“Instrumentation amplifiers (IAs) are the workhorse of inspection applications. This article will explore some ways to take advantage of the balance and excellent DC/low frequency common-mode rejection (CMR) characteristics of in-amps, allowing in-amps to be used with resistive sensors (such as strain gauges) that are physically separated from the amplifier. This article will present some ways to improve the noise immunity of such gain stages while reducing their susceptibility to power supply variations and component drift. The article also provides measured performance values and results to demonstrate the range of accuracy for quick evaluation of end-user applications.
From electrified vehicles, digital health, to instrumentation, smart industry, energy sustainability, and demanding applications in any high-precision signal chain, the precision technology signal chain plays an important role, a good precision technology signal Chains allow for easy transition between various design tradeoffs to create a premium end solution for the application.
01 Remote detection using high precision instrumentation amplifiers
Instrumentation amplifiers (IAs) are the workhorse of inspection applications. This article will explore some ways to take advantage of the balance and excellent DC/low frequency common-mode rejection (CMR) characteristics of in-amps, allowing in-amps to be used with resistive sensors (such as strain gauges) that are physically separated from the amplifier. This article will present some ways to improve the noise immunity of such gain stages while reducing their susceptibility to power supply variations and component drift. The article also provides measured performance values and results to demonstrate the range of accuracy for quick evaluation of end-user applications.
Shielded twisted pair cables are not immune to all interference on long cable runs. In this case, a well-balanced input from the instrument cannot be relied on to eliminate the CM effect. Interference picked up by long cables has an uneven effect on the positive and negative amplifier inputs, and the inputs contain uncorrelated signals that CMR cannot cancel. Therefore, as shown in Figure 1, it is not surprising to find significant noise at the output of the circuit due to an unbalanced response to CM noise (as it appears to be).
Figure 1. Troubled amplifier output with 120Hz noise (0.1V/div, 2 ms/div).
To successfully extract very small bridge differential voltages from CM (DC and interference), one solution is to use two pairs of shielded or unshielded twisted pair (UTP). In this way, the two inputs of the instrumentation amplifier are balanced and are equally affected by CM noise, and devices such as the LT6370 have excellent low frequency CMR (120dB), which can reliably reject the noise that plagues the IA input. As a result, the output waveform is clean at long distances, even in noisy environments.
Figure 2. Remote Detection Using Two Unshielded Twisted Pair Cables
The bridge sensor is far from the signal processing amplifier and requires an instrumentation amplifier to extract a clean measured differential voltage. The characteristics of the LT6370 instrumentation amplifier allow it to successfully process signals from remote sensors over long cables. The LT6370 manufacturing process invokes on-chip heaters to guarantee temperature drift values during production testing, further enhancing the LT6370’s suitability for remote monitoring applications and extending its service life and product life in hard-to-service equipment.
02 Composite Amplifier: High-Precision High-Output Drive Capability
It’s normal, almost reasonable, for an app to be developed that doesn’t seem to have a solution. To meet application requirements, we needed to come up with a solution that exceeded the performance of existing products on the market. For example, an application may require an amplifier with high speed, high voltage, high output drive capability, and may also require excellent DC accuracy, low noise, low distortion, etc.
Amplifiers that meet speed and output voltage/current requirements as well as amplifiers with excellent DC accuracy are readily available in the market, and indeed many are. However, all of these requirements may not be met by a single amplifier. When faced with such a problem, some people think that we cannot meet the requirements of this type of application, we must settle for a mediocre solution, choose either a precision amplifier or a high-speed amplifier, possibly sacrificing some requirements. Fortunately, this is not entirely true. One solution to this is to use a composite amplifier, and this article will show how this is accomplished.
A composite amplifier consists of two separate amplifiers configured in such a way that one can realize the advantages of each amplifier while attenuating the disadvantages of each amplifier.
Figure 3. Simple composite amplifier configuration.
When first encountering a composite amplifier, the first question may be how to set the gain. To solve this problem, it helps to think of the composite amplifier as a single non-inverting op amp contained within a large triangle: imagine the large triangle is black and we can’t see what’s inside, then the gain of the non-inverting op amp is 1 + R1 /R2. Uncovering the composite configuration inside the big triangle didn’t change anything, the gain of the whole circuit is still controlled by the ratio of R1 and R2.
Figure 4. Composite amplifier treated as a single amplifier
One of the main advantages of implementing a composite amplifier is wider bandwidth compared to a single amplifier configured for the same gain.
03 The new generation of SAR ADC solves the difficulties of precision data acquisition signal chain design
Many applications require a precision data acquisition signal chain to digitize analog data for accurate data acquisition and processing. Precision system designers are under increasing pressure to find innovative ways to increase performance and reduce power consumption while accommodating higher circuit densities on small PCB circuit boards. The purpose of this article is to discuss common difficulties encountered in precision data acquisition signal chain design and how to solve these difficulties with a new generation of 16-bit/18-bit, 2MSPS, precision successive approximation register (SAR) ADCs.
Figure 5. Typical precision data acquisition signal chain.
Applications requiring sophisticated data acquisition systems, such as automated test equipment, machine automation, and industrial and medical instrumentation, exhibit common trends that are often considered technically conflicting. For example, system designers are forced to compromise on performance to maintain tight system power budgets, or to reserve a small area on the board to achieve high channel density. System designers of these precision data acquisition signal chains face common challenges on several fronts: driving SAR ADC inputs; protecting ADC inputs from overvoltage events; reducing system power consumption with a single supply; Microcontrollers and/or digital isolators for higher system throughput, etc.
The AD4000/AD4003 family are fast, low-power, single-supply, 16-bit/18-bit precision ADCs based on the SAR architecture.
Figure 6. Key benefits of AD4000/AD4003 ADCs.
The AD4000/AD4003 precision ADC family uniquely combines high performance with ease of use to reduce system complexity, simplify the signal chain BOM, and dramatically reduce time to market. With this family, designers can solve the system-level technical challenges of precision data acquisition systems without making major compromises. For example, features such as longer acquisition times left to the user, high input impedance (Z) mode, and span compression mode are combined in the AD4000/AD4003 ADC family to reduce the challenges associated with ADC driver stage design and increase the flexibility of ADC driver selection sex. This reduces overall system power consumption, increases density, and shortens customer design cycles. Most of the easy-to-use features can be enabled/disabled by writing to the configuration registers via the SPI interface. Note that the AD4000/AD4003 ADC family is pin compatible with the 10-lead AD798x/AD769x ADC family.
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