This post was last edited by cruelfox on 2019-12-8 12:17
In the previous article Analog Discovery 2 Review (5) Acquisition System Hardware Analysis , I introduced the analog/digital conversion part of Analog Discovery 2. This article will talk about its waveform generator part - how to generate analog signals for testing.
The core device of the waveform generator is ADI AD9717, a dual 125Msps 14-bit low-power DAC.
The output of this DAC is in the form of differential current, and the full scale can be set from 1mA to 4mA. The analog output of Analog Discovery 2 requires voltage, so there must be an I/V conversion circuit behind it, as well as a buffer to enhance the output drive capability. The analog output circuit behind the DAC in the manual is as follows:
The I/V conversion is mainly performed by AD8058, and the output center voltage is 0V. The AD8067 then performs 11 times inversion amplification on this voltage and superimposes a controllable DC voltage (VOFF_AWGx control). An auxiliary DAC: AD5645 is also used to generate the DC voltage (4 DACs, the remaining two are for the adjustable DC power supply).
The full-scale current setting of AD9717 is realized by external resistors. Two gears are set in the Analog Discovery 2 circuit, and the resistor (8k or 32k) is switched by analog switch.
The two voltage output ranges corresponding to the DAC are +/-5.45V and +/-1.36V (the switching is managed by the software itself, and I did not find an option on the interface), and then a settable DC voltage is superimposed. However, the voltage range of the analog output terminal is limited by the power supply voltage of the op amp and cannot exceed the range of +/-5.5V.
In addition, Analog Discovery 2 also has a 3.5mm stereo output socket for connecting headphones to "listen to the signal". The DAC output is driven by another op amp and then output with a gain of 2 times and AC coupled output.
The basic usage of a waveform generator is to generate a periodic waveform with a given frequency, amplitude and predefined shape, that is, a function generator. The most commonly used is the sine wave, and with this 14-bit DAC, the accuracy is acceptable. Using the instrument's own ADC loop test, the THD of the signal below 5MHz from the waveform generator is less than -60dB, which meets general test requirements. The following figures show the results of two waveform generators generating 1.44MHz and 5MHz 1V sine signals respectively, and directly connected to two analog inputs for FFT analysis.
Although the waveform generator supports a maximum output frequency of 25MHz, because the DAC sampling rate is fixed at 100MHz, if the signal frequency is too high, there will be few points in one cycle, and the waveform will not look beautiful. When the signal output frequency is too high (above 10MHz), the amplitude will drop significantly due to the limitation of the analog circuit, and the usability will be reduced.
However, I was surprised by the frequency resolution of the sine signal output by the waveform generator. I was able to accept the input of 9.99999999MHz in the software. I used another channel to output 10MHz and observed the waveforms of the two signals at the same time. I could see the relative movement of the peak positions. I couldn't imagine how Analog Discovery 2 could achieve such a fine frequency setting while maintaining a good THD. Due to the limitation of USB transmission rate, it is impossible to send 100Msps 14-bit data in real time from the PC calculation. The FPGA must be involved in the calculation.
In addition, the waveform generator also supports frequency ramp (linear sweep) and amplitude ramp; supports simple FM and AM modulation.
In the configuration options of Analog Discovery 2, we can see that the waveform generator has a buffer. This allows the user to compile waveform data and generate "arbitrary" waveforms - but the length is limited. In the Custom waveform dialog box, there are many ways to write waveform data, such as importing from a text file, generating with mathematical expressions, and directly entering the value of the sampling point.
In the above dialog box, you can set up to 32768 points of data. However, the device's buffer capacity is 4K. I don't understand why it can support data that is 8 times the buffer capacity? I have tested that it is indeed possible to output this custom waveform completely at a rate of 100Msps, and it can also be repeated periodically without interruption.
If you need to generate a long-term non-periodic custom waveform, 32768 points are still too few. The waveform generator also has a "Play" mode, which generates a long-term custom waveform by continuously sending data from the PC. However, this is at the expense of reducing the sampling rate, and the maximum sampling rate is limited to 1MHz. In the version of the software I use, the maximum number of sampling points that can be imported is 1M, but since it is Play, there should be a way to not limit the length (such as using the SDK to write a program yourself).
When the waveform generator is used as the stimulus signal provider, it can also trigger other functional components of Analog Discovery 2 to work together. For example, when observing the time domain waveform synchronously (not only does it save a trigger channel of the virtual oscilloscope), the time axis of the stimulus and response are aligned, which is helpful for circuit analysis. The figure below is an example of testing the step response of an LC series network, using a single pulse from the waveform generator. The trigger source of the scope is set to Wavegen 1.
NEXT >>> Analog Discovery 2 Review (7) Implementation of Impedance Analyzer
This content was originally created by EEWORLD forum user cruelfox . If you want to reprint or use it for commercial purposes, you must obtain the author's consent and indicate the source
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