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Author Topic: Help about Arbitrary Waveform Generator  (Read 3062 times)
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telfo2007
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« on: September 16, 2013, 10:36:53 22:36 »

Hi friends,
I'm developping an Arbitrary Signal Generator in verilog, using a cheap altera FPGA board ($15) similar to this:
http://www.ebay.es/itm/ALTERA-FPGA-Cyslonell-EP2C5T144-Minimum-System-Learning-Board-Development-Board-/200942146349?pt=LH_DefaultDomain_0&hash=item2ec915d32d

and a homemade 12bit DAC (based on AD9872 AD9742).

I'm reaching until 200MSa/s  with useable 10MHz sine, ramp, triangle, square, etc... from a standard 50MHz clock with until 8kB sample RAM. But I'm lost with output buffer circuit with amplifier and offset (actually only getting 0.8Vpp on 50ohm at output). Someone could help? I'm trying to use AD8011 without too much luck...

Thanks on advance friends.
« Last Edit: September 17, 2013, 05:11:34 17:11 by telfo2007 » Logged
Checksum8
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« Reply #1 on: September 16, 2013, 11:54:40 23:54 »

Here is a couple of examples with of DDS's with output buffers. Just google "DDS signal generator"
I could not find data on the "AD9872"?  It must be AD9782?

http://midnightdesignsolutions.com/dds60/

http://www.radiolocman.com/shem/schematics.html?di=128164
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optikon
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« Reply #2 on: September 17, 2013, 12:32:01 00:32 »

Hi friends,
I'm developping an Arbitrary Signal Generator in verilog, using a cheap altera FPGA board ($15) similar to this:
http://www.ebay.es/itm/ALTERA-FPGA-Cyslonell-EP2C5T144-Minimum-System-Learning-Board-Development-Board-/200942146349?pt=LH_DefaultDomain_0&hash=item2ec915d32d

and a homemade 12bit DAC (based on AD9872).

I'm reaching until 200MSa/s  with useable 10MHz sine, ramp, triangle, square, etc... from a standard 50MHz clock with until 8kB sample RAM. But I'm lost with output buffer circuit with amplifier and offset (actually only getting 0.8Vpp on 50ohm at output). Someone could help? I'm trying to use AD8011 without too much luck...

Thanks on advance friends.

are you trying to just copy another design? are we supposed to guess about your problem. why dont you post a schematic of what you built if you want help?
« Last Edit: September 17, 2013, 04:49:26 04:49 by optikon » Logged

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telfo2007
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« Reply #3 on: September 17, 2013, 02:31:27 14:31 »

Sorry, I commited a mistake in DAC number. It's a AD9742. I'm writing down my own verilog code, but I'm not so expert on analog circuits.
Thanks all for your help, on advance
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koseyel
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« Reply #4 on: September 17, 2013, 06:45:00 18:45 »

Sorry, I commited a mistake in DAC number. It's a AD9742. I'm writing down my own verilog code, but I'm not so expert on analog circuits.
Thanks all for your help, on advance

As Optikon said, if you need help, you'll have to show what you've done. It really isn't sufficient showing an ebay link of a device similar to what your are developing.
Please post your schematic, if you need help.
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telfo2007
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« Reply #5 on: September 17, 2013, 10:52:54 22:52 »

This is my simplified output circuit. I'm looking for a amplifier circuit based on i.e. AD8011 power with 3.3V or 5V, with gain and offset control. This is the point where I'm locked. Thougth DAC is 12bits, don't worry if amplifier circuit is a little noisy and at output I'll get 8 bits.

Thanks again, friends.
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Vineyards
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« Reply #6 on: September 17, 2013, 11:01:19 23:01 »

Is that an RMS value at the output? Measure the value at the output with an RMS multimeter. If you need voltage gain use an opamp based gain circuit (inverted or non-inverted) if the output amplitude is OK at high Z just buffer it with a voltage follower. You will need to check the datasheets for parameters such as slew rate. Since your output frequency can reach 10MHz, you will need an opamp with a slew rate that will make it possible for you to obtain the desired voltage swing at that high frequency. Most opamps can output 20mA. If you need anything higher than that you should either choose an opamp with a higher current output or you should use a transistor stage after the opamp.

Posted on: September 17, 2013, 10:57:15 22:57 - Automerged

For offset / gain control you could use an opamp with offset control or build your own simple circuit based on the examples in datasheets. There are programmable gain opamps which you can control with an MCU. Or you can fiddle with digital pots to adjust gain/offset.
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koseyel
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« Reply #7 on: September 18, 2013, 10:25:35 10:25 »

Is another opamp other than AD8011 an option? If yes, what is the minimum slew rate needed?
« Last Edit: September 18, 2013, 10:34:05 10:34 by koseyel » Logged
Vineyards
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« Reply #8 on: September 18, 2013, 02:45:13 14:45 »

There is a handy tool here: http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/a741p3.html
You should make it a habit to check out datasheets for each component you intend to use. You could also browse component producers' websites and list their components based on the parameter you need the most.
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bigtoy
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« Reply #9 on: October 06, 2013, 04:20:48 16:20 »

You might find this project helpful:
http://www.frankvh.com/dds-signal-generator.html
This guy uses an AD8051 op-amp for his output stage. Full schematic is there.
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