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 Forum index » DIY Hardware and Software
Can a S/H have Gain?
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richardc64



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PostPosted: Tue Oct 10, 2006 1:39 pm    Post subject: Can a S/H have Gain? Reply with quote  Mark this post and the followings unread

Would this work? I have to attenuate the voltage into the analog switch to not exceed its supply rails, then boost the voltage back to its original level. I'd rather not use 2 opamps. Thanks in advance.


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mosc
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PostPosted: Tue Oct 10, 2006 3:52 pm    Post subject: Reply with quote  Mark this post and the followings unread

Looks like it would work but it depends on the application. Give it a try.
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yusynth



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PostPosted: Wed Oct 11, 2006 2:53 am    Post subject: Reply with quote  Mark this post and the followings unread

It should work.
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richardc64



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PostPosted: Thu Oct 12, 2006 2:16 am    Post subject: Reply with quote  Mark this post and the followings unread

Wow. 2 opinions not to be taken lightly. Thanks guys.

The application is to catch the right voltage at the right time from a DAC that's cycling through a keyboard scan counter. Adding in a 1V octave shift, or a pitch-bend, or tuning voltage -- which must be done at the DAC -- the final CV could exceed the analog switch supply voltages, which must be +/-5V. So I'm thinking scale the DAC output to 0.5V/Octave and bring it back to 1V/Octv after the switch.

I hope to post this idea circuit soon.
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yusynth



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PostPosted: Thu Oct 12, 2006 2:29 am    Post subject: Reply with quote  Mark this post and the followings unread

Well considering your application I would suggest that you separate the gain stage from the buffer stage, it costs another OPA to do so.

Cheers

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mosc
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PostPosted: Thu Oct 12, 2006 12:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

yusson wrote:
Well considering your application I would suggest that you separate the gain stage from the buffer stage, it costs another OPA to do so.

I agree. This is one case where viewing an op amp as an ideal element can lead to subtile problems. In most cases it is safe to assume the op amp to have infinite input impedance and zero input offset voltage, but in any real device that isn't the case. The input resistance and/or offset voltage might change as you adjust the gain. Probably not a problem, but it might bite you.

BTW, the original Moog Modular keyboard had very poor S/H circuits. There were several variations over time, but on all of them that I tested there was a voltage drop or boost when the key was released. I mention this because a keyboard S/H is one case where it pays to get it right - don't economize. There are times where even the tinyist error shows up.

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richardc64



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PostPosted: Fri Oct 13, 2006 1:55 am    Post subject: Reply with quote  Mark this post and the followings unread

Dang it! The point was to keep the parts count down. There's going to be 2 of these S/Hs in the project. Back to the drawingboard.

There's already going to be another opamp(s) after the S/H(s) for portamento, so change the analog switch to a potentiometer and change the question to: Can a Lag have gain?
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mosc
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PostPosted: Fri Oct 13, 2006 9:25 am    Post subject: Reply with quote  Mark this post and the followings unread

You can have gain in virtually any op amp circuit. Most of the time you can view the op amp as an ideal element, but when you are using it to measure a voltage on a capacitor, then the non-ideal effects might show their ugly head. I think it would be much safer to add gain to a slew processor. This is because the capacitor is probably being charged by an active circuit. On the S/H, the charge on the capacitor is "floating".
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Blue Hell
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PostPosted: Fri Oct 13, 2006 1:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

mosc wrote:
On the S/H, the charge on the capacitor is "floating".


Yes, but I don't see how adding amplification would alter the circuit.

The idea is that the feedback circuit regulates the outpus in such a way that the negative input gets the same voltage as the positive input has. It does not matter here what the amplification is as long as the output does not clip. All input voltages and currents will be the same as in the non-amplyfying case.

So IMO, as long as the output does not clip, there can not possibly be any difference depending on the amplification of the buffer.

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mosc
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PostPosted: Fri Oct 13, 2006 1:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
mosc wrote:
On the S/H, the charge on the capacitor is "floating".


Yes, but I don't see how adding amplification would alter the circuit.


When you change the gain of the amplifier you change the input offset voltage of the op amp. It may not matter, but it could. The characteristics of the leakage of the charge on the capacitor will change as you change the gain. I would at least put a jfet or mosfet voltage follower in there.

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Blue Hell
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PostPosted: Fri Oct 13, 2006 1:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

mosc wrote:
When you change the gain of the amplifier you change the input offset voltage of the op amp.


As I reasoned all currents and voltages in/on the opamps inputs must essentially be the same (as long as the output doesn't clip), so I fail to see how the offset voltage could change. Could you explain this ?

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PostPosted: Fri Oct 13, 2006 2:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

See this web page about voff in these kinds of op amp circuits.

http://www.ecircuitcenter.com/Circuits/op_voff/op_voff.htm

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Blue Hell
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PostPosted: Fri Oct 13, 2006 2:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

That's not quite the point here (1), supposedly bad things were going to happen with the capacitor charge, I still don't see that.


(1) Indeed the offset voltage will be present on the output as an amplified signal - so stronger for less feedback (more gain). But that will happen in any opamp circuit and so adding a second independent gain stage will not improve things. No matter if its put before the holding capacitor or after the the buffer amp. And indeed in general it is not a good idea to first attenuate a signal and then amplify it again, but this was a design constraint here, certain circuit parts have to operate on a limited suppy voltage and so the attenuation has to be done.

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PostPosted: Sat Oct 14, 2006 9:17 am    Post subject: Reply with quote  Mark this post and the followings unread

The effect can be subtile, but in keyboards even the tinyest error can cause unwanted and audible errors. An error of a millivolt could be a disaster.

I'm sorry I can't explain this adequately. When you change the operating parameters (such as gain, power supply, temperature) of a non-ideal op amp the ioc and voc change.

Do a search on sample and hold circuits and you'll see it is common practice to buffer the inputs and outputs with stable amplifiers.

http://www.du.edu/~etuttle/electron/elect53.htm
http://www.intersil.com/data/an/an517.pdf

Also, putting a pot in the feedback path of an op amp is to be avoided because pots are noisey and when put in the feedback path, the noise can be amplified, but that's another topic.

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