Precision measurement is the foundation of high-accuracy and high-performance products being developed today. The precision of measuring signals has seen spectacular improvements. Precision measurement devices now make it possible to detect a hidden anomaly in a patient's heartbeat, thus enabling accurate diagnosis. Modern industrial, automotive, and IoT systems require accurate and high-performance analog devices, which form the building blocks of a wide variety of mission-critical applications, instrumentation, and automation products. This Essentials learning module explores precision measurement and focuses on precision measurement ICs such as analog-to-digital converters (ADCs), digital-to-analog converters (DACs), audio amplifier ICs, high-precision low-noise op amps, high-accuracy current-sense amplifiers (CSAs), and highly stable voltage references. These all have a crucial role in analog design.
Upon completion of this module, you will be able to:
Measurement systems are designed to sense and measure temperature, pressure, speed, or other physical parameters. These parameters are converted to a readable form and used for monitoring, alarming, and control. Precision measurement is a necessity where precision and accuracy are mandatory, as in medical instruments, IoT devices, and battery-operated equipment.
Precision and accuracy are cornerstones of any precision measurement system. Precision deals with repeatability of a result, while accuracy refers to how closely a quantity's measured value corresponds to its "true" value. A measurement can be categorized in the context of precision and accuracy, as in Figure 1, which illustrates different scenarios for these parameters. The black dots indicate the measured results and center red point indicates the true value.
All measurements are susceptible to error, contributing to an uncertain result. An error can be human-made or technical. A technical error can be further classified as a random error or a systematic error. Random errors are caused by uncontrolled variables with no recognizable design. Conversely, systematic errors occur if the measurement system has an inherent problem corresponding to design or components fitted in the system. Precision measurement ICs are specially engineered to minimize these errors, and play a critical role in precision circuitry design.
This learning module covers the specific features and performance of analog devices used in precision measurement systems design. The scope is limited to the key parameters, characteristics, and applications of precision measurement analog ICs, including ADCs, DACs, voltage references, op-amps, audio amplifiers, and current sense amplifiers.
Precision measurement is the capability of a device to show an identical reading when operated each time. Precision is vital when we work with low voltage or low current signals. In complete systems with multiple devices, it is also linked to accuracy, sensitivity, speed, and many more factors that may influence full system performance. For example, the voltage reference, ADC, and other design components in a data acquisition system must be precise and efficient to generate a compatible digital output. An audio amplifier must be accurate during the amplification of a low-level signal. Figure 2 shows a measurement signal chain. It consists of essential analog components such as an ADC, DAC, op-amp, and a voltage reference.
- 4.1 Characteristics
Let's discuss the characteristics of precision analog components such as ADCs, op-amps, DACs, voltage references, audio amplifiers, and current sense amplifiers that designers use when designing analog precision circuitry.
Analog-to-Digital Converter (ADC)
Analog-to-digital converters (ADCs) are used in a wide range of electronic devices. There are different types of analog-to-digital converters (ADCs) with different ADC architectures. Let’s overview them briefly:
Successive-approximation-register analog-to-digital converters (SAR ADCs) are frequently the architecture of choice for medium-to-high-resolution applications, typically with sample rates fewer than 5 megasamples per second (Msps). SAR ADCs most commonly range in resolution from 8 bits to 20 bits and provide low power consumption as well as a small form factor. This combination makes them ideal for a wide variety of applications, such as automatic test equipment, battery-powered equipment, data acquisition systems, medical instrumentation, and more.
Delta-sigma analog-to-digital converters (Δ∑ADCs) are used predominately in lower speed applications requiring a trade-off of speed for resolution by oversampling, followed by filtering to reduce noise. 24 bit delta-sigma converters are used in applications such as automation test equipment, high-precision portable sensors, and medical and scientific instruments.
Integrating ADCs provide high resolution and can provide good line frequency and noise rejection. The integrating architecture provides a novel yet straightforward approach to converting a low bandwidth analog signal into its digital representation. These type of converters often include built-in drivers for LCD or LED displays and are found in many portable instrument applications.
