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Design >> Analog Design >> single-ended to fully differential conversion, AD8476.
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Message started by Jacki on Aug 10^th, 2013, 8:56am

Title: single-ended to fully differential conversion, AD8476.
Post by Jacki on Aug 10^th, 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 12^th, 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 12^th, 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 12^th, 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 13^th, 2013, 1:54am

aaron_do wrote on Aug 12^th, 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 13^th, 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 13^th, 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 14^th, 2013, 8:27am

Sheldon,

this wasn't actually my question, but perhaps you care to elaborate?

Quote:

If you read the product description or the app note most of your
questions will be answered

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:

be careful not to confuse common mode gain (rejection) for
differential to single ended conversion. These two characteristics
are different.

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 15^th, 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?