“As industrial systems increasingly shift from mechanical to Electronic controls, manufacturers see improvements in product quality and worker safety. The main reason for this is that the latter offer greater protection to workers in harsh environments. However, it is these harsh environments, extreme temperatures, and electrical noise and electromagnetic interference (EMI) that make good signal conditioning critical to maintaining the stability and sensitivity of circuits that are required for industrial machinery over the operational lifetime All you need to achieve reliable, precise and accurate control.
Author: Jeff Shepard
As industrial systems increasingly shift from mechanical to electronic controls, manufacturers see improvements in product quality and worker safety. The main reason for this is that the latter offer greater protection to workers in harsh environments. However, it is these harsh environments, extreme temperatures, and electrical noise and electromagnetic interference (EMI) that make good signal conditioning critical to maintaining the stability and sensitivity of circuits that are required for industrial machinery over the operational lifetime All you need to achieve reliable, precise and accurate control.
A key element in the signal conditioning chain is the op amp, a high-gain DC differential amplifier that acquires and amplifies the desired signal. Standard op amps are susceptible to temperature drift and have limited precision and accuracy; therefore, to meet industry requirements, designers add some form of system-level auto-calibration. The problem is that this calibration function can be complicated to implement and it can increase power consumption. Additionally, it requires more board space and increases cost and design time.
This article will review signal conditioning requirements in industrial applications and what designers need to focus on. Next, ON semiconductor‘s high-performance zero-drift op amp solutions will be introduced, and why and how they can be used to meet industrial signal conditioning requirements. Other related features of these devices, such as high common-mode rejection ratio (CMRR), high power supply rejection ratio (PSRR), and high open-loop gain, will also be discussed.
Industrial Signal Conditioning Applications
Low-side current sensing and sensor interfaces are frequently used in industrial systems. Because the differential signals associated with these circuits are so small, designers need high-precision op amps.
Figure 1 shows a low-side current sensing circuit used to detect an overcurrent condition, which is often used for feedback control.A low-value sense resistor shown in the figure
Figure 1: Low-side current sensing circuit showing the op amp interface between the sense resistor and the ADC. (Image credit: ON Semiconductor)
Sensors used to measure strain, pressure, and temperature in industrial and instrumentation systems are often configured in a Wheatstone bridge configuration (Figure 2). The sensor voltage changes that provide the measurement can be quite small and must be amplified before entering the ADC. Precision zero-drift op amps are often used in these applications because of their high gain, low noise, and low offset voltage.
Figure 2: Precision op amps are often used with a Wheatstone bridge to amplify signals from strain, pressure, and temperature sensors before sending that signal to an ADC. (Image credit: ON Semiconductor)
Key Parameters of Precision Operational Amplifiers
Offset voltage, offset voltage drift, susceptibility to noise, and open-loop voltage gain are key parameters that limit the performance of op amps in current sensing and sensor interface applications (Table 1).
Table 1: Key parameters of precision op amps that affect precision and accuracy. (Image credit: ON Semiconductor)
The input offset voltage (represented by VOS or VIO, depending on the manufacturer) results from imperfections in the semiconductor manufacturing process, resulting in a differential voltage between VIN+ and VIN-. This is a part-to-part variation that drifts with temperature and can be positive or negative, making it difficult to calibrate. Efforts by designers to reduce the skew or drift of standard op amps not only add complexity but in some cases lead to increased power consumption.
For example, consider using an op amp in a differential amplifier configuration for current sensing (Figure 3).
Figure 3: Current sensing using an op amp in a difference amplifier configuration. Low offset voltage is critical because the input offset voltage is amplified by noise gain, creating an offset error at the output (represented as “error due to VOS”). (Image credit: ON Semiconductor)
The output voltage is the sum of the signal gain term (VSENSE) and the noise gain term (VOS), as shown in Equation 1.
As an internal op amp parameter, the input offset voltage is multiplied by the noise gain rather than the signal gain, resulting in an output offset error (“Error due to VOS” in Figure 2). Precision op amps utilize various techniques to minimize offset voltage. In zero-drift op amps, this is especially true for low frequency and DC signals. Compared with general-purpose op amps, precision zero-drift op amps can have offset voltages that are more than two orders of magnitude lower (Table 2).
Table 2: Comparing the maximum offset voltage of a selected general-purpose op amp and a chopper-stabilized zero-drift op amp, the offset voltage of a precision zero-drift op amp can be more than two orders of magnitude lower. (Image credit: ON Semiconductor)
Zero-drift op amp
With its improved performance, designers can use zero-drift op amps to meet the signal conditioning requirements of industrial applications. ON Semiconductor’s NCS325SN2T1G and NCS333ASN2T1G are two examples of zero-drift op amps with different performance levels. Designers can use the NCS325SN2T1G device for precision applications, benefiting from its 50 microvolt (µV) offset and 0.25µV/°C drift, while the NCS333ASN2T1G family is suitable for the most demanding precision applications, offering 10µV offset and 0.25µV/°C drift. Only 0.07µV/°C drift. The two op amps use different internal architectures to achieve zero drift.