Flash analog-to-digital converters, also known as parallel ADCs, are a fast way to convert an analog signal to a digital signal. They are suitable for applications requiring very large bandwidths. However, flash converters consume a lot of power, have relatively low resolution, and can be quite expensive.
Pipeline analog-to-digital converters (ADCs) are used for sampling rates from a few megasamples per second (MS/s) up to 100MS/s+, with resolutions from 8 to 16 bits. They offer the resolution and sampling rate to cover a wide range of applications, including CCD imaging, ultrasonic medical imaging, and digital receivers.
Two Step analog-to-digital converters (ADCs), also known as subranging converters and sometimes referred to as multi-step or half flash, are a cross between a Flash ADC and pipeline ADC. They can achieve higher resolution or smaller die size and power for a given resolution needed vs. a Flash ADC.
SAR and delta-sigma ADCs are often used as precision ADCs. They can execute measurements with very high accuracy and resolutions up to 24 bits, making them a critical analog IC in the precision measurement signal chain. They are also utilized in precision measurement applications because they are highly integrated (such as with integrated programmable gain amplifiers and precision voltage references), thereby reducing part counts and simplifying board layouts, and they have fast sample rates and very low power consumption.
Figure 3: The MAX11168 is a 16-bit, 500ksps, SAR ADC offering excellent AC and DC performance with true bipolar input range, internal reference, 2.3dB SNR and -101dB THD at 10kHz, and small size.
Digital-to-Analog Converter (DAC)
Digital-to-analog converters (DACs) generally incorporate either a resistor-string architecture or an R-2R ladder architecture. When a resistor string is used, the DAC's inputs control a set of switches that divide the reference voltage through matched series resistors. A DAC R-2R ladder divides down a positive reference voltage by switching individual resistors between a positive reference voltage and the negative side of this reference voltage (usually ground), thus generating a current. A voltage-output DAC converts this current back to an output voltage through an output amplifier. A current-output DAC either routes the R-2R ladder current to the output directly, or uses an amplifier to buffer the output.
Selecting a DAC involves a number of choices, including: serial vs. parallel interface, resolution/number of bits, number of input channels, voltage or current output, cost, and relative accuracy.
A DAC communicates digitally with either a serial or a parallel interface. Serial interfaces send data sequentially. Parallel interfaces send all the data bits at the same time and require a separate pin/connection for each bit. For applications where speed is important, parallel interfaces are preferred. Where size and cost are important, 3-wire and 2-wire serial interfaces are a good option, since they require fewer pins and often cost less.
The highest resolution DACs are designed with 16- or 18-bit inputs to provide resolution down to the microvolt range. An 18-bit DAC, for example, with a 2.5V reference has a least significant bit (LSB) weight of 9.54µV. This degree of resolution is important for industrial designs such as robots or motors.
DACs are available with many data converters integrated in a single package. DAC outputs sink and source current, which provides additional flexibility for designers. In addition to current outputs, some DACs have the amplifier connections bonded out to allow additional output control. These DACs are known as force/sense DACs. Force/Sense DACs are unique because they provide user access to the inverting node of the output buffer amp in addition to the conventional output. These DACs are interesting because they provide flexibility to create custom DAC gains, or other useful circuits by simply adding a few simple components.
Precision amplifiers are op amps that have better specifications and are more accurate than the standard op amp. Precision amplifiers can have better specs with precision offset, zero-drift over time, lower internal noise (nV/√Hz), and input bias current. Precision op amps have precisely matching resistors etched into the substrate at the chip level.
Precision and low-noise op amps are often used to condition the signal coming from a sensor before it enters an analog-to-digital converter (ADC). In such a role, two particular op amp specifications are crucial for good system resolution: the input offset voltage and the input voltage noise. The input offset voltage is defined as the voltage that must be applied between the two input terminals of the op amp to obtain zero volts at the output. The input voltage noise is the voltage fluctuation at the input of an otherwise noise-free amplifier with shorted inputs.
The most common error is input offset voltage. This characteristic is caused by small differences in the input differential transistors or any related resistors. This error is usually very small, and in many applications, it can be ignored as it doesn't cause any detrimental effects. However, in applications where very small input signals are to be amplified with very high gain, this unwanted error voltage is amplified along with the desired input. The output, therefore, isn't representative of the true input. In addition, the input offset voltage error varies with temperature, introducing further obfuscations of the true signal.
