Defect in Analog Devices Spice model for the AD734

pdf

urrent output limits at something between +200uA and +280uA.

d the voltage at the W and Z1 outputs get high enough to be interesting.

at 15k.

to -350uA.

clamped a lot higher, at about +800uA, when the negative currents were get ting down to -2.4mA.

Spice model (which would interest Jim Thompson, who wants to sell Analog De vices better Spice models).

bout +/-10mA output current limits (as it does on page 13).

. Any advice will be gratefully received. I probably should have raised thi s with Analog Devices directly, but the price they charge for the AD734 mea ns that they can't be selling many of them, which doesn't suggest that I'd get a prompt response.

Oops. It looks as if the defect wasn't in the Analog Devices model, but in my circuit diagram - a connection to +15V seems to have been edited out at some point and the circuit was getting it's positive power supply from its inputs, making the model inconveniently realistic - I've had that happen on real circuits, and it can take a while to work out what's going wrong.

My apologies to one and all.

AD734 running with a current output - see Figure 25 on page 13 of the Rev E AD734 data sheet

current output limits at something between +200uA and +280uA.

did the voltage at the W and Z1 outputs get high enough to be interesting.

A at 15k.

n to -350uA.

ts clamped a lot higher, at about +800uA, when the negative currents were g etting down to -2.4mA.

about +/-10mA output current limits (as it does on page 13).

ng. Any advice will be gratefully received. I probably should have raised t his with Analog Devices directly, but the price they charge for the AD734 m eans that they can't be selling many of them, which doesn't suggest that I' d get a prompt response.

n my circuit diagram - a connection to +15V seems to have been edited out a t some point and the circuit was getting it's positive power supply from it s inputs, making the model inconveniently realistic - I've had that happen on real circuits, and it can take a while to work out what's going wrong.

LTSpice is an example of why engineers should NOT design CAD. I've never se en such absolute crap anywhere. The program should have flagged the schemat ic entry error.

When running a roughly 15kHz sine wave through the device, the positive current output limits at something between +200uA and +280uA.

Dunno. Seems as though it wouldn't be that easy in general to distinguish a missing flag from a decoupling network such as a cap multiplier, or an externally boosted amp (common-emitter BJT wraparounds, with their bases driven by resistors in the op amp supply lead). It would be crazy-making to have to design around the quirks of a buggy warning system.

What algorithm would you suggest?

Something like an assertion in C might be possible, but it's just as easy to treat a misbehaving sim like a misbehaving proto--the first thing you check is the power supplies, the second thing is the enable lines, the third thing is the CM limits, which can lead to startup problems, etc., etc.

Cheers

Phil Hobbs

Spice simulators can't detect faults in circuit schematics. All they can do is detect floating nodes. Apparently LTspice doesn't concern itself with "VP" of a symbol, since there's probably some path to ground within the model. Generally PSpice would call that a float, and balk. ...Jim Thompson

LTspice does detect floating nodes, but it doesn't know that a net that goes to the VCC pins of two op amps is actually floating. Danglers are easy to spot.

Cheers

Phil Hobbs

You missed my point... LTspice calls a path thru a symbol as non-floating... for symbols PSpice would call that floating.

However, if the part were a "block", PSpice would recognize the path.

It's a moot point, bizarre results call for careful examination of your schematic... GIGO ;-) ...Jim Thompson

Even something like the tuning pin of a 555, or an offset adjust on an op amp?

Cheers

Phil Hobbs

He said "-a connection to +15V seems to have been edited out at some point-" - looks like a NO CONNECTION to me, and on a power supply pin, seems straightforward.

In PSpice, at least in original flavor PSpice Schematics before the OrCAD flawed abortion, when you construct a symbol you can define float conditions for each pin.

In a "block" (actually a "hole" from whence to descend into another schematic) paths to ground are seen.

...Jim Thompson

Well, it depends. LTspice will complain about floating nodes, but that doesn't cover all the possibilities by a lot. I gave a few examples that would meet Bill's description but would make such an automatic check very hard to implement.

Cheers

Phil Hobbs

My first completely mindless thought on that one would be to message the user on a potential problem and let him/her either abort the sim to fix it or ignore and proceed. This is not rocket science.

I'm not so sure it's that easy. Annoying pop-up messages about non-errors would get old pretty fast. Something written to the log file might be okay, like a compiler warning.

Cheers

Phil Hobbs

If the disconnected power supply connection was simulated accurately, then Spice did good. Spice should simulate what you draw, not question it. It does check for a few physically appropriate things, like floating nodes, singularities, and such.

I sometimes connect opamp power pins to things that are not supply rails. I wouldn't want or expect Spice to prevent that.

I don't think there are so many power supply pins that it would become an a nnoyance. Then there are all those friggin floating ground errors the pos ( any SPICE) doesn't tell you about, except by displaying screwy results or l ocking up in some infinite loop.

he AD734 running with a current output - see Figure 25 on page 13 of the R ev E AD734 data sheet

D734.pdf

ive current output limits at something between +200uA and +280uA.

ail.

down to -350uA.

rents clamped a lot higher, at about +800uA, when the negative currents wer e getting down to -2.4mA.

D734 Spice model (which would interest Jim Thompson, who wants to sell Anal og Devices better Spice models).

alk about +/-10mA output current limits (as it does on page 13).

doing. Any advice will be gratefully received. I probably should have raise d this with Analog Devices directly, but the price they charge for the AD73

t in my circuit diagram - a connection to +15V seems to have been edited ou t at some point and the circuit was getting it's positive power supply from its inputs, making the model inconveniently realistic - I've had that happ en on real circuits, and it can take a while to work out what's going wrong .

seen such absolute crap anywhere. The program should have flagged the sche matic entry error.

That would be some other program but not SPICE, SPICE is simple.

In LT Spice, connect two voltage sources. Or leave a current source unloaded. Or make a double assignment in a .PARAM expression. You'll get warnings.

It doesn't seem to mind one end of a cap floating, but that's physically OK. It sensibly complains about a totally floating component or circuit.

Overall, it behaves pretty well.

l the AD734 running with a current output - see Figure 25 on page 13 of th e Rev E AD734 data sheet

s/AD734.pdf

sitive current output limits at something between +200uA and +280uA.

e rail.

h 15k did the voltage at the W and Z1 outputs get high enough to be interes ting.

nt down to -350uA.

currents clamped a lot higher, at about +800uA, when the negative currents were getting down to -2.4mA.

e AD734 Spice model (which would interest Jim Thompson, who wants to sell A nalog Devices better Spice models).

t talk about +/-10mA output current limits (as it does on page 13).

re doing. Any advice will be gratefully received. I probably should have ra ised this with Analog Devices directly, but the price they charge for the A D734 means that they can't be selling many of them, which doesn't suggest t hat I'd get a prompt response.

but in my circuit diagram - a connection to +15V seems to have been edited out at some point and the circuit was getting it's positive power supply f rom its inputs, making the model inconveniently realistic - I've had that h appen on real circuits, and it can take a while to work out what's going wr ong.

ver seen such absolute crap anywhere. The program should have flagged the s chematic entry error.

Maybe, but that has nothing to do with simulating an IC with power pin open .

It works just like real life!

That was my problem - the circuit contains two AD734s and the +15V pins on both were connected - to each other - but not to anywhere else.