The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> single-ended to fully differential conversion, AD8476. https://designers-guide.org/forum/YaBB.pl?num=1376150195 Message started by Jacki on Aug 10th, 2013, 8:56am |
Title: single-ended to fully differential conversion, AD8476. Post by Jacki on Aug 10th, 2013, 8:56am Hi, I want to design a single-ended to differential converter with resistive feedback and fully differential Opamp (here I am not interested in the CG-CS noise cancel or CS-CS noise cancel architecture). I find AD8476 from ADI company could achieve the single-ended to differential conversion very well. Is anybody know what the main function of AD8476? Is it a very good fully differential Opamp, or it is not just an Opamp but with some other complex functions? Thank you. Jacki |
Title: Re: single-ended to fully differential conversion, AD8476. Post by Jacki on Aug 12th, 2013, 2:29am Hi, Maybe I can ask the question in another way, is it possible to converter the singled-ended input signal to differential output signals from the architecture shown below? |
Title: Re: single-ended to fully differential conversion, AD8476. Post by Jacki on Aug 12th, 2013, 2:59am I think it is possible to use this architecture to get the single-ended to differential conversion. But how to make it as idea as possible? Like decreasing the distortion? Jacki. |
Title: Re: single-ended to fully differential conversion, AD8476. Post by aaron_do on Aug 12th, 2013, 7:59am Hi, If you treat the single-ended input as a common-mode + differential signal, then you just want to reject the common-mode part. So i guess you want to make sure the amplifier has very good common-mode rejection. Also, you should check the common-mode output impedance. If it is high compared to those resistor values, then the effectiveness might depend on the loading to the op-amp. just my thoughts... Aaron |
Title: Re: single-ended to fully differential conversion, AD8476. Post by Jacki on Aug 13th, 2013, 1:54am aaron_do wrote on Aug 12th, 2013, 7:59am:
Thank you very much Aaron, Cancel the common mode voltage is one side, other side is I really want to get the differential output. Because when I see the document of AD8476, they really show a measured figure, both negative and positive outputs could output the symmetric signals. It looks very nice. But still thank you for the comments on common mode rejection. Jacki. |
Title: Re: single-ended to fully differential conversion, AD8476. Post by aaron_do on Aug 13th, 2013, 7:01pm Hi Jacki, I'm not sure if you understood my meaning. If you cancel the common-mode signal, all you will have left is a differential-mode signal. i.e. you will get perfect single-ended to differential conversion. That's in an ideal world however. I had a look at the datasheet, and to me its not obvious what kind of common-mode cancellation you will get. Maybe somebody else can shed some light. If I had to guess, I would say that there are 2 factors you need to take into account. 1. The common-mode gain. For the common-mode gain, I think it is a resistive divider between output impedance of the device, and (RFP + RFG). So at 1 MHz it would be 20/20020 = -60 dB. 2. The output balance error. At 1 MHz its -40 dB. I assume that it means that if you provide a perfect differential signal, how much is converted to common-mode at the output. In this case, -40 dB is much worse than -60 dB, so you can just look at the output balance error. Remember that it is frequency dependent. regards, Aaron |
Title: Re: single-ended to fully differential conversion, AD8476. Post by sheldon on Aug 13th, 2013, 9:15pm Aaron, If you read the product description or the app note most of your questions will be answered. http://www.analog.com/en/specialty-amplifiers/differential-amplifiers/ad8476/products/product.html In answer to your final questions, be careful not to confuse common mode gain (rejection) for differential to single ended conversion. These two characteristics are different. Sheldon |
Title: Re: single-ended to fully differential conversion, AD8476. Post by aaron_do on Aug 14th, 2013, 8:27am Sheldon, this wasn't actually my question, but perhaps you care to elaborate? Quote:
I read the product description (and skimmed through the data sheet) and it says that it is suitable for single-to-differential conversion, but I wasn't able to identify which parameter describes the single-ended to differential performance. What have you seen that I didn't? Quote:
Common-mode gain normally means common-mode input to differential output conversion gain, but what I was referring to was common-mode input to common-mode output gain. The simple calculation I included should have made that obvious. I realise I used the wrong terminology, but honestly, the way I was using it makes more sense, and the normal meaning should be called common-mode to differential gain or something like that. Whatever the case, the datasheet doesn't say anything about common-mode input to common-mode output gain. Is this the confusion you were talking about? Aaron |
Title: Re: single-ended to fully differential conversion, AD8476. Post by avlsi on Aug 15th, 2013, 8:02am single ended input - swing is Vpp single ended to differential output - swing is 2Vpp I see putting one end of the input to Vcm will give single ended to differential operation. This is what your circuit also shows. Did you simulate? |
The Designer's Guide Community Forum » Powered by YaBB 2.2.2! YaBB © 2000-2008. All Rights Reserved. |