Any application that involves the amplification of very low signal levels is a candidate for zero-drift amplifiers, because any significant input offset voltage will introduce errors. Zero-drift precision op amps are specialized op amps designed for applications that require high output accuracy due to small differential voltages. Not only do they feature low input offset voltage, but they also have high CMRR, high PSRR, high open loop gain, and low drift over temperature and time. These features make them ideal for applications such as low-side current sensing and sensor interface, particularly with very small differential signals. Some of the primary applications include bridge amplifiers using strain gauges or other sensors, current shunt measurement, thermocouples, IR sensors, electronic scales (load cells), and medical instrumentation. Other uses are ADC input buffer amplifiers and DAC output amplifiers.
Figure 4: The MAX40100 is a low-power, zero-drift operational amplifier available in a space-saving, 6-bump, wafer-level package (WLP). Picture is a high-input impedance 2-op amp instrumentation amplifier.
An audio amplifier increases the amplitude of a small signal to a useful level, all the while maintaining the smaller signal's detail. This is known as linearity. The greater the amplifier linearity, the more the output signal is a true representation of the input. With the ever-changing performance requirements for amplifiers, there have been many advances in audio amplifier topologies. Let's briefly go through them.
Class A Amplifiers: The simplest type of audio amplifiers is Class A. Class A is the most linear type of audio amp, but it has low efficiency. Consequently, these amps are used in applications that require high linearity and have ample power available.
Class B Amplifiers: these amplifiers use a push-pull amplifier topology. The output of a Class B amp incorporates a positive and negative transistor. To replicate the input, each transistor only conducts during half (180°) of the signal waveform. This allows the amp to idle with zero current, thereby increasing efficiency compared to a Class A amp. There is a trade-off that comes with a Class B amp: the increased efficiency degrades audio quality. This happens because there is a crossover point at which the two transistors transition from the on state to the off state. Class B audio amps are also known to have crossover distortion when handling low-level signals. They are not a good choice for low-power applications.
Class AB Amplifiers: A compromise between Class A and Class B amplifier topologies is the Class AB audio amp. A Class AB amp provides the sound quality of the Class A topology with the efficiency of Class B. This performance is achieved by biasing both transistors to conduct a near zero signal output, i.e., the point where Class B amps introduce nonlinearities. For small signals, both transistors are active, thus functioning like a Class A amp. For large-signal excursions, only one transistor is active for each half of the waveform, thereby operating like a Class B amp. Class AB speaker amps offer high signal-to-noise (SNR), low THD+N, and typically up to 65% efficiency. This makes them ideal choices as high-fidelity speaker drivers.
Class D Amplifiers: The popularity of handheld mobile audio devices, such as smartphones, MP3 players, and portable docking stations brings power consumption into greater focus. Now, it is necessary to reduce power consumption to increase battery life. Class D amplifiers use pulse-width modulation (PWM) to produce a rail-to-rail digital output signal with a variable duty cycle to approximate the analog input signal. These amps are highly efficient (often up to 90% or higher) because the output transistors are either fully turned on or fully turned off during operation. This approach completely eliminates the use of the linear region of the transistor that is responsible for the inefficiency of other amplifier types. Modern Class D amps also achieve fidelity comparable to Class AB amps.
Figure 5: The MAX98390 is a high-efficiency mono Class-D DSM smart amplifier that features an integrated boost converter, integrated Dynamic Speaker Management, and FET scaling for higher-efficiency at low output power.
Class G Amplifiers: Class G amplifiers are similar to Class AB amps, except that they use two or more supply voltages. When operating at low signal levels, Class G amps select a low supply voltage. As the signal level increases, these amps automatically select the appropriate supply voltage. Class G amps are more efficient than Class AB amps because they use the maximum supply voltage only when required; in contrast, Class AB amps always use the maximum supply voltage.
Class DG Amplifiers: A Class DG amplifier uses PWM to produce a rail-to-rail digital output signal with a variable duty cycle. In this respect, a Class DG amp is the same as a Class D amp. The Class DG amp, however, also uses a multilevel output stage to sense the magnitude of the output signal. It then switches the supply rails, as needed, to supply the required signal power more efficiently.
