Analog mux poor rise and fall time

I can't see anything wrong with it. Are the inputs all within the supply range of the 4067? Other than that, I would suspect you've got the pins wrong somehow. The real inputs are just floating?

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John Devereux

On a sunny day (Fri, 16 Jan 2009 09:55:17 -0800 (PST)) it happened " snipped-for-privacy@hotmail.com" wrote in :

Use a decent mux, like the 74HC4053. Switching times is measured in nanosconds (a lot of those), and Ron is about 150 Ohm. And there are 3 in one package.

There's something else wrong I think.

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John Devereux

as expecting

Dear John & Jan,

thanks for the quick responses. The inputs are all within the supply range. Vdd and inputs are 5V max. The pins are right as far as I can tell - mature board, signals vary at the output when varied at the input as expected. I am increasing the performance in a mature product which has run a lot slower in the past. From the datasheet I hadn't expected I had already reached its limit (: I'll try another board on Monday just in case there's something dodgy with this one e.g. ESD.

In what way is the HC4053 a "decent mux" compared to the 14067? I am aware the HC part is better comparing datasheets but the plain vanilla CMOS part ought to be good enough by a long way already - I'm only looking for rise and fall times of the order of 20us. I need 16 channels so I can't use an HC405x. I've looked in Farnell for an 74HC4067 but they only stock stupidly small outline (SSOP) and my board has small outline (SO). I'd really, really like to avoid a PCB relayout! When I next have a large order I'll buy some from Digikey (I'm in the UK) who wack a hefty handling charge for orders under =A350.

One other thing I tried without effect was to disable the mux using the INH input for ~5us either side of switching the logic input forcing break before make and to reduce any charge transfer,

regards, Colin

What are the power supply voltages? Those antediluvian COS/MOS MUX circuits don't work so well at low supply voltages. They work even less well if you accidentally leave the supply pins floating.

Daft question. Have you checked the supplies and switching waveform?

On a sunny day (Fri, 16 Jan 2009 13:58:13 -0500) it happened Spehro Pefhany wrote in :

Make you wonder if that chip is really executing a 'break before make' switching sequence, at that supply voltage. The couple of nF on the input would explain a lot, if not.

Supply voltages?

Scope probe capacitance?

John

Wow, a 4067--I last used that in 1981, to implement a 5-input logic block using data selector logic. From the symptoms, you might have forgotten to connect the power supply to the 4067.

Cheers,

Phil Hobbs

That would also be my guess, missing supply connection. Along the lines of "is it plugged in?" ... SCNR.

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Regards, Joerg

http://www.analogconsultants.com/

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Better still max 4619....

How is that better? He's feeding into an opamp from a 1k resistance so

150R or 10R on resistance isn't going to make a difference. You just end up paying more for no improvement.

If you're going to change the design make U2 a virtual earth with U1 feeding the -ve input. If you use a 4053 feed the signal into the common pin and ground the terminal not going to U2. This gives a T configuration and can take signals outside the supply range of U1.

Yep, in fact it's *worse* because you have got from a multi-sourced commodity part to Maxim! I always try to go the other way...

I can't see anything much wrong with the original circuit, as drawn, I think he must have just done something silly somewhere.

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John Devereux

I was

Dear All,

thanks for your help - issue solved. The general concensus has been it's something simple and obvious, there's nothing unusual about the part or my application. So this morning I've checked the power - fine. The logic select inputs

- ah, eh, they've got the same rise and fall as the analogue signals. It's the bleeping 'scope probe compensation, idiot, embarrassed sheepish red faced grin on my part.

Good point John raised about sourcing, engineering isn't just about electronics, it's a multidimensional discipline isn't it? My only worry about the 4067 rather than the HC part is long term availability, and then I expect to be able to just change over, I'm only using 5V. This analogue input structure has been a part of many of my products for 6 years,

regards, Colin