Common topologies for AFEs are often designed for one application. On the other hand, new models with integrated programmable amplifiers are more flexible.
Wherever it is necessary to amplify high-sensitivity analog signals from analog sensors and convert them into digital signals, analog front-ends (AFE) are used. JRC’s AFE combines several functions in one device: a programmable instrumentation amplifier with two fully-differential inputs, a sigma-delta based A / D converter (ADC), a digital interface for communication with the MCU, and a configuration register.
With high differential and integral linearity, the AFE processes almost any analog signal in the μV or mV range with a good signal-to-noise ratio (SNR) and low distortion. It is, therefore, suitable for applications in the field of embedded and IoT, such as in pressure sensors or battery diagnostics for the smallest signals in the AC range, such as DC-free signals in the AC impedance measurement. For the automotive industry, you must have the best oscilloscope for automotive to analyze your automotive parts.
Requirements for analog front-ends
Operational amplifiers (OPs) used in analog front-ends should always have the following parameters:
- a low offset voltage: Offset and offset drift amplify non-linear voltage components in the output of the OP and thus distort the amplified signal
- low voltage noise: The noise voltage per hertz depends on the bandwidth of the signal (nV / √ (Hz)); it too is amplified with the signal
- High Common Mode Rejection Ratio (CMRR) to suppress electrical noise on both signal inputs of the OR so that they do not affect the measurement signal
- Rail-to-rail in the input and output (RRIO, rail-to-rail input and output), especially in circuits with small supply voltages, to obtain the maximum interference voltage gap and thus error-free input signals to the negative or positive supply transfer
There are three different topologies for operational amplifiers:
- The no inverting amplifier is the simplest form of surgery. Its simple construction goes hand in hand with low component requirements and low power consumption. The problem is, however, that non-inverting amplifiers also amplify common-mode signals as well as the noise at the input of the OP. In the industrial sector, interference and noise can be so significant that they superimpose the useful signal. Therefore, it is recommended to use a filter that suppresses the signal components that are present at both OP inputs.
- The differential amplifier only amplifies the input signal and, in contrast to the non-inverting amplifier, blocks the common-mode signals. Due to the differential signal processing, higher interference suppression can be achieved, but the common-mode suppression at higher interference frequencies is not satisfactory. Besides, the circuit requires exactly matched resistors with a tolerance of 0.1 percent and better. If this is not the case, the inputs operate with different gain, so that the common-mode rejection deteriorates significantly.
- The instrumentation amplifier with three OPs avoids the disadvantages of the other two topologies. High input impedance, as well as the two amplification stages, ensure improved common-mode rejection. But even this construction has disadvantages: The increased energy consumption makes the system unsuitable for low-power applications.
In addition to the choice of OPs and the corresponding topology, it is necessary to determine the distribution of the amplification level. It depends on several factors, including the desired Gain Bandwidth Product (GBP), the amplification of the output error of the OPs used, and the limitation on the input and output voltage ranges of the first and second stages. If the circuit operates with high gain or low supply voltage, it will be challenging to find a good compromise here.
Digitization with A / D converters
After the operational amplifier amplifies the sensor signal analogously, it converts an A / D converter into a digital signal. Developers must select their bit resolution according to the desired accuracy of the application and the reference voltage. For example, at 4.096 V, a 12-bit A / D converter achieves one mV accuracy.
Furthermore, developers must consider the Nyquist-Shannon sampling theorem. This states that the maximum signal frequency should be less than half the maximum sampling rate. It follows that the input signal must be bandwidth-limited by suitable filtering. Quantization of the analog signal frequency also causes quantization errors. To keep these as low as possible, developers should choose a high quantization, i.e., a high sampling rate. This is important to note in SAR A / D (Successive Approximation Register) converters, but this is not critical for sigma-delta A / D converters due to high oversampling.
Integrated solutions are often the better option
With all of these requirements and conditions, it’s often better to resort to a pre-built, integrated solution. Because integrated solutions not only achieve greater precision but also reduce the time and cost involved in development. Especially in the field of precision sensor signal conditioning, such a solution is usually inevitably necessary. Precision here means to achieve maximum gain linearity independent of gain factor, the common-mode component of the sensor signal and the temperature.
A solution with integrated PGA (Programmable Gain Amplifier) is, for example, the NJU9103 from JRC. The analog front-end can process analog signals with a gain of G = 512. Due to its large input voltage range and high sampling rate, it can amplify and process even very small μV and mV sensor signals as well as signals in the 100 mV range and up to signal frequencies in the kHz range. Together with its wide range of settings, it provides good gain for pressure and flow sensors and is also suitable for use with thermostats, digital displays, PLC and PLC applications. In addition to its large input voltage range, the compact housing (DFN8 / SSOP8) also contributes to flexibility.
JRC’s front-end 16-bit internal sigma-delta A / D converter (ADC) has sample rates ranging from 0.814 kbps to 6.51 kbps, single-ended, differential and a pseudo-differential input. One of the advantages of sigma-delta A / D converters is their oversampling architecture. Oversampling means that the sampling frequency of the switched integrator (Sigma) and the clock frequency of the modulator represent an extremely high oversampling. This has two effects: First, the noise is distributed over a full frequency band. On the other hand, it serves as an alternative to a more sophisticated and expensive anti-aliasing filter, as is typically necessary with SAR ADCs. Due to a much higher sampling frequency than essential according to the sampling theorem, a single-pole low pass is usually sufficient.
The PGA also keeps the ADC working within the ideal dynamic range. For example, if gain 128 and sensor offset are ten mV, the PGA would operate in the limit. To avoid this, an internal reference voltage generates a compensation voltage that is opposite to the offset voltage of the sensor. This returns the PGA output to within the dynamic range. This makes the NJU9103 the only available AFE with sensor offset compensation.
Further advantages result from the high RF immunity, which causes much less malfunction due to high-frequency noise such as from mobile phones. Added to this are the ease of configuration and the fast data rate of more than 1 kbps, which offers many possibilities for processing high-frequency measurement signals. Besides, the NJU9103 is the first AFE with a PGA that achieves a gain of more than 256 and a maximum of 512. Current solutions from other manufacturers only achieve a factor of 128.
Cost-effective oscilloscope alternative
The clou of the NJU9103: together with a microcontroller, it can replace an oscilloscope for low-frequency signals: Signal analysis and signal synthesis can be realized with the associated evaluation boards of the front-end and the microcontroller and only a few passive components. The sine wave signal can be visualized using a connected display – a smart concept at a low cost.