horrible development

I've run into that too. Some parts connect all their outputs together when you exceed the supplies on one input. :-(

Could you raise the impedance of the voltage dividers a whole lot? Or replace one of the 4 resistors with a small cap and adjust the frequency of the test signal?

Cheers,

Phil Hobbs

Might be a easy solution, current limit the ESD protection. You got an OPamp so Zin can be quite high. See my edit in your circuit. R5

Cheers

WHAT WERE THEY THINKING?

A couple of schottkies, clamping the low-ratio divider to +-5, seems to work. Not too ugly a kluge.

John

The BAT54 comes in series pairs, making it one part. If it has to be super small even in SOT-523:

http://59.120.39.77/mccsemi/up_pdf/BAT54T,AT,CT,ST(SOT-523).PDF

See my discrete diode kluge in abse.

Actually, a sot-23 dual series schottky might be nice. If I solder pin

John

Nice solder job. Do I see a tin whisker south of TP57? Just kidding...

The BAT54S comes in SOT-23:

About $0.03 in qties which is why I like them. Digikey has 72,000 of them in stock, which is kind of normal, which is why I like them even more. They'll still be around when I put my teeth in a jar. Imagine, a component that lasts a whole career.

On a sunny day (Sun, 25 Feb 2007 16:00:32 -0800) it happened John Larkin wrote in :

You are using it out of spec, inputs should always be within clipping. If it ever worked you were lucky! Cannot you add a MOSFET or other switch to clamp the too high input to ground?

That designers would read the spec-sheet? It's not like there are lots of chips that /don't/ dump over/under signal voltages to their common rail.

Or flip the 4051 backward so you pick a tap on a single divider... (after all the pins are called I/O ;-)

...Jim Thompson

Ooops! Strike that second suggestion :-(

...Jim Thompson

The spec sheet names a maximum esd diode clamp current. It doesn't mention that the chip becomes non-functional at about 1% of that current.

John

I think it is in spec, or at least that the spec is woefully misleading. It's screwing up at about 1% of the epecified esd clamp current.

John

Dang, you beat me to it.

But you can do a tapped divider and use the mux to ground taps on a series-string divider, sort of like what you suggest.

But why do these chips do this? How does the esd diode current find its way into the output? Incidentally, the effect is almost perfectly symmetric for high and low clmping.

John

I could be pompous and remind everyone how many times I've said DON'T DO IT ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

GND is the substrate (NMOS bodies _are_ the substrate).

VDD has all the PMOS bodies tied to it.

There is nothing there to keep currents from going all over hell and back when ESD is forward-biased.

...Jim Thompson

Maybe because the marketeers decided that such mention wouldn't look good.

But it's soooo tempting....

Sno-o-o-ort!

I just realized it's not the ESD that's causing your fault.

Hand-holding available upon request ;-)

...Jim Thompson

So maybe it's driving the source enough to turn on the series fet? At

It does make sense from the behavior I'm seeing. I wonder if an old CD4051 would have higher thresholds, and work?

There are some parts that don't do this, and in fact let unselected inputs go far beyond the rails. But none are 4051 pinout, I think.

We were in the midst of a pretty serious crisis when we designed this board (we had to completely redesign the fpga and firmware for a screwed-up digital delay generator) and we knew we hadn't spent enough time checking and reviewing. The fact that we can ship the first rev, with one wire and two diodes, ain't too shabby.

John

Yep. For 74HC... it's _likely_.

I don't have enough familiarity with the CD stuff to know, but I'm sure the threshold is higher. I don't know what the ESD structure is like though in these devices.

If you're only doing two channels, my 74HC4052 suggestion should work.

...Jim Thompson