Class H Amplifiers: Class H amplifiers modulate their supply voltage to minimize the voltage drop across the output stage. Implementations range from using multiple discrete voltages to an infinitely adjustable supply. Though similar to the Class G technique of reducing dissipation across output devices, the Class H topology does not require multiple power supplies. Class H amps are generally more complex than other audio amplifier designs. These amps require extra control circuitry to predict and control the supply voltage.
Today's space-saving precision audio amplifiers are highly efficient operational amplifiers, precisely amplifying the input audio signals to power levels and voltage as needed by their speaker element. They offer improved THD, PSRR, and power savings, while eliminating potential issues with EMI, RF interference, and audio artifacts such as clicks and pops.
Current Sense Amplifier (CSA)
Current sensing is a fundamental requirement in a wide range of electronic applications. Current-sense amplifiers are sophisticated ICs used in electronic equipment that monitors load currents in real time. System controllers use this load information to implement power-management algorithms that modify the load-current characteristic itself, and to implement flexible overcurrent protection schemes.
Current-sense amplifiers (CSAs) monitor current flow through a shunt (sense) resistor and provide closed-loop feedback of system loads. The precision gain of the current-sense amplifier reduces, for a given current value, the voltage burden imposed by the sense resistor on the line in which current is measured, since less drop in the sense resistor is required for the output voltage needed to measure it. A current-sense amplifier therefore complies with the basic description of a voltage instrumentation amplifier (IA): it is a precision-gain differential amplifier.
Most current-measurement applications employ either the low-side principle, in which the sense resistor connects in series with the ground path, or the high-side principle, in which it connects in series with the hot wire. These two approaches pose some trade-offs. The low-side resistor adds undesirable extraneous resistance in the ground path. However, the circuitry associated with the high-side resistor must cope with relatively large common-mode signals.
Precision current-sense amplifiers allow for the measurement of multiple decades of current. They can decrease sense resistor impedance, improve system efficiency, and achieve faster signal processing and acquisition time for improved AC performance.
Analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) need a stable reference voltage to accurately measure or generate analog signals. Voltage references play an important part in driving the total system error. In other words, voltage references set the precision standard. Key internal factors that dictate the overall performance of voltage references are the IC design architecture, design techniques, and fabrication process. Also important are specifications including noise, thermal hysteresis, temperature coefficient, and long-term drift (LTD).
In theory, an ideal voltage reference has a perfect initial accuracy and maintains its voltage independent of changes in load current, temperature, and time. In the real world, a designer must make tradeoffs such as: initial voltage accuracy, voltage temperature drift and hysteresis, current source and sink capability, quiescent current (or power dissipation), long-term stability, noise, and cost.
The two most common types of references are zener and bandgap. Zeners are usually used in two-terminal shunt topologies. Bandgap references are usually used in three-terminal series topologies. Zener diodes are diodes optimized for operation in the reverse-bias breakdown region. Because breakdown is relatively constant, it can be used to generate a stable reference by driving a known current in the reverse direction. Buried zener diodes are a specific type of zener that are more stable than a regular zener, due to their structure, which places them below the surface of the silicon.
The key difference between a shunt and series reference is that a three terminal series-mode voltage reference does not require an external resistor and has significantly lower quiescent power. The most common form is the ubiquitous bandgap reference.
A bandgap reference develops two voltages: One has a positive temperature coefficient (tempco) and one has a negative tempco. Together, they have a zero-tempco sum at the output. The positive tempco is usually derived from the difference of two VBEs running at different current levels. The negative tempco uses the naturally negative tempco of the VBE voltage. In practice, the tempco sum is not exactly zero. Depending on design details like the IC circuit design, packaging, and manufacturing test capabilities, these devices can usually achieve a VOUT tempco between 1 and 100ppm per degree C.
