IDAC Shunt Resistor

Tip / Sign in to post questions, reply, level up, and achieve exciting badges. Know more

cross mob
Anonymous
Not applicable
        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.   
0 Likes
9 Replies
Bob_Marlowe
Level 10
Level 10
First like given 50 questions asked 10 questions asked

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

0 Likes
Anonymous
Not applicable
        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.   
0 Likes
Anonymous
Not applicable
        This assumes my supply operating range is 2-3V.   
0 Likes
Bob_Marlowe
Level 10
Level 10
First like given 50 questions asked 10 questions asked

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

0 Likes
Anonymous
Not applicable
        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.   
0 Likes
Anonymous
Not applicable
        OK. That helps. Thanks.   
0 Likes
Bob_Marlowe
Level 10
Level 10
First like given 50 questions asked 10 questions asked

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

0 Likes
Anonymous
Not applicable
        Makes sense. I just had a "well duh" moment. Thanks again for you help. Phil   
0 Likes
ETRO_SSN583
Level 9
Level 9
250 likes received 100 sign-ins 5 likes given

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.

0 Likes