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Altera_Forum
Honored Contributor I
1,269 Views

DC coupled ADC (HSMC)

I bought a "Terasic Cyclone V GX Starter Kit" and I have to process a analog signal. I bought a "Terasic Highspeed AD/DA Card" and connected it via HSMC. Unfortunatly I realized that the inputs are transformer coupled but I need the absolute voltage (DC coupling). For my application I need a ADC with >40 MSPS and >=10 Bit resolution. Are there any advises which board to buy? Or is there a way to make my AD/DA card DC coupled? BTW: My input signal is 0-2 V and I'm not sure if a 2 Vpp input (specification of the AD/DA card) is suitable.

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Altera_Forum
Honored Contributor I
88 Views

See this previous threads 

http://www.alteraforum.com/forum/showthread.php?t=39308 

http://www.alteraforum.com/forum/showthread.php?t=3304 

 

I posted a simple schematic bypassing the transformers, achieving a 0.5 to 2.5 V DC input range.
Altera_Forum
Honored Contributor I
88 Views

 

--- Quote Start ---  

See this previous threads 

http://www.alteraforum.com/forum/showthread.php?t=39308 

http://www.alteraforum.com/forum/showthread.php?t=3304 

 

I posted a simple schematic bypassing the transformers, achieving a 0.5 to 2.5 V DC input range. 

--- Quote End ---  

 

 

Thank you for the answer. And happy new year BTW. 

I browsed the manual of the AD and not really understood how the 0.5 to 2.5 V range is resulting for the board in combination with your adaptions. Could you please explain where this range is coming from? And is there a way to change it to 0-2V? 

Additionally I have an differental signal coming via the SMA connector. On an oscilloscope I can see the signal moves between 0 and 2V. Is there any way to use this differential signal?
Altera_Forum
Honored Contributor I
88 Views

Thank you for the detailed answer. Could you please explain why the input range is 0.5 - 2.5V? I browsed through the ADs datasheet but I'm not sure what VREF and REFB etc. is in case of the Terasic card. And can I also achieve 0-2 V somehow? Would your modification allow a differential signal to be processed? 

And just for understanding: Does single ended means, that I exchange the mass of the input signal with the mass of the board? And does this mean that a -1 - +1 V signal will become 0-2 V? 

 

And regarding your modified schematics: does the crossed out parts have to be removed? And what does the external R33 mean? Do I have to put an additional resistor in there, or I just need to connect C1 with R33? And what is the second green wire for?
Altera_Forum
Honored Contributor I
88 Views

 

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Could you please explain why the input range is 0.5 - 2.5V? 

--- Quote End ---  

 

More exactly 0.65 - 2.65 for the present circuit. Because the input is biased to 1.65 V with an ADC input range of +/- 1 V. According to the AD9248, single ended operation with 0 to 2 V input range can be achieved by biasing VINn to 1 V. The voltage divider has to modified respectively.
Altera_Forum
Honored Contributor I
88 Views

 

--- Quote Start ---  

More exactly 0.65 - 2.65 for the present circuit. Because the input is biased to 1.65 V with an ADC input range of +/- 1 V. According to the AD9248, single ended operation with 0 to 2 V input range can be achieved by biasing VINn to 1 V. The voltage divider has to modified respectively. 

--- Quote End ---  

 

 

 

Thanks for the explanation. The span is clear now, but where does the 1.65V come from? Which voltage divider do you mean? Unfortunately I also don't understand from the datasheet what is the native input range of the ADC. With 2V span I would admit that it's -1 to +1V biased to 0V. But why is an input biased at 1.65V required? Sorry for the probably dump question. 

 

Edit: Does it have something to do with VCCAD33 (which is probably 3.3V) which is divided by the voltage divider (on the right of the schematics) by a factor of 2? Does this mean that I have to connect VINn to (0V) ground to get an input range of -1 to 1V?
Altera_Forum
Honored Contributor I
88 Views

The 1.65 V bias is generated by dividing 3.3 V supply with 1k + 1k voltage divider, see ADDA card schematic. It's usual to bias differential input ADC at the center of analog supply voltage for best linearity. 

 

 

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Does this mean that I have to connect VINn to (0V) ground to get an input range of -1 to 1V? 

--- Quote End ---  

 

That's not possible, you need to consider ADC input voltage range. 0 V (or may be -0.2 or -0.3 V) is the most negative voltage that can be processed by an ADC input pin. 

 

If you read the datasheet thoroughly, you'll notice that single ended operation gives reduced performance anyway. For DC capable operation of a differential input ADC, you preferably use a fully differential amplifier, see e.g. AD9248 evaluation board schematic.
Altera_Forum
Honored Contributor I
88 Views

 

--- Quote Start ---  

The 1.65 V bias is generated by dividing 3.3 V supply with 1k + 1k voltage divider, see ADDA card schematic. It's usual to bias differential input ADC at the center of analog supply voltage for best linearity. 

 

 

That's not possible, you need to consider ADC input voltage range. 0 V (or may be -0.2 or -0.3 V) is the most negative voltage that can be processed by an ADC input pin. 

 

If you read the datasheet thoroughly, you'll notice that single ended operation gives reduced performance anyway. For DC capable operation of a differential input ADC, you preferably use a fully differential amplifier, see e.g. AD9248 evaluation board schematic. 

--- Quote End ---  

 

 

Thanks for the explanation. Do you know what is the original input range of the Terasic board? The manual only says 2Vpp span. Do you have an idea what simple circuit can be used to make the board suitable for 0-2V DC signaling? Even if the performance is not perfect? I would like to leave the ADC on the board like it is, but I could completely change the circuit to VINp and VINn. I searched a lot for available boards with DC coupling and a suitable interface, but without luck. I know decided to modify the board, but I'm not sure about all the voltage levels etc.
Altera_Forum
Honored Contributor I
88 Views

The circuit I suggested in the previous thread, changing the bias voltage form 1.65 to 1.0V, e.g. by adding a parallel resistor to R3. 

 

https://alteraforum.com/forum/attachment.php?attachmentid=11278&d=1444749511
Altera_Forum
Honored Contributor I
88 Views

 

--- Quote Start ---  

The circuit I suggested in the previous thread, changing the bias voltage form 1.65 to 1.0V, e.g. by adding a parallel resistor to R3. 

 

https://alteraforum.com/forum/attachment.php?attachmentid=11278&d=1444749511 

--- Quote End ---  

 

 

Thanks! This sounds very promising. Could you please giveme an advise, which resistor to choose? And why would you make it parallel? Could exchanging of R3 also be an option?
Altera_Forum
Honored Contributor I
88 Views

Sorry for maybe a dump question: If I want to reduce the voltage, don't I have to build in a resistor parallel to R2?

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