ELVISmx interface to Multisim

[Justin.e.snodgrass]

I'm having trouble getting multisim to report an accurate voltage while using the ELVIS multimeter virtual instrument in this situation:  

In the first instance measuring across the 10MΩ resistor R1 gives 9.31V In the second instance measuring across R2 gives 7.5V. Neither of the answers are correct considering the 11MΩ input impedance of the meter. However... in simulating I can understand if the meter is assuming an infinite impedance. If that is the case shouldn't it be reading 7.5V in both cases measuring across R1 and across R2?

[Fernando_D · Accepted solution]

Hello,

The NI ELVIS DMM (hardware) has an input impedance of 11MΩ, however, the ELVISmx DMM behaves differently in simulation mode.

The ELVISmx DMM has 3 terminals: V, COM and A; when you take a voltage measurement, a large resistance is used for the simulation and, for this instrument, this value depends on the RSHUNT resistance, which is a SPICE parameter. The defaul value is 1e+012Ω.

On the other hand, for current measurements this instrument introduces a small resistance (1nΩ) in the SPICE netlist. In your circuit, if you open the SPICE Netlist Viewer (View menu) you will see this:

.subckt XXLV1_88615056 1 2 3
Rprobe 3 2 1e-9
.ends

This means that even though you are not taking a current measurement, a 1nΩ resistor is present in the SPICE netlist. In your circuit the size of resistors spans 16 orders of magnitude (10MΩ resistor, 1nΩ resistor), and this is a range that can't be internally represented by the Multisim solver, that's why the simulation result is incorrect.

The Multisim Multimeter uses a different approach and it will give you the correct measurements. Let's use this Multimeter to clarify the behavior of the

ELVISmx DMM.

This is the equivalent circuit when you place the ELVISmx DMM:

R3 is the 1nΩ resistance introduced by the A terminal of the ELVISmx DMM. As you can see in the previous image, the simulation result is incorrect. If you remove R3 everything is back to normal:

To sum up, this particular scenario is a limitation of the ELVISmx DMM. I tried different values of resistors and was able to get good results around 500KΩ.

Hope this helps.

[Justin.e.snodgrass]

Great explanation! Thank you.

[gy7785]

I just installed Multisim 12. During install I got a message saying that Elvismx file could not be foond on this volume, please insert CD. There is no CD. I have no instruments available to me when I build a circuit. It says my manufacturer's database is empty for any instrument that I select. There are no Multisim instruments.

[Fernando_D]

Hello,

First of all, do you have the actual ELVIS workstation? If not you don't need to install the NI ELVISmx software driver.

You mentioned you don't have any virtual instruments? Are they grayed out?

Can you place components from the Master Database?

What's your operating system?

Regards,

[gy7785]

I found the problem. I had not noticed the Multisim instrument toolbar. It was inconspicuous on the side somewhere. Thanks for the response.

[Fernando_D]

Glad to hear the issue has been resolved.

Have a nice day.