[sdiy] LF398 and droop

[Tom Wiltshire]

On 17 May 2007, at 08:07, Oakley Sound wrote:

> Don't forget about layout too. A guard ring around the cap does  
> make a small difference. The datasheet on the 398 talks about this.
>
> Another thing I found was that one should not build precision s/h  
> on Veroboard/stripboard. The paper boards are likely to absorb  
> moisture.
>
> I should also say that there are better s/h circuits around too if  
> you are prepared for a bit more complexity.
>
> Lastly, Philips used to make an improved 398, called the NE5537,  
> but I don't know if they still do.
>
> Tony


I'd just like to second this. There are many layouts for the basic  
voltage follower circuit, and impedances vary from 10's of KOhms to  
10's of MOhms. Obviously you want the top end of this range, where  
guard rings are common practice. I've got a book of Op-amp designs  
(Op Amp Circuits, R.M.Marston - great book) which includes one  
example with a 741 which claims 500MOhm impedance with a guard ring,  
or 50MOhm without. The guard ring in this case is round the amp's +ve  
input.