How to increase the output of the 6704 analog current output to 40mA

[andy_3141593]

Hi There,

I've ordered a PXI-6704 board, and I would like to increase the current output range from 20mA to 40mA.  At the moment I'm thinking a simple opamp would do the job.  Thoughts?

Also, I'd like to know more about the analog current output signal.  Does it control current with voltage, duty cycle, or something else?  What is the voltage range?  If duty cycle, what is the voltage range and frequency of the pulse?  Are there any NI documents where I could read more about the details... the circuit, signal diagram, etc.

Andy

[dmysliwiec13]

 Hi Andy, 

This output current range is 0.1 to 20.2 mA, so using an opamp on this output should do the trick. 

I am not sure what the output current is controlled by. Is there a specific reason you are needing to know this?

The voltage output range is -10.1V to 10.1V. In the link below you can find the data sheet for this module's information, along with various other specifications. Please reference pages 4 and 5:

http://www.ni.com/datasheet/pdf/en/ds-116

Below you will also find the user manual for this device. Please reference pages 2-2 and 2-3 for the pinout diagrams:

http://www.ni.com/pdf/manuals/372110d.pdf

Hopefully this answers your questions!

Best, 

Danielle

[Henrik_Volkers]

Use two channels to double the current?

A classic way to double a current is an extended current mirror.

AoE will help 🙂

However WHY do you want to double the current? 

Do you need it controlled or just on/off?

[andy_3141593]

Hi Henrik,

Good questions.  Two channels is a possibility, thought I would have to buy another card as I need 14 channels.  See NI reference on this.  If that works, that would be the easiest solution. http://digital.ni.com/public.nsf/allkb/B990B1BB8C6228F286256DDA005526E0

I did a quick wikipedia on current mirror.  I'm concerned about linearity and temperature affects.  Do you have a specific current mirror circuit in mind?  I'm not sure what "AoE" is.

I've got a customer that has a 20mA input on a device, but would like to fault to 35mA.

You might be right, customer might just need the 35mA switched in for testing.  Maybe just a fixed resistance would get the fault current they are looking for.  NI salesman suggested an amplifier perhaps thinking that if a current amplifier was easy to make, then that would be the most general solution.  Perhaps not!

Andy

[Henrik_Volkers]

AoE - (Book)  Art of Electronics 😉

Using one OP-Amp (and two resistors) per Channel and an additional powersupply is the low material cost way. But needs to be designed and build and qualified 🙂 .

Documentation, spare parts .....     no big deal if you have to design a custom ouput/setup anyway , but for a 'simple' & traceable 40mA output I would vote for a second channel ( and second unit if no of channels needed).

[Henrik_Volkers]

Update: I did a small simulation

While it seems to work 🙂  I found one important point:

As your client/customer about the max applied voltage needed 🙂

If you choose a current doubler like the schematic or two current channels, keep in mind that the 6704 has a max output voltage  of 10V.  Depending on the load/DUT you migth not be able to apply the 40mA! 

Plot legend:

blue: current in R_load

green: current in R1 (or output current of current source I1 -> your 6704)

red: voltage at the current output I1  (with a total load of 410 Ohm you get  ~24.4 mA and 10V)

[andy_3141593]

Hi Henrik.  We decided to parallel another 6704, and that did the job.  I am still interested in this circuit though. Conceptually, I understand in a simple way how this works.  I've got an ancient 741 opamp laying around the office that I thought that I might breadboard just to do some learning.  Before doing that, I thought I would try to simulate it.  Based on an article I read, I chose the LM308 as a similar op amp.  Adjusting for the power supply, the simulatation yielded strange results.  I guess that I haven't learned about all the practicalities of op amp choices.  Can I ask you this: how did you choose the LT1366?  Thanks!

[Henrik_Volkers]

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@andy_3141593 wrote:

Hi Henrik.  We decided to parallel another 6704, and that did the job.  I am still interested in this circuit though. Conceptually, I understand in a simple way how this works.  I've got an ancient 741 opamp laying around the office that I thought that I might breadboard just to do some learning.  Before doing that, I thought I would try to simulate it.  Based on an article I read, I chose the LM308 as a similar op amp.  Adjusting for the power supply, the simulatation yielded strange results.  I guess that I haven't learned about all the practicalities of op amp choices.  Can I ask you this: how did you choose the LT1366?  Thanks!

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You want a R2R (rail to rail) output and a supplyvoltage range of the supply voltage choosen and a max current that fit's your needs. RTFMdatasheet 😉

I picked the first one I happend to find in LT-Spice by using the linear search functions...   but distributors like digikey have nice search/filter pages for OPs...

[andy_3141593]

Hey, thanks Henrik.  That was where my thinking was going also, but thought there might be more magic than that.  I see now why my attempt with LM308 didn't work.  I chose LM308 b/c I had a 741 here in my office to play with.  I didn't think anybody makes a 741 any more, so LM308 looked like the closest.  The data sheet didn't really specify an output current, but it did specify a 0.3mA TYP supply current, so I'm guessing 40mA at the output was over driving it by a lot.  I attached the simulation if you'd like to take a look.