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Circuits from the Lab

Circuits from the Lab reference circuits are engineered and tested for quick and easy system integration to help solve today's analog, mixed-signal, and RF design challenges.

July 2014

CN0357: Low Noise, Single-Supply, Toxic Gas Detector, Using an Electrochemical Sensor with Programmable Gain TIA for Rapid Prototyping

This single supply, low noise, portable gas detector, uses an Alphasense CO-AX electrochemical sensor to detect carbon monoxide. Electrochemical sensors offer several advantages for instruments that detect or measure the concentration of many toxic gases. Most sensors are gas specific and have usable resolutions under one part per million (ppm) of gas concentration. The circuit uses the ADA4528-2 dual auto-zero amplifier, which has a 2.5 V maximum offset voltage at room temperature and an industry leading 5.6 V/√Hz voltage noise density, and the AD5270-20 programmable rheostat, allowing for rapid prototyping of different gas sensor systems without changing the bill of materials. The ADR3412 precision, low noise, micropower reference establishes the 1.2 V common-mode, pseudo ground reference voltage with 0.1% accuracy and 8 ppm/C drift.

CN0328: Completely Isolated 4-Channel Multiplexed HART Analog Output Circuit

This circuit combines the AD5755-1 four-channel voltage and current output DAC with dynamic power control and the AD5700-1 HART modem, to impement a completely isolated multiplexed HART analog output solution. Power can be provided either from the transformer isolated power circuit provided on the board or from external power supplies connected to terminal blocks. This circuit is suitable for use in programmable logic controllers (PLCs) and distributed control system (DCS) modules that require multiple HART-compatible 4 mA to 20 mA current outputs, along with unipolar or bipolar voltage outputs. External transient protection circuitry is also included, which is important for applications located in harsh industrial environments.

May 2014

CN-0350: 12-bit, 1-MSPS, Single-Supply, Two-Chip Data-Acquisition System for Piezoelectric Sensors

This 12-bit, 1-MSPS data-acquisition system uses only two active devices. The system processes charge input signals from piezoelectric sensors using a single 3.3-V supply and has a total error of less than 0.25% FSR after calibration over a 10C temperature range, making it ideal for a wide variety of laboratory and industrial measurements. The circuits small footprint makes this combination an industry-leading solution for data-acquisition systems where accuracy, speed, cost, and size play a critical role.

April 2014

CN0343: Ultrasonic Distance Measurement

This self-contained distance sensor utilizes an ultrasonic transmitter and sensitive analog receiver in conjunction with a precision analog microcontroller to provide distance measurements. Unlike complicated PLL-based receivers, the sensor uses a sensitive window comparator circuit, thereby minimizing real estate and cost. The approximate range is from 50 cm to 10 m with a resolution of about 2 cm. Temperature compensation is provided by the integrated temperature sensor and ADC contained in the microcontroller. For sensing the levels of thick liquids or foamy water, the ultrasonic level sensor is a better choice than capacitance, reed, or float sensors. In very dusty or corrosive environments, the ultrasonic sensor is the sensor of choice.

CN0304: Low Power DDS Waveform Generator

This 75-MHz low-power (25 mW total) direct digital synthesis (DDS) waveform generator includes an output buffer and anti-imaging filter to provide improved spectral performance, making it suitable for frequency generation or clocking applications requiring sine wave, triangular wave, and square outputs up to 18 MHz. As sampled data devices, low-power DDS devices must be followed by a suitable anti-imaging filter to remove spectral images, but their maximum current output is approximately 4 mA into a recommended 200-Ω load, so an op amp buffer at the DDS output is required to provide a low-impedance drive source for a high-quality 50-Ω filter. The combination of the DDS, output buffer, and seventh-order elliptic low pass filter provides high-quality spectral performance.

