Jim Thompson wrote...
On 18 Sep 2006,
[email protected] wrote:
[snip]
So what you are trying to say is that, when the above circuit switches
the feedback, there might be some time of inestability in the circuit.
If that's what you meant there is no problem, there were relays before
that took forever to change the feedback, there's no problem with that
kind of inestability. How's this make before break circuit you mention ?
Did you miss my post...
http://analog-innovations.com/SED/VoltageGainSwitch.pdf
That doesn't Glen's address transient situations, does it? But
yes, I agree there isn't really a problem. The hc405x series
in fact switches so fast that ordinary opamps don't have time
to respond to open-switch situations, as they might with other
slower high-voltage, breakdown-protected CMOS switches. While
Glen's comments are on target for many if not most traditional
analog switch families, the hc405x switch family was first and
foremost a logic switch, with low-resistance low-capacitance
MOSFETs, and it responds rapidly, appropriate to that task.
These are also readily-available low-cost parts, and attractive
if they're buried deep inside a circuit, where static discharge
and low maximum-operating voltages may not be an issue, like
Leo's amplifier.
That said, there's still the issue of charge injection, where an
output pulse appears from the transient current injected into the
opamp's feedback loop from the MOSFET-switch gate-voltage swing.
Using low-value feedback divider resistors minimizes this affect.
Using low-capacitance switches also helps, but these are slow,
and therefore can exacerbate the open-switch problem.