Designing a precision measurement system is a complex task, as system requirements vary with the application. The design engineer has to be careful in analyzing the signals and errors at various stages of the signal chain. To better understand the use cases of high-precision analog ICs, let us look at some precision measurement applications using Maxim devices:
- 5.1 Improving Ultrasound Images With the High Signal-to-Noise ADC
Ultrasound is widely used in medical imaging, catheter guidance, bone densitometry, blood flow measurement, and cancerous lesion detection. In medical ultrasound applications, image clarity can make a crucial, life-saving difference, like a cardiologist successfully spotting a problem with turbulent blood flow in a heart or missing the complication altogether. Doppler Imaging techniques to measure blood velocity and direction can be impaired due to small return signals from blood flow. All these challenges need an extremely high signal-to-noise ratio (SNR) ADC with low total harmonic distortion (THD). Since this application would merge a high sampling rate and high resolution with little total harmonic distortion and good SNR, Maxim's MAX11905 fully differential SAR ADC is suitable for this type of application. The MAX11905 is a 20-bit, 1.6Msps, single-channel, fully differential SAR ADC with internal reference buffers. The MAX11905 provides excellent static and dynamic performance with best-in-class power consumption that directly scales with throughput. The device has a unipolar differential ±VREF input range. Supplies include a 3.3V supply for the reference buffers, a 1.8V analog supply, a 1.8V digital supply, and a 1.5V to 3.6V digital interface supply. This ADC achieves 98.3dB SNR and -123dB THD, guarantees 20-bit resolution with no-missing codes and 6 LSB INL (max). The MAX11905 communicates data using an SPI-compatible serial interface. The MAX11905 is offered in a 20-pin, 4mm x 4mm, TQFN package and is specified over the -40°C to +85°C operating temperature range.
- 5.2 Battery Management with Current Sense Amplifier
Smartphones, tablets, and other portable devices require increasingly smaller form factors and lower power consumption rates to extend battery life. Figure 8 shows the MAX40016 current-sense Amplifier. It is a very wide range current sense amplifier (CSA) with internal sense element that senses from less than 300µA to greater than 3A current range. The 4-decade sensed current functions with 1% (typical) gain error and offers three multiplexed programmable output ranges in order to interface with 12-bit ADCs. Having an integrated sense element has the extra advantage that the entire current measuring path can be factory-trimmed, saving the user from having to calibrate independent sense resistors and CSAs. The MAX40016 (WLP package) drops a typical of 60mV at 3A from the voltage input to load output.
The MAX40016's integrated current-sensing element saves the space and cost of an external high-power, precision current sense resistor. The MAX40016 is offered in an ultra-tiny, 1.98mm x 1.31mm, 15-bump wafer level package (WLP), further reducing board space. The MAX40016 is also available in a 4mm x 4mm 16-pin TQFN package. The MAX40016 also includes a committed on-board amplifier with an internal gain of 1.5V/V. The MAX40016 operates over the -40°C to +125°C temperature range.
- 5.3 Precision Comparator Using a Bandgap Voltage Reference
Comparators are widely used in ADCs, level detectors, Schmitt triggers, window detectors, and on-off controls. For making an exact measurement, a precise voltage reference is used with comparators that make a perfect trip point. The behavior and accuracy of the comparator circuit depends on the voltage reference. This application is an ideal use for the MAX6279.
The MAX6279 is a precision, two-terminal shunt mode, bandgap voltage reference available in fixed reverse breakdown voltage of 1.225V. Ideal for space-critical applications, the MAX6279 is offered in an 8-Pin ceramic package. Laser-trimmed resistors ensure precise initial accuracy. With a 25ppm/°C temperature coefficient, the device is offered in three grades of initial accuracy ranging from 0.1% to 0.5%. The MAX6279 has a 70μA to 12mA shunt current capability with low-dynamic impedance, ensuring stable reverse breakdown voltage accuracy over a wide range of operating temperatures and currents. The MAX6279 does not require an external stabilizing capacitor while ensuring stability with capacitive loads. The MAX6279 is a higher precision device than the LM4040/LM4050.
Bandwidth: Bandwidth describes band frequencies over which Amplifier gain remains constant (with 3dB maximum deviation). The output may suffer distortion if the amplifier's bandwidth fails to cover the input signals spectrum.
