After experiencing the Analog Discovery Simulation Kit, the rabbit has experienced the powerful function of this small black box, and 3 oscilloscopes, signal generators, and logic analyzers are integrated together. But just use it is not enough, why not learn from yourself to design a virtual instrument? In terms of a general DIY oscilloscope, the USB oscilloscope can not be displayed and processed without the structure of the MCU + FIFO and FPGA + RAM, and the acquired data can be displayed and processed by the PC software, but it also requires high requirements. Data bandwidth and real-time. What is the hardware structure of ANALOG DISCOVERY to achieve these features and performance? Then this rabbit will disassemble Analog Discovery and explore the internal design.
The mechanical structure of Analog Discovery is very simple, with only four screws in the back, and the two will be done. You can also see that DIGILENT is sponsored by ADI and Xilinx sponsor from ADI and Xilinx, and of course sponsorship can refer to providing chip and design technology.
Disconnect the black box, there is only one PCB inside, and the visually is at least 6 layers. The main device is at a glance: the master chip is the number I / O of the Xilinx Spartan6 Series FPGA XC6SLX16, 16 channels of the FGPa. In addition, the FPGA is also connected to the AD9648 to implement two differential inputs of the oscilloscope. USB communication uses FTDI's USB-to-Serial / FIFO chip FT232HQ. The entire system is powered by a USB single power supply, and the various voltages required on the board are mainly provided by the ADI DC / DC.
The rear density is high, and the main device is two DA chips, including the AD9717 and a secondary AD5645R for realizing the signal generator dual output, and the remaining is a large number of analog circuits and power supply circuits.
The XC6SLX16 of Taiwan's substitute is available with 8x8mm CPG196 package, with 14,579 logic units, 576kbits RAM, and 106 available I / O. The entire PCB does not find a set of RAM and Flash for configuring FPGAs, so the FPGA should not bear the main storage work (it does not need, the PC has such a large memory), and the FPGA should also be configured by USB online configuration. of. Such flexibility of the entire system greatly increases, in addition to basic oscilloscopes, logic analyzers, etc., Analog Discovery can also configure any custom devices you need (as long as software support).
FT232HQ supports USB 2.0 Hi-Speed (480Mbps) and FULL SPEED (12Mbps) protocols, and the FPGA interface can be configured to UART, SPI, I2C, JTAG, and 245 FIFO. When configured as synchronous 245FIFO mode, the maximum rate can reach 40Mb / s, which can meet the high upstream data bandwidth requirements of the USB virtual instrument.
The following figure shows the AD9648 of the most expensive IC-worth 51 in the entire device. Analog Discovery Nominal AD is 105MSPS, the actually used chip supports 125MSPS sampling rate, supports two differential inputs and 1.8V CMOS / LVDS output (CMOS interface should be used in PCB), 14-bit resolution The rate provides a higher longitudinal axis performance. Its input bandwidth can reach 650 MHz and supports multiple parameters such as clock and offset through the SPI interface. A radio system such as GSM, CDMA, radar is written in its application.
If you count the commercial version of $ 199, the chip accounts for 25% of the sales price. If you consider $ 99 special version of US students, this IC has reached more than 50% of sales prices, which is really awkward. However, Digilent will definitely get this price from Adi, otherwise it will be lost :).
The DA chip AD9719 is located on the back of the PCB. It is also 125MSPS, the same 14-bit resolution, supports differential coupling or single-ended output, and the price is relatively cheaper. The lower left corner of the picture should be a digital I / O ESD protection diode.
In fact, the whole system has the highest amount of gold or analog front end portion, and it is estimated that it is designed by the engineer of Adi. Below is one of the ways of the oscilloscope two-way full-time input, which looks very dizzy. The red visual is high-frequency compensation capacitance, which is followed by the attenuation circuit controlled by the ADG612 analog switch. It is an amplified circuit composed of op amps. The rabbit speculation may have an impedance transformation (pure guess, you need to measure the circuit verify). Finally, the AD Buffer chip ADA4940 is buffered and sent to the ADC.
Then the rabbit made a brief analysis for each part (half of the measuring half guess), plot the imaginary block diagram:
It should be mentioned that many chips used on Analog Discovery are 10-pin or 8-pin small package ICs, and there is no writing chip model, and only three simple silk screens made of three letters. The rabbit is also doing what you can find on the ADI website, and eventually list the BOM and cost of the main chip (the power IC and individual ICs are not there).
Part name
Description
Price ($)
XC6SLX16-CPG196
Spartan-6 FPGA, 14, 579LCS, 106I / OS
twenty one
AD9648BCPZ-125
14-Bit, 125MSPS, 1.8V Dual ADC
51
AD9717BCPZ
14-Bit, 125MSPS, Dual DAC
10
FT232HQ
Hi-Speed USB to Multipurpose UART / FIFO IC
2.2
ADA4051-2 X2
1.8V, μPower, Zero-Drift, RRIO DUAL AMPLIFIER
3
AD5645R
Quad, 14-Bit I2C Nanodac with on-Chip Reference
6
ADA4940-2 X2
UltraLow Power, Low Distortion Adc Driver
5.2
ADF4360-9
Clock Generator PLL with INTEGRATED VCO
3
ADG612 x2
Quad spst switches
3
AD8592
CMOS Single Supply RRIO DUAL OP AMP
0.4
AD5623R
Dual 12-Bit Nanodac with on-Chip Reference
2.1
AD8066 x2
High Performance, 145 MHz Fastfet OP amp
4.6
AD8058
Low Cost, Dual, High Performance Voltage Feedback, 325 MHz Amplifier
1.6
AD8067
HIGH GAIN BANDWIDTH Precision Fastfet OP amp
1.9
Total
115
After reading the BOM, the rabbit found that Digilent's equipment is already a cabbage, and DIY can save more. Of course, this is just a description of the imagination that is seen when the internal price is, and if you do it yourself, you will not take it, but will use the cost-effective solution according to the needs of the demand.
Finally, I have to mention the problems in Analog Discovery, that is, the high sample rate and ultra-low analog input bandwidth are serious and uncoordinated. In fact, the op amps adopted in the AD and the analog front end have a high bandwidth. If the impedance transformation is excluded, the maximum factor affecting the bandwidth is to use a normal pin as an analog input connector (not to Tu Du ").
Friends who have foreign countries have found that 9MHz below 9MHz can maintain a good integrity, which is much higher than nominal, but still causing many controversies. If the rabbit really wants DIY, it will inevitably select the high-bandwidth connectors such as BNC, and equipped with an oscilloscope probe, although some costs are added, but there will be no small improvements in performance.
Reference link
Analog Discovery Simulation Design Suite Reviews
Analog Discovery Simulation Design Suite Academic Price Application Please click to buy
Adi internal information, the simulation engineers must know those who must know
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