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need help with opamp DC servo
need help with opamp DC servo
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Old 22nd February 2019, 04:49 AM   #11
EUVL is offline EUVL  Europe
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Try OPA445.


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Old 23rd February 2019, 02:41 AM   #12
PRR is offline PRR  United States
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Quote:
Originally Posted by snowcatmaiden View Post
...[under +-23V supply]...
Most opamps are rated +/-18V MAX.

A servo certainly does not need super voltages.
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Old 23rd February 2019, 06:27 AM   #13
snowcatmaiden is offline snowcatmaiden  Hong Kong
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Quote:
Originally Posted by EUVL View Post
Try OPA445.


Patrick
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Old 23rd February 2019, 06:28 AM   #14
snowcatmaiden is offline snowcatmaiden  Hong Kong
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Quote:
Originally Posted by PRR View Post
Most opamps are rated +/-18V MAX.

A servo certainly does not need super voltages.
yes i know.
But I can save some space for regulating low voltage if so.
I run whole pcb with +-24V.
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Old 23rd February 2019, 09:13 PM   #15
MarcelvdG is offline MarcelvdG  Netherlands
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Maybe you are all barking up the wrong tree.

The annotated input and output voltages of the main amplifier in post #1 make no sense at all: positive input at 0 V DC, negative input at -9.5 V and output at -14 V. Maybe the main amplifier suffers from polarity inversion when the negative input goes too far negative? Either that or some oscillation messes up the bias point.
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Old 23rd February 2019, 09:23 PM   #16
MarcelvdG is offline MarcelvdG  Netherlands
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By the way, the voltages around R2-R4-R5 imply that the main amp draws 8.65 mA from the voltage divider, which seems very high. Considering the voltages on the other ends of R2, R4 and R5, the -9.5 V voltage should be -1.63636363... V if the input DC current of the main amp were negligible.
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Old 23rd February 2019, 09:48 PM   #17
Johno is offline Johno  Australia
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need help with opamp DC servo
Select your servo op amp with care and read the data sheets

- very low input bias current and offset voltage (could be fet input but not necessarily) so the capacitor accurately monitors the error voltage

- rail to rail input voltage range so that you know the correction voltage can remain in operating control range

- rail to rail output voltage swing for the same reason


This should help you understand why some op amps do not work as expected.
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Old 23rd February 2019, 10:20 PM   #18
MarcelvdG is offline MarcelvdG  Netherlands
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But then why doesn't the main amp's inverting input invert?
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Old 23rd February 2019, 10:40 PM   #19
Johno is offline Johno  Australia
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need help with opamp DC servo
Hard to tell from a distance. Possibly the op amp is damaged?
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Old 24th February 2019, 02:18 AM   #20
EUVL is offline EUVL  Europe
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Nothing wrong with the basic circuit as far as I can tell from Spice.
And it also works with ADA4522.

What am I missing ?


Patrick

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