|Home Analog Devices Feedback Subscribe Archives 简体中文 日本語|
Ultrahigh-Performance Differential-Output Programmable-Gain Instrumentation Amplifier for Data Acquisition
Data-acquisition systems and programmable-logic controllers (PLCs) require versatile high-performance analog front ends that interface with a variety of sensors to measure signals accurately and reliably. Depending on the particular type of sensor and the magnitude of the voltage or current being measured, the signal may need to be amplified or attenuated to match the full-scale input range of the analog-to-digital converter (ADC) used for further digital processing and feedback control.
Typical voltage measurement spans in data-acquisition systems range from ±0.1 V to ±10 V. By choosing the correct voltage range, the user implicitly changes the system gain to maximize the amplitude of the sampled voltage at the input of the analog-to-digital converter (ADC), which, in turn, maximizes the signal-to-noise ratio (SNR) and measurement accuracy. In typical data-acquisition systems, signals that require attenuation and signals that require amplification are processed by different signal paths. This usually results in a more complex system design, requires extra components, and uses more board space. Solutions that offer attenuation and amplification in the same signal path generally use programmable-gain amplifiers and variable-gain amplifiers, but these amplifiers do not usually offer the high dc precision and temperature stability required by many industrial and instrumentation applications.
One way to build a powerful analog front end that provides both attenuation and amplification in a single signal path—and differential outputs to drive high-performance analog-to-digital converters—is to cascade a programmable-gain instrumentation amplifier (PGIA), such as the AD8250 (gain of 1, 2, 5, or 10), AD8251 (gain of 1, 2, 4, or 8), or AD8253 (gain of 1, 10, 100, or 1000), with a fully differential funnel (attenuating) amplifier, such as the AD8475, in a circuit similar to that shown in Figure 1. This solution offers simplicity, flexibility, and high speed—along with excellent precision and temperature stability.
The aforementioned programmable-gain instrumentation amplifiers,
featuring 5.3-GΩ differential input impedance and –110-dB total-harmonic distortion (THD), are
ideal for interfacing with a wide variety of sensors. At a gain of 10,
guaranteed specifications for the AD8250 include: 3-MHz bandwidth,
18-nV/√Hz voltage noise, 685-ns settling time to 0.001%, 1.7-μV/°C offset
The AD8475 high-speed, fully differential funnel amplifier with integrated precision resistors provides precision attenuation of 0.4 or 0.8, common-mode level-shifting, single-ended-to-differential conversion, and input overvoltage protection. This easy-to-use, fully integrated precision gain block is designed to process signal levels up to ±10 V using a single +5 V supply. As a result, it can match industrial-level signals with the differential input voltage range of low-voltage high-performance 16-bit and 18-bit successive-approximation (SAR) ADCs with sampling rates of up to 4 MSPS.
The AD825x and AD8475, working together as shown in Figure 1, provide a flexible high-performance analog front end. Table 1 shows the gain combinations that can be achieved, depending on the input and output voltage range requirements.
Figure 1. Data-acquisition analog front end that uses the AD825x PGIA and AD8475 differential-output funnel amplifier.
Table 1. Input Voltage Ranges and Gains Possible with the AAD8475 in Combination with the AD8250, AD8251, and AD8253
Capabilities: Input Voltage Range and Bandwidth
To determine the combination of gain settings required in any system, consider the full-scale input voltage of the ADC (VFS) and the minimum/maximum current or voltage levels expected from the sensors.
The speed and bandwidth of this analog front end is exceptional given its level of precision and functionality. The speed and bandwidth capabilities of this circuit are determined by the following combination of factors:
Many data-acquisition and process-control systems measure pressure, temperature, and other low-frequency input signals, so the dc precision and temperature stability of the front-end amplifiers are critical to the system performance. Many of these applications include multiple sensors that are multiplexed to the amplifier inputs in a polling fashion. Typically, the polling frequency is much greater than the bandwidth of the signal of interest. When the multiplexer switches from one sensor to the next, the voltage change seen by the amplifier inputs is unknown, so the design must accommodate the worst-case scenario: a full-scale voltage step. The amplifier must be able to settle from this full-scale step within the time allotted to switching. This settling time also needs to be lower than the settling time required by the ADC to sample and acquire the signal.
An antialiasing filter (AAF) is recommended between the AD8475 and the ADC’s inputs. The AAF band-limits the signal and the noise presented to the ADC inputs to prevent undesirable aliasing effects and to improve the system SNR. Additionally, the AAF absorbs some of the ADC input transient currents, so the filter also provides some isolation between the amplifier and the ADC’s switched-capacitor inputs. Typically, the AAF is implemented using a simple RC network as shown in Figure 1. The following equations describe the filter bandwidth:
In many cases, the filter’s R and C values are optimized empirically to provide the necessary bandwidth, settling time, and drive capability for the ADC. Refer to the ADC data sheet for specific recommendations.
See Circuit Note CN-0180, Precision, Low Power, Single-Supply, Fully Integrated Differential ADC Driver for Industrial-Level Signals for additional information on the AD8475 as a driver for a precision successive-approximation ADC.
Copyright 1995- Analog Devices, Inc. All rights reserved.