March 2014

CN0337: 12-Bit, 300 kSPS, Single-Supply, Fully Isolated RTD Temperature Measurement System with 3-Wire Compensation

This completely isolated 12-bit, 300-kSPS RTD temperature measuring system uses only three active devices to process the output of a Pt100 RTD. An innovative circuit provides lead-wire compensation using a standard 3-wire connection. The circuit operates on a single 3.3-V supply. The total error after room temperature calibration is less than 0.24% FSR for a 10C change in temperature, making it ideal for a wide variety of industrial temperature measurements. Both data and power are isolated, making the circuit robust to high voltages and ground-loop interference often encountered in harsh industrial environments.

CN0335: 12-Bit, 300 kSPS, Single-Supply, Fully Isolated, Data Acquisition System for 10 V Inputs

This completely isolated 12-bit, 300-kSPS data acquisition system uses only three active devices to process 10-V input signals using a single 3.3-V supply. The total error after room temperature calibration is less than 0.1% FSR over a 10C temperature change, making it ideal for a wide variety of industrial measurements. Both data and power are isolated, thereby making the circuit robust to high voltages and ground-loop interference often encountered in harsh industrial environments.

CN0314: Configurable 4-20 mA Loop Powered Transmitter/Receiver using a Micropower Instrumentation Amplifier

This configurable 4 mA-to-20 mA loop-powered transmitter is based on an industry-leading micropower instrumentation amplifier. Total unadjusted error is less than 1%. It can be configured with a single switch as either a transmitter that converts a differential input voltage into a current output, or as a receiver that converts a 4 mA-to-20 mA current input to a voltage output. Optimized for precision, low noise and low power industrial process control applications, the circuit can accept 0 V to 5V or 0 V to 10 V input range as a transmitter. As a receiver it can provide 0.2 V to 2.3 V or 0.2 V to 4.8 V output range compatible with ADCs using 2.5 V or 5 V references. The supply voltage can range from 12 V to 36 V as a transmitter and 7 V to 36 V as a receiver. The circuit is configurable, so a single hardware design can be used as a backup for both transmitter and receiver at the same time, minimizing inventory requirements.

February 2014

CN0336: 12-Bit, 300 kSPS, Single-Supply, Fully Isolated, Data Acquisition System for 4-20 mA Inputs

This completely isolated 12-bit, 300-kSPS data-acquisition system uses only three active devices to process 4-mA to 20-mA input signals using a single 3.3-V supply. The total error after room temperature calibration is 0.06% FSR over a 10C temperature range, making it ideal for a wide variety of industrial measurements. The small footprint makes it an industry-leading solution for 4-mA to 20-mA data acquisition systems where accuracy, speed, cost, and size play a critical role. Both data and power are isolated, making the circuit robust to high voltages and ground-loop interference often encountered in harsh industrial environments.

January 2014

CN0348: 16-Bit Single-Supply Buffered Voltage Output Digital-to Analog Conversion with Less Than 1 LSB Integral and Differential Nonlinearity

This complete single-supply,16-bit buffered voltage output DAC maintains 1 LSB integral and differential nonlinearity by utilizing a CMOS DAC followed by an innovative amplifier that has no crossover distortion. The circuit eliminates the crossover nonlinearity associated with most rail-to-rail op amps that can be as high as 4 LSBs to 5 LSBs in a 16-bit system. This industry leading solution is ideal for industrial process control and instrumentation applications that require a compact, single-supply, low-cost, highly linear 16-bit buffered voltage source. Total power dissipation for the three active devices is less than 25 mW typical when operating on a single 6-V supply.

CN0341: Magnetoresistive Linear Position Measurement

This contactless, anisotropic magnetoresistive (AMR) linear position measurement solution features 0.002-inch accuracy over a 0.5-inch range, making it ideal for applications where accurate, high-speed, non-contact length and position measurements are critical. It provides all the signal conditioning, including instrumentation amplifiers, buffers, and a 2-channel ADC, required to efficiently process the low-level AMR sensor bridge outputs. The result is an industry leading position measurement solution suitable for valve and flow measurement, machine tool speed control, motor speed measurement, and other industrial or automotive applications.

