Negative staircase wave with DAC0808 and PIC

[picchiedi]

Hello.

In the enclosed simple circuit, my aim was to manage positive and negative staircase and sawtooth waves for a transistor curve tracer.
Let's take for instance the staircase (but the same is for the sawtooth): for the positive wave (from 0 to +7 V step 1 V) no problem, since I connect to pin 14 Vref+ a voltage of +15V (as per datasheet indication).
In my "crazy" attempts to have also negative staircases with the same circuit and firmware(from 0 to -7V step -1V), to test other transistor types, I connected -15V to the Vref+ pin 14 (against all reasonable indications), and I found that the simulation works good, but not the real circuit.
The real DAC0808 circuit has only a current entering pin 4, according to the working principle of DAC0808.

Any suggestion/explanation would be appreciated.

Thank you

[Ettore]

This model is as older as Proteus version 5.0 and while it could be possibly fixed this is certainly not a priority.
A simple diode in series with pin 14 (cathode to VREF+) would fix it and possibly would provide you with the same behaviour as the real world part.
You may pick such the diode from Modelling Primitives->Analog (SPICE) and type in the property N=0.001 in the property field.

However you need to understand that a number of component in the library may simulate only partially the real world component. For example a number of opamps or many digital components won't simulate the supply current, just to say one. The reason of this depends by technical considerations in terms of model efficiency or complexity or whatever.

As a result it's always user's responsability to comply with datasheet ratings and specification of the component. In this specific case you would have prevented any false reasult if followed and complied with Absolute Maximum Ratings of DAC0808 specified in the datasheet.

[picchiedi]

Thanks a lot ,Ettore, for your kind answer and explanation, as usual!
Bye.

[Ettore]

This has now been fixed and will be in the next Proteus service pack or release.