Common Mode Rejection Ratio (CMRR): When an identical input voltage is applied to both op-amp input terminals, it operates in common mode-configuration with common-mode gain Acm = (Vo cm/Vcm). CMRR is defined as the ratio of differential gain AD to common-mode gain Acm; thus, CMRR = (AD/Acm) Where AD equals op-amp internal gain A. The CMRR value, in most cases, is large and consequently specified in decibels (dB): CMRR (dB) = 20 log (AD/Acm)
Dropout voltage: Dropout voltage refers to the difference between input and output voltage, permitting the reference to maintain the specified accuracy. This voltage is critical to battery operated and low-voltage equipment.
Initial Accuracy: The initial accuracy of any voltage reference is described as its worst-case tolerance after the full production process. This specification, at room temperature, for a defined input voltage and load current, helps to determine the precision of the ADC conversion.
Input Bias Current: The ideal op-amp does not draw current from its input terminals. However, a small DC is conducted by the input terminals to bias the input transistors. It is Input Bias Current and is meager for precision Op-Amps.
Input Offset Voltage: The Op-Amps output ideally should be at zero volts during grounded inputs. Still, in practicality, the input terminals have somewhat different voltage, and output does not show zero volts. The input offset voltage is the minimum voltage to be applied to op-amp input to make output voltage zero. Offset minimization is achieved by resistor networks, and offset correction circuit techniques are integrated into the Op-Amp.
Load Regulation: Load regulation is variation in output voltage due to alteration in reference load current and is specified by ppm/mA, %/mA, or percent change from a no-load to full load. This parameter is crucial if reference load current varies while the reference works.
Load Transient Response: Load transient response, also known as output settling time, is response characteristic to a sudden load variation. It is the time needed for the output voltage to revert to its particular value after it fell or rose.
Noise: Noise in voltage reference is the random signal generated by passive and active devices inside the IC, affecting accuracy. The MAX6173AASA+ and MAX6173BASA+ are high-precision voltage, low-noise references.
Power Supply Rejection Noise (PSRN): It is the ratio of power supply voltage variation and the resulting delta differential output due to the change. PSSR ideally indicates the circuit's ability to provide accurate and stable output despite supply variations.
Resolution: The number of possible output levels the DAC can reproduce. For example, an 8-bit DAC reproduces 256 levels. This value dictates audio bit depth in audio applications and color depth in video applications.
Settling Time: Settling time is hiatus between a command updating its output value and the moment it arrives in the final output. Settling time is influenced by the output amplifier's slew rate, ringing, and signal overshoot.
Signal-to-Noise Ratio: The signal-to-noise ratio is the variance between the output signal and noise level, as measured in dB. A high parameter implies a low noise introduced by the Amplifier, with a concurrent rise in sound quality.
Temperature Drift: Temperature drift is output voltage change apropos temperature and is widely used to examine voltage reference performance. This drift is caused by nonlinearities and imperfections in-circuit elements and stated by ppm/°C.
Total harmonic distortion (THD): It is the RMS sum of all harmonics in the output signal introduced due to signal nonlinearity of the amplifier at various input and output conditions like amplitude, frequency, and load. This value is generally lower than 0.1% and is specified for an entire band or a test frequency.
Total harmonic distortion and noise (THD+N): Measurement of distortion and noise imported to the signal by the DAC is expressed as a percentage of the total power of unwelcome noise and harmonic distortion, which accompanies the wanted signal.
Turn-on/Turn-off Settling Time: The turn-on settling time is the schedule needed to stabilize the reference output voltage after the first switch-on of power. The output must be stable and not necessarily reached the specified accuracy. Turn-off settling time is time for the reference output voltage to touch virtually zero volts. Both parameters depend on input and output capacitor values.
*Trademark. Maxim Integrated is a trademark of Maxim Integrated Inc. Other logos, product and/or company names may be trademarks of their respective owners.
Test Your KnowledgeBack to Top
Are you ready to test how much you've learned from this Precision Measurement Essentials? Then take this 15-question quiz to see how much you've learned.
To earn the Analog Electronics I Badge, read through the learning module, attain 100% on the Quiz, leave us some feedback in the comments section, and give the learning module a star rating.