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 Author Topic: Help - Schmitt Trigger with common offset  (Read 235 times) 0 Members and 1 Guest are viewing this topic.
Dubai, UAE
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 « on: January 19, 2013, 08:38:39 am » Bigger Smaller Reset

Hi,
Can I use a single voltage divider to apply a DC Offset to four op-amp inverting schmitt triggers ?

The four op-amps are provided by a single LM324 so common supply/ground. At the moment, a single stage of my circuit looks like this -

(image source - wikipedia - http://en.wikipedia.org/wiki/Schmitt_trigger)

I am using a potentiometer or r-r as a voltage divider to set a virtual ground on the inverting input. If I try to share this across schmitt triggers - will it work or will R1-R2 feedback into the divider and upset the common reference ?

Thanks for any help

Duane B
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South Africa
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 « Reply #1 on: January 19, 2013, 09:51:14 am » Bigger Smaller Reset

I assume you are using the non-inverting circuit?  If so I guess the only thing you need to consider is the leakage current of the input pin.  In the wiki link you sent there is a circuit where a resistor is connected to the inverting input specifically to limit the impact of leakage current on the inverting input voltage.  Not sure this will help when connected to a voltage divider since there will be an additional voltage drop over this resistor.  Perhaps just make sure that the current through the voltage divider is much larger than the input leakage current of the op amps.
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Anaheim CA.
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 « Reply #2 on: January 19, 2013, 09:56:34 am » Bigger Smaller Reset

Certainly If the combined loading of the other sections doesn't load the divider appreciably certainly or degrade isolation between stages, If Not a voltage follower is in order to provide an isolation buffer for the divider.

Bob
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Dubai, UAE
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 « Reply #3 on: January 19, 2013, 10:07:49 am » Bigger Smaller Reset

Hi,
Its the inverting circuit I am using.

At the moment I have 10K + 10K as the divider, the feedback/input resistors are R2 22K, R1 10K.

I only have the one stage running at the moment, if feedback becomes a problem with 4 schmitt triggers connected to the common reference, am I correct in understanding the suggestion that I can reduce the divider resistance so that the feedback current is smaller in comparison to the current through the reference divider ?

Thanks both,

Duane B

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 « Reply #4 on: January 19, 2013, 08:45:48 pm » Bigger Smaller Reset

The inverting circuit loads it's virtual ground so no you cannot share without considering cross-talk.

Either buffer the virtual ground with another opamp in voltage follower mode or use the non-inverting
schmitt-trigger which presents a very high impedance to its virtual ground.

Or, if only worried about ac crosstalk you can add decoupling to the virtual ground to make it much stiffer.
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Dubai, UAE
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 « Reply #5 on: January 20, 2013, 01:12:49 am » Bigger Smaller Reset

Thanks,

I am using 4 out of 4 op/amps in the package - it would have been nice to have one spare for a buffered reference.

Based on the responses, I will drop the idea of a common divider for all four op-amps and instead use individual dividers.

Thanks all

Duane B
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