The NCS333ASN2T1G uses a chopper-stabilized architecture, which has the advantage of minimizing offset voltage drift over temperature and time (Figure 4). Unlike traditional chopper architectures, this chopper-stabilized architecture has two signal paths.
Figure 4: The NCS333ASN2T1G has two signal paths: The second path (below) samples the input offset voltage and is used to correct the offset at the output. (Image credit: ON Semiconductor)
In Figure 4, the lower signal path is where the chopper samples the input offset voltage, which is then used to correct the offset at the output. Offset correction occurs at a frequency of 125 kilohertz (kHz). The chopper-stabilized architecture is optimized for optimal performance at frequencies up to the relevant Nyquist frequency (1/2 the offset correction frequency). Since the signal frequency exceeds the Nyquist frequency of 62.5kHz, aliasing may appear at the output. This is an inherent limitation of all chopping and chopper-stabilized architectures.
Nonetheless, the NCS333ASN2T1G op amp has minimal aliasing up to 125 kHz and remains low up to 190 kHz. ON Semiconductor’s patented method uses two cascaded, symmetrical resistor-capacitor (RC) notch filters tuned to the chopping frequency and its fifth harmonic frequency to reduce aliasing effects.
Another way to implement a zero-drift op amp is to use an auto-zero architecture (Figure 5). The auto-zero design has a main amplifier and a return-to-zero amplifier. It also uses a clock system. In the first stage, the switched capacitor holds the offset error of the previous stage at the output of the return-to-null amplifier. In the second stage, the offset of the main amplifier is corrected with the offset of the output of the nulling amplifier. ON Semiconductor’s NCS325SN2T1G uses an auto-zero architecture.
Figure 5: Simplified block diagram of an auto-zero op amp with switched capacitors like the NCS325SN2T1G. (Image credit: ON Semiconductor)
In addition to the above differences in offset voltage and drift between the NCS333ASN2T1G (chopper-stabilized architecture) and NCS325SN2T1G (auto-zero architecture), the different architectures also produce differences in open-loop voltage gain, noise performance, and aliasing sensitivity. The open-loop voltage gain of the NCS333ASN2T1G is 145 decibels (dB), while the open-loop voltage gain of the NCS325SN2T1G is 114 dB. Taking noise into account, the CMRR of the NCS333ASN2T1G is 111dB and the PSRR is 130dB, while the CMRR of the NCS325SN2T1G is 108dB and the PSRR is 107dB. Both rated well, but the NCS333ASN2T1G outperformed the NCS325SN2T1G.
The NCS333ASN2T1G family of op amps also have minimal aliasing. This is because ON Semiconductor’s patented method uses two cascaded, symmetrical RC notch filters tuned to the chopping frequency and its fifth harmonic frequency, reducing aliasing effects. In theory, the auto-zero architecture will exhibit a greater degree of aliasing than the chopper-stabilized counterpart. But aliasing effects can be very different and not necessarily specified. The designer needs to understand the aliasing characteristics of the specific op amp being used. Aliasing is not a defect of sampling amplifiers, but a behavior. Understanding this behavior and how to avoid it allows zero-drift amplifiers to perform optimally.
Finally, op amps also have varying degrees of EMI susceptibility. The semiconductor junction can receive and rectify the EMI signal, creating an EMI-induced voltage offset at the output, adding another component to the total error. The input pins are the most sensitive to EMI. The high-precision NCS333ASN2T1G operational amplifier integrates a low-pass filter to reduce susceptibility to EMI.
Design and Layout Considerations
To ensure optimal op amp performance, designers must follow good board design practices. Precision op amps are sensitive devices. For example, it is important to place 0.1 microfarad (µF) decoupling capacitors as close to the power pins as possible. In addition, when making a shunt connection, the printed lines on the circuit board should be of equal length and size, and should be as short as possible. The op amp and shunt resistor should be on the same side of the board, and for applications requiring the highest level of accuracy, a four-terminal shunt, also known as a Kelvin shunt, should be used. A combination of these techniques will reduce EMI susceptibility.
Be sure to follow the shunt manufacturer’s recommendations when making connections. Improper connections can add unnecessary stray lead and induced impedance to the measurement and increase error (Figure 6).
Figure 6: Two-terminal shunt resistor (RLead and RSense) connections depicting stray resistance. (Image credit: ON Semiconductor)
Accuracy may be affected by temperature-dependent offset voltage differences at the input pins. To minimize these differences, designers should use metals with low thermoelectric coefficients and prevent temperature gradients from heat sources or cooling fans.
There is an ever-increasing need for precise, accurate signal conditioning in a variety of industrial applications. Accompanying this increase is the need for low-power, compact solutions. The op amp is a critical element in signal conditioning, but designers need to add automatic calibration and other mechanisms to ensure system time and temperature stability, thus increasing system complexity, cost, and additional power consumption.
Fortunately, designers can turn to high-performance zero-drift op amps that feature continuous auto-calibration, very low offset voltage, and near-zero drift over time and temperature. In addition, they have low power consumption over a wide dynamic range, are compact, and feature the key features of high CMRR, high PSRR, and high open-loop gain required by all industrial applications.