THS4151: dc coupling to ADS54j40

Here is the schematics.

Hello Giora,

I wouldn't recommend using the THS4151 as an attenuator, as our FDAs are typically speced to be stable at unity gain. You can try using the method described in this article: 

Best,

Hasan Babiker

Morning Giora, 

This is an area I have supported for many years, 

1. The FDA works fine as an attenuator, with some compensation considerations. 

2. A single 5V supply solution can do what you are trying to do easily (the CM loop will level shift a bipolar input up into range on the input pins) 

3. Probably the THS4541 is the right part, it has a section I put in there on designing attenuators - looks a little complicated but is not - it essentially shapes the noise gain up at higher F, with that peaked noise cut off by your post filter normally, 

4. The article Hasan attached is probably the least attractive approach - those 3 articles should be purged. Karki essentially attenuated first to get a unity gain FDA stage to work - that of course is a DC to high F noise degradation - remarkably crude and unnecessary. 

There is quite a range of articles I have published touching on the various topics here, but lets see where you go from here first. 

Dear Michel,

Thanks.

You quote" Probably the THS4541 is the right part, it has a section I put in there on designing attenuators - looks a little complicated but is not - it essentially shapes the noise gain up at higher F, with that peaked noise cut off by your post filter normally, "

to what section and where you implied?

Rקעשרגדת

Gןםרש

Hu Michael,

" Probably the THS4541 is the right part, it has a section I put in there on designing attenuators - looks a little complicated but is not - it essentially shapes the noise gain up at higher F, with that peaked noise cut off by your post filter normally, "

To what section and where do you imply?

Regards, Giora

just open up the datasheet and do a control F for "designing attenuators"

Michael,

Did you mean the THS4151 data sheet?

I did the search and find none.

Giora

The THS4151 is quite old, I meant the THS4541 datasheet, one of the most recent FDA's particularly suitable for pulse oriented DC precision ADC driving. 

Michael,

Thanks.

I can not use the THS4541 in my circuit. 

Does it will work ok with +/-3 pulse input.

See schematics.

You are suffering from a typical FDA misconception, that 2.1V output CM will level shift the input summing pins up above ground - think of the lower FDA output swinging your desired swing - divide that back to ground by that feedback divider - that is a CM swing that is always above ground, there is some discussion of these issues in this article - the original lost the expand plots feature, so here is the pdf 

One of the magic things about all FDA's is they can take an input bipolar swing and level shift everything around an output CM voltage and operate single supply. 

2nd 1/2 of this article, 

2816.The Signal Sped Up - I_O range for high speed CFA and FDA amplifiers Oct27 2018.pdf

Michael,

The 2.1V on the OCM pin is for interfacing the ADS54J40.

DC simulation of my circuit shows this: