[EE] 24-bit A/D. Are We in the Twilite Zone Here?
Dan Michaels email (remove spam text)
Dave Van Horn wrote:
>>Sounds like you could use some front-end RF filtering too. Maybe
>>ferrite beads in the input signal lines, or inductors in the Vcc
>BTDT. None of that made a noticable difference without changing the op-amp.
>I'm going in again now, and re-evaluating all that in case it makes a
>difference, but it was too small to observe before.
I bet what fixed the problem is that the LMC6464 only has 50 Khz BW.
>>And I've looked at dozens of op amp specsheets in the past couple
>>of days. Turns out all of the JFET types have very bad PSRR- at 1
>>Mhz, only 0 to 20 dB. Even true for Natl LM6152 which is 75 Mhz GBW.
>Use the best you can afford, and then fix as much as possible on the layout.
>RC filters specifc to the op-amp will help a lot, they will knock 1 MHz
>Those Murata/Panasonic EMI filters are great down there, and cheap too $0.25.
Good suggestion, I've got just enough room to add these EMI
filters in the layout. So far, I've heard about 50 suggestions on
this thread, and about all of them are on the pcb now. I'm gonna
poop badly if the noise doesn't get any better after all this
wheel spinning :-).
- Dan Michaels
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