CN0332: Magnetoresistive (MR) Rotational Speed Sensor

This low-cost, high-speed, single-supply magnetoresistive (MR) signal conditioner features a minimum PCB footprint. The complete signal conditioning solution amplifies the small output voltage of the magnetoresistive sensor and converts it into a digital output signal with less than 5 ns rise and fall times and approximately 100-ps rms jitter. An excellent alternative to Hall effect sensors, the circuit provides a compact, cost effective, robust solution for high-speed rotational sensing in industrial and automotive applications.

November 2013

CN0340: True RMS RF Detector with 95 dB Detection Range

This true rms responding power detector uses a variable gain amplifier (VGA) and a power detector to provide a 95-dB wide detection range, making it useful for accurate measurement of signals with diverse or varying crest factors, such as those found in GSM/EDGE, CDMA, WCDMA, TD-SCDMA, and LTE receivers and transmitters. The 65-dB detection range of the ADL5902 rms detector is extended to 95 dB by the addition of the AD8368 linear-in-dB VGA.

CN0320: An IQ Demodulator-Based IF-to-Baseband Receiver with IF and Baseband Variable Gain and Programmable Baseband Filtering

This flexible, frequency agile, direct conversion IF-to-baseband receiver features a fixed 5-dB conversion gain to reduce the cascaded noise figure. Variable baseband gain adjusts the signal level, and a programmable low-pass filter eliminates out-of-channel blockers and noise. The filter bandwidth can be dynamically adjusted as the input signal bandwidth changes, ensuring full use of the available dynamic range of the driven ADC. The core circuit is an integrated I/Q demodulator with fractional-N PLL and VCO. With a single variable reference frequency, the PLL/VCO can provide a local oscillator (LO) between 750 MHz and 1150 MHz.

CN0276: High Performance, 10-Bit to 16-Bit Resolver-to-Digital Converter

This complete high-performance resolver-to-digital (RDC) circuit accurately measures angular position and velocity in automotive, avionics, and critical industrial applications where high reliability is required over a wide temperature range. The innovative resolver rotor driver circuit has two modes of operation: high performance and low power. In high-performance mode, the system operates on a 12-V supply and can supply 6.4 V rms (18 V p-p) to the resolver. In low-power mode, the system operates on a 6-V supply and can supply 3.2 V rms (9.2 V p-p) to the resolver, while drawing less than 100 mA. Active filtering is provided in both driver and receiver to minimize the effects of quantization noise. The maximum tracking rate is 3125 rps in 10-bit mode (resolution = 21 arc min) and 156.25 rps in 16-bit mode (resolution = 19.8 arc sec).

CN0269: 18-Bit, 1.33 MSPS, 16-Channel Data Acquisition System

This high-performance multichannel data acquisition circuit has been optimized for fast channel-to-channel switching with industrial signal levels. It can process 16 single-ended channels or eight differential channels, with 18-bit resolution at sampling rates up to 1.33 MSPS. A 250-kHz channel-to-channel switching rate provides 16-bit performance, making it an ideal solution for multichannel data acquisition in process control and power line monitoring applications.

October 2013

CN0323: Magnetoresistive Angle Measurement

This contactless, AMR (anisotropic magnetoresistive) angle measurement solution provides 1 angular accuracy over a 180 range, making it ideal for applications that require accurate, high speed, non-contact angle measurements. The full signal chain, which includes instrumentation amplifiers, buffers, and a dual channel ADC, efficiently processes the AMR sensors low level bridge outputs. The result is an industry leading angle measurement solution suitable for machine tool speed control, crane angle control, motor speed measurement, and other applications.

