9 Replies Latest reply on Jun 8, 2015 8:29 AM by DaKn_263916

# IDAC Shunt Resistor

How do I go about selecting a shunt resistor for the IDAC in order to get a voltage output. The trial and error method worked, but I don't know the trade-offs.
• ###### 1. Re: IDAC Shunt Resistor

Just two things to observe: Ohm's law and the maximum current the output of the IDAC may deliver/sink.

What voltages would you like to have?

Bob

• ###### 2. Re: IDAC Shunt Resistor
Well I assume that the supply voltage will be the limiting factor. If I run off battery (2 AAA) then 2V would be the rail.
• ###### 3. Re: IDAC Shunt Resistor
This assumes my supply operating range is 2-3V.
• ###### 4. Re: IDAC Shunt Resistor

There is an internal resistance for the switch to connect the IDAC to the pin. In the .cydwr view choose the "Analog" tab. With a lright-click on your IDAC you may select the ohm meter. When clicking on the pin you will see a resistance of ~200Ohms. Take that into your shunt calculation. So a shunt of ~ 6.2k will give you ~2V if I am not mistaken.

Bob

• ###### 5. Re: IDAC Shunt Resistor
I guess my basic question is can I damage the IDAC if the shunt resistor is too small. Also, does the max current change with supply voltage.
• ###### 6. Re: IDAC Shunt Resistor
OK. That helps. Thanks.
• ###### 7. Re: IDAC Shunt Resistor

Even when you use a zero-ohm resistor the current will be not more than what the IDAC is programmed to, Only when R is too high, the volatge will reach its max before the required current can flow.

Look at IDAC datasheet for variance of I over the voltages. Comparably stable output of I.

Bob

• ###### 8. Re: IDAC Shunt Resistor
Makes sense. I just had a "well duh" moment. Thanks again for you help. Phil
• ###### 9. Re: IDAC Shunt Resistor

Keep in mind a current source has a limiting R value due to its

necessary limit on complience. That is there is some min V required

across the current source to keep it in its active region regulating

current.

On the low R size no limits per se but then route and pcb parasitics

come into play.

Regards, Dana.