigma ADCs (22-bit resolution).  When designing with the MCP355X ADC, the initial step should be to evaluate the sensor performance and then determine what steps (if any) should be used to increase the overall system resolution when using the MCP355X.  In many situations, the MCP355X devices can be used to directly digitize the sensor output, eliminating any need for external signal conditioning circuitry.  This saves Space and Cost while maintaining Accuracy…all of which are critical system requirements.

Using the absolute pressure sensor in our altimeter example, the NPP-301 device has a typical full scale output of 60 mV when excited with a 3V battery.  The pressure range for this device is 100 kPa.  The MCP3551 has a output noise specification of 2.5 μVRMS.  The following equation is a first order approximation of the relation ship between pressure in pascals (P) and altitude (h), in meters.

Log(P) ≈ 5 – (h/15500)

Using 60 mV as the full scale range and 2.5 μV as the resolution, the resulting resolution from direct digitization in meters is 0.64 meters or approximately 2 feet.  It should be noted that this is only used as an example for  discussion; temperature effects and the error from a first order approximation must be included in final system design.

Some advantages to this ADC system configuration are the following

• High resolution removes the need for signal gain block
• Differential input reduces noise
• Ideal for slower signals such as a pressure applications
• Low current consumption during conversion (~120µA)

Benefits; cost savings, board space, accuracy, battery life

See more in reference to high resolution delta sigma ADCs

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MCP3551 22-bit Delta-Sigma Converter

Silicon Temperature Sensors

Silicon based temperature sensors are gaining popularity in many applications across a variety of market segments.  These devices can be categorized by their output type logic, voltage and serial outputs.  IC sensors integrate many useful features that allow system designers to implement the design that best meets the requirement of their application.  Temperature-sensor ICs require minimal design effort, and the integrated features can decrease overall system cost and minimize design effort.

Logic-Output Temperature Sensors

Logic-output temperature sensors typically function as a thermostat, notifying the system that a minimum or maximum temperature limit has been reached.  Sometimes referred to as “temperature switches,” these devices can be used to turn-on either a fan or warning light when high- or low-temperature conditions are detected.  As the output is typically not latched, the switch will turn off when the temperature falls below or rises above the temperature set point.  Most logic-output sensors have an integrated hysteresis of few degrees Celsius, to prevent output chatter.

Logic-output temperature sensors are available in either a “hot” option, where the output toggles as temperature increases; or a “cold” option, where the output toggles as temperature decreases. The hot and cold options ensure that the hysteresis is in the appropriate position, either below or above the temperature set point.

Voltage-Output Temperature Sensors

Voltage-output temperature sensors develop an output voltage proportional to temperature, with a typical temperature coefficient of 6.25 mV/° C, 10 mV/° C or 19.5 mV/° C.  Temperature-to-voltage converters can sense a -55° C to +150° C temperature range and feature temperature offset which allows reading negative temperatures without requiring a negative supply voltage.  Typical operating currents are in the tens of µAs, which minimizes self heating and maximizes battery life.

Serial-Output Temperature Sensors

Typically, serial-output temperature sensors use a two- or three-wire interface to the host microcontroller.  These devices have an ADC onboard that converts the analog output of the internal sensing element to a digital output.  They can achieve temperature accuracies as high as ± 0.5° C, with a measurement resolution of 0.0625° C.
Many serial-output temperature sensors provide user-programmable functions, such as over and under temperature alerts and onboard EEPROM.  These features can be used to simplify a design, and increase its flexibility, improve temperature-sensing accuracy, and lower overall system cost.

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Temperature Sensors