CN0302: Ultra-Fast Settling PLL with RF to 13 GHz

This PLL circuit uses a 13-GHz fractional-N synthesizer, wideband active loop filter, and VCO, to achieve phase settling time of less than 5 μs to within 5 for a 200-MHz frequency jump. The performance is achieved using an active loop filter with 2.4-MHz bandwidth. This wideband loop filter is enabled by the 110-MHz maximum frequency of the ADF4159s phase-frequency detector (PFD); and the 145-MHz gain-bandwidth product of the AD8065 op amp. The AD8065 can operate on a 24 V supply voltage, allowing control of most wideband VCOs having tuning voltages from 0 V to 18 V.

CN0280: Robust Completely Isolated Current Sense Circuit with Isolated Power Supply for Solar Photovoltaic Converters

This completely isolated current sensor with isolated power source is highly robust and can be mounted close to the sense resistor for accurate measurements and minimum noise pickup. The 16-MHz output data stream from a sigma-delta modulator is processed by a DSP using a SINC3 digital filter. Ideal for monitoring the ac current in solar photovoltaic (PV) converters, the circuit can handle peak ac voltages of several hundred volts and currents between a few mA and 25 A.

September 2013

CN0326: Isolated Low Power pH Monitor with Temperature Compensation

This completely isolated low power pH sensor signal conditioner and digitizer with automatic temperature compensation achieves 0.5% accuracy for pH between 0 and 14, with greater than 14-bits of noise-free code resolution, making it suitable for a variety of industrial applications such as chemical, food processing, water, and wastewater analysis. It supports a wide variety of pH sensors with internal resistance that can range from 1 MΩ to several GΩ. Digital signal and power isolation provides immunity to noise and transient voltages often encountered in harsh industrial environments.

CN0277: High-Precision, 18-Bit, 5 MSPS, Low Power Data Acquisition Signal-Chain

This 18-bit, 5-MSPS, low-power, low-noise, high-precision, complete data-acquisition signal-chain solution dissipates only 122 mW. The reference, reference buffer, driver amplifiers, and ADC provide an optimized solution with industry-leading 99-dB SNR and −117-dB THD. Its low power and small PCB footprint make it ideal for portable applications.

August 2013

CN0290: Extending the Low-Frequency Range of a High-Performance Phase Locked Loop

This high-performance phase locked loop (PLL) uses high-speed clock buffers and low-noise LDOs to maintain low phase noise even at low reference and RF frequencies. For example, the ADF4106 PLL specifies a 20-MHz minimum reference frequency and a 500-MHz minimum RF input frequency. This frequency range can be lowered to a 10-MHz reference frequency and a 100-MHz RF input frequency using additional clock buffers.

CN0287: Isolated 4-Channel Thermocouple/RTD Temperature Measurement System with 0.5C Accuracy

This completely isolated 4-channel temperature measurement circuit, optimized for performance, input flexibility, robustness, and low cost, supports all types of thermocouples with cold-junction compensation and 2-, 3-, or 4-wire RTDs with resistances up to 4 kΩ. The RTD excitation current is programmable for optimum noise and linearity. RTD measurements achieve 0.1C typical accuracy, and Type-K thermocouple measurements achieve 0.05C typical accuracy. The circuit uses a 4-channel AD7193 24-bit sigma-delta ADC with on-chip PGA for high accuracy and low noise. Input transient and overvoltage protection are provided by low-leakage transient voltage suppressors and Schottky diodes. The SPI-compatible digital inputs and outputs are isolated to 2500 V rms, and the circuit operates on a fully isolated power supply.

CN0239: Broadband 6 GHz Active Mixer with a Glueless Local Oscillator Interface

This 10-MHz to 6-GHz wideband active mixer has a direct interface to a frequency synthesizer-based low phase noise local oscillator (LO). It offers an optimum solution for wideband applications that require frequency conversion to higher or lower frequencies. The two-chip circuit covers a broad LO frequency range from 35 MHz to 4400 MHz. The LO interface is simple and glueless, eliminating the need for a balun, matching network, and LO buffer. In addition, the mixer bias adjust function allows optimization of IP3, noise figure, and supply current based on the application requirements or on the size of the input signal.


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