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Loading the output of a VCVS filter

I've got a circuit where the signal goes through one of two low pass VCVS filters, and the output of one of them is selected by an analog switch (DG419).

One filter uses 1nF 2% PPS capacitors - there are various constraints like trying to minimise the variety of components on the board so this value was chosen, though it could be changed if there was a good reason. I note that the DG419 has a typical channel capacitance of 35pF, which is 3.5% of 1nF. My question is, does this matter, i.e. will it upset the filter: do relatively low capacitances hanging on its output affect the feedback loop? The op amp is a NatSemi LMP7702.

Thanks,

Nemo

Reply to
Nemo
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A bit of capacitance on the output of the opamp shouldn't matter. Most opamps can drive ~100pF, without too many worries. But check the spec sheet. (I don't know the LMP7702) If you're still worried how 'bout testing a prototype... hang more C on the output and see where it starts to 'wig out'.

George H.

George H.

Reply to
George Herold

What filter configuration? Sallen-Key is notoriously sensitive to component variations, ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

Good question. At the time I wrote the above, I had a VCVS design. Today I found some how-to's which implied a multiple feedback / Rauch design would be more tolerant of output conditions, so I decided to go for a few more component types - not many needed as it happens - and went for that instead. I'd still be interested in advice though as I've never done a MFB filter before. I'm aiming for a Bessel response.

Thanks,

Nemo

Reply to
Nemo

If this...

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is what you have in mind, both are versions of Sallen-Key.

Write out the equations and you'll see what I mean by component "sensitivity".

However, independent of filter configuration, ALL OpAmps will exhibit some phase shift due to capacitive loads.

What is the load after the DG419? If high impedance simply connect from OpAmp to DG419 with a resistor to isolate the capacitance.

Hard to nail down exactly without some specs on filter frequency and Q.

You could write out the equations ;-) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

      Remember: Once you go over the hill, you pick up speed
Reply to
Jim Thompson

Thanks. I hadn't appreciated that MFB's are Sallen-Key types too. The resistors will be 0.5% or better low tempco, and the caps 2%, so hopefully the only thing to worry about is the loading. The frequency is about 100kHz. The load beyond the DG419 is 1kohm. Sounds like I need to knock together a test circuit, then.

Reply to
Nemo

Use a buffer after the DG419 and before your load?

Looks like you pay dearly in capacitance to get low Ron :-( ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

      Remember: Once you go over the hill, you pick up speed
Reply to
Jim Thompson

By VCVS, do you mean opamp Sallen-Key? I don't understand why the analog switch changes the value of one of the caps. Got a schematic?

One thing to be careful about: charge injection from an analog switch can mess up an opamp for a while, microseconds to milliseconds. Even if the switch is on the opamp output. If you switch rarely, that may not be a problem.

The opamp alone won't mind a 35 pF load.

Expect a fair amount of distortion using this kind of opamp in a S-K filter, or in fact any non-inverting configuration.

John

Reply to
John Larkin

See diagram 16-19 here: focus.ti.com/lit/ml/sloa088/sloa088.pdf

this then feeds into the input of a DG419 analog switch.

What concerned me was that this would therefore place ~35pF in parallel with one end of the 1nF 2% capacitor from the op amp output back towards the input. On reflection, this was probably a silly concern as the signal at the point that the 1nF feeds back to is not going to be affected by something happening at the output of the op amp.

Not an issue here - users will switch manually, see what "looks best" for their application, and leave it there.

Hmm. I had been using a passive LC filter but decided it was silly to throw away half my signal that way, and the wound components were long lead time, so I went to an active filter. Signal levels at this point are about a volt but I've chosen film (PPS) capacitors. I thought that would eliminate significant distortion. Is there a better way of filtering I've not thought of? I'm after a low pass Bessel at about 100kHz.

Reply to
Nemo

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