J
John Larkin
Guest
On Thu, 8 Oct 2020 21:18:46 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
Nice. But they don\'t mention offset as a function of capacitance at
the input pins.
The general issue is that a cmos analog switch has, as it switches, an
interaction between its changing switch resistance and its changing
charge injection. Both of those vary as a function of common-mode
voltage. That\'s pretty messy.
<pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2020-10-08 16:31, John Larkin wrote:
On Thu, 8 Oct 2020 08:56:45 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:
On 2020-10-08 08:15, Piotr Wyderski wrote:
John Doe wrote:
How can an input inject current into a circuit?
Asking this question implies that you have selected the red pill, Neo.
All opamps do that, but in this case the current is exceptionally high
(by the standards I am used to). In the case of, say, LT6242 it is 1pA,
while the LT8262 has 2700000pA -- value high enough to visibly distort
the circuit driving it.
ADC inputs are also outputs--they kick out enough charge to seriously
discombobulate some op amps. That RC on the input is not an optional extra.
I have a fairly swoopy Krohn-Hite tunable filter box with plugins that
I\'d use a lot more if its kickout weren\'t so hideous. Over the years
I\'ve learned to connect the inputs of any new instrument to a scope to
spot problems like that.
Cheers
Phil Hobbs
Zero-offset chopamps often shoot nasty spikes out of their input pins.
The result can be a huge (compared to the specs) DC offset that is a
function of the capacitance that the pins see. That can ruin your
afternoon.
Yup. One reason that I\'m fond of the OPA2188 is that TI opened the
kimono unusually wide. See the TI presentation at
https://www.electrooptical.net/News/random-resources-from-my-sed-posts/
Nice. But they don\'t mention offset as a function of capacitance at
the input pins.
The general issue is that a cmos analog switch has, as it switches, an
interaction between its changing switch resistance and its changing
charge injection. Both of those vary as a function of common-mode
voltage. That\'s pretty messy.