We are building an Educational Embedded Computing System for a Road Profiler. I am incharge of taking care of the ADC interface to the DE2-115 board. Can anyone please suggest a suitable 16-bit standalone ADC which has on-chip flash memory, 4-8 Channels, Minimum Sampling Rate 1Mhz, Simultaneous Sampling Possible with all channels active and acquiring data, and that can be interfaced to the DE2-115 board using any protocol say I2C or using the HSMC interface? There are cards available by Terasic, but they only have a 2 Channel 14-bit ADCs which can be interfaced to the HSMC connector. Also, we will be connecting a laser through the Ethernet Connector on the board and processing data. Is it possible to have Parallel Processing of the Data where all the data from the 4 Channels on the ADC and the data from the Laser can be simultaneously processed? We are completely new to the FPGA, including my professor. Hence we are all faced with tough challenges. Can somebody please help me out ASAP!!!

I have used the LTC1407A-1 from Linear Tech and interfaced it to Cyclone III and Cyclone IV many times on various boards I've designed over the years. They also make a LTC1407A, which is a unipolar part, while the LTC1407A-1 is a bipolar type part. You just need SCLK, CONV, and SDO for each ADC. Each ADC Simultaneously samples two channels at 1.5 MSPS. This is an excellent part, I have had good experience with it. I'd be happy to help you with the VHDL driver.:) James

Thanks a lot for the valuable information James. This looks to be a good part, but it still is 14-bits. I have valuable data coming in from an accelerometer. So this data needs to be sampled to at least a 16-bit resolution. So for a 16-bit, I tried to look this one up: I tried to look up LTC1865 . Considering the sampling rate that I require, which is definitely less than 250khz, I guess this can be better than LTC1470. I have looked up these too, TI has the ADS8556 , which is good Analog Devices has AD7606-6 But I havent worked with the Verilog/VHDL before much, just simple gate/logic/mux designing and am completely new to the NIOSII, so I do not know how I am going to interface this to the FPGA. I definitely know the SPI and I2C buses, but then, what about the driver for these ADCs and their initialization? I appreciate your consideration in helping me out for the VHDL driver, which I must say will be extremely valuable to me, since I am a complete beginner at everything. Wont the specific ADCs be required to be initialized using their specific firmware from the Manufacturer? Also, I will have to design a board myself for interfacing this as a peripheral through the GP I/O lines on the DE2-115 board, right? Because this just comes in as a chip and I will have to put it on a GP board and come up with the Pull Ups and the Noise Cancelling Caps etc? Excuse me for being to naive and asking stupid questions, but I am a complete beginner and have been assigned this task, which although doesn't have a deadline, needs to be completed at a satisfactory pace, since this is applied research for professor that I work for, and he hasn't worked on FPGAs ever before. Hope you can help me more. Thanks a lot again!! :)

--- Quote Start --- I have valuable data coming in from an accelerometer. So this data needs to be sampled to at least a 16-bit resolution. Considering the sampling rate that I require, which is definitely less than 250khz, I guess this can be better than LTC1470. --- Quote End --- Ok, so you have two important pieces of information here; you require 16-bits of dynamic range over a bandwidth of less than 125kHz (Nyquist bandwidth for sampling at 250kHz). If the signal-to-noise (SNR) of the signal you are sampling is dominated by quantization noise, then you can oversample the signal, then digitally filter and decimate the signal. For every oversampling factor of 4, you gain about 1-bit in SNR, so to achieve 2-bits of SNR you need to oversample by 16x, i.e., 16 x 250kHz = 4MHz. Don't the Terasic ADC boards operate at close to 100MHz? I think you should try to look for an existing ADC board that can meet your requirements before trying to attempt to build your own. For example, what does the signal you want actually look like? Go and use an oscilloscope or an ADC board and capture a few MB of data, then Fourier transform that data to look at its spectral response. How much dynamic range do you really have, is it 16-bits? Cheers, Dave

In general, to interface the ADC to Nios, you would need to write a driver for the ADC and then make the data available on the Nios bus. I tend not to use Nios much, as I write at the microarchitecture and finite state machine level. But you will need to study the datasheet of whatever ADC you pick, and then create an appropriate interface. This mean adhering to the setup and hold times admonished in the data sheet, whether it be from TI, ADI, or Linear Tech, etc. For a serial ADC interface, the VHDL (or System Verilog) will typically have counters, shift registers, and a state machine. What I recommend you to do is to pick an ADC, write the driver, and write the outputs of the ADC to a simple PWM DAC. Then use Signal Tap to scope the output of your ADC driver, using various analog inputs, to verify the driver is working correctly. This would be academic after performing the digital design with a proper testbench. Best, James

@Dave: Thanks a lot. Yes, Terasic boards operate at very fast speeds. I am not trying to construct my own board for the time being. My professor too told me to look into existing boards and buy one out. I will test the signal that you are talking to me about and check its spectral response and see if I actually need a range of 16 bits. If not necessary, then I can definitely go for ADCs which are more accurate or I can go by your idea of oversampling the signal for a better SNR. Thank you Sir again for your valuable information. @James: I havent used either much, this is after 2 years that I am even getting back to Verilog programming. I appreciate all your help and advice. I will however be selecting the correct ADC and will soon begin work with it and keep in mind your advice about the the testing and design. Also, I might ask really lame and stupid questions or ask for help from time to time. Hope you dont mind helping me out. Thanks a ton Sir! Yash

4-8 Channel ADC for Altera DE2-115 using Simultaneous Sampling

13 Replies

Altera_Forum
Honored Contributor
13 years ago
@ ames,

Can you let me know the circuitry involved for interfacing the LT-1407? I tried to find an evaluation board by LT, but couldnt find one. I guess you must've designed your own board and interfaced it to the Cyclone.

I have a very basic and stupid doubt. Hope you can tend to it. Like you've read in my post earlier
http://www.alteraforum.com/forum/showthread.php?t=36131
that I have data coming in from 4 analog channels, which I need to sample simultaneously, I forgot to mention the fact that I need the digital data out of it simultaneously too.

Can you tell me how this would be possible using a SPI bus? Forgive my really basic questions, but my knowledge is still unclear.

I am currently trying to understand the following datasheets of the ADCs:

LT-1408
AD7606
ADS8556

I need to decide between them which one is best for my application.
Altera_Forum
Honored Contributor
13 years ago
Yash,

Linear Tech does have an eval board for the LTC1407, the link for the board is shown below:

http://www.linear.com/demo/dc1082a-a

This would allow you to interface this board to something like the DE-NANO board from Terasic or similar.

Also the circuitry is simply SPI; I would recommend using a pull up resistor on the CONV signal; otherwise SCLK and SDATA can be directly connected.

Best, James
Altera_Forum
Honored Contributor
13 years ago
Stuart Sutherland's book are all very good on System Verilog, very readable. I suggest you also take a look at VHDL-2008. -James

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4-8 Channel ADC for Altera DE2-115 using Simultaneous Sampling

13 Replies

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