In the case of instability due to capacitive loading on the output a capacitor from the output to the inverting input will not work because the capacitive load makes the op amp not unity gain stable.
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Yes, that's why I always place a resistor between the opamp output and the capacitive load. Also for protection because an opamp that constantly muscles temporary overloads around might not enjoy a very long life. Unless the datasheet explicitly allows that I don't do it.
An upper end "peaker" circuit adds noise up there which isn't always desired.
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The proper way is shown on that page at...
"In-loop compensation: If RL is either unknown or dynamic, the effective output resistance of the gain stage must be kept low. In this circumstance, it may be useful to connect RX inside the overall feedback loop, as shown below. With this configuration, dc and low-frequency feedback comes from the load itself, allowing the signal gain from input to load to remain unaffected by the voltage divider, RX and RL."
...Jim Thompson
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| James E.Thompson, P.E. | mens |
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I love to cook with wine Sometimes I even put it in the food
ADG1604 has lots of capacitance! It's ratings are based on a 50 ohm system. Probably won't work for a gain control element in an opamp due to high capacitances.
I've been using Fairchild's FSA66 / FSA266 series of switches. Around
10pF, 7 ohms. The power supply rails may be an issue for you since this is a 5V part. These switches have very low charge injection, about 10 times lower than any other IC analog switch. From email correspondence with Fairchild, the FSA266 has the same charge injection specs as the FSA66 (FSA266 data sheet is missing charge injection specs).-- Mark
[...]
Yup. I'll second that opinion.
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Neat! Thanks, Mark. I just wonder why they are all non-stock at Digikey. Hopefully it doesn't go the way of the SD5400.
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More complete charge injection specs versus power supply voltage for the FSA266 (and FSA66).
Mouser has stock.
-- Mark
Doesn't take much switch capacitance to make the low code ranges inaccurate at higher frequencies. I tend to do R-2R type things with FET switches to ground.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal ElectroOptical Innovations 55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net http://electrooptical.net
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