Help:Digitally controlled current source

[Roff]

I sense that you do not understand the concept of switching sense resistors during the dead time between pulses. It requires a little more coding, and one more control line (to the multiplexer).

 

[abicash]

In fact i understood the concept thoroughly.I have used a similar concept earlier with 2 channels of an Adc to get 2 ranges of a signal.

 

[Roff]

In fact i understood the concept thoroughly.I have used a similar concept earlier with 2 channels of an Adc to get 2 ranges of a signal.

OK, great.

 

[abicash]

Is 300uA max error at 50mA acceptable?

Oh i just checked and accuracy for test current should be 0.5%.That makes it 250µA

Also,can you put some thoughts on post #28 ?

Awaiting precious reply

Thanks and regards

 

[Roff]

Oh i just checked and accuracy for test current should be 0.5%.That makes it 250µA

Also,can you put some thoughts on post #28 ?

Awaiting precious reply

Thanks and regards

I already told you why the TLC4502 won't work in your output amplifier circuit.
It is difficult to recommend solutions when you have restrictions on op amp availability.

 

[abicash]

Hi Ron thanks again.
I have already discarded TLC4502.Looking for other opamps.As i said i cant get the Linear part.So my choice is either OP07(cheap with dual supply) or AD620(expensive with single supply).Other question is How do i get unity gain with the instru amp? Is there something you can share?

 

[Rajaneesh]

Hello

I am in the thought process for a new project where one of the modules is to design a Digitally controlled constant current source (50mA-1A in steps of 10mA).
I am thinking about using the Silab's C8051F021 with a 100ksps sampling rate 12-bit-ADC and 12-bit-DAC.

I am looking for some starters/pointers on designing the CC Digital control.

Please help as usual.

Thanks and regards

What exactly you wanted to control (load) ? If you wanted to adjust current digitally, take a look at this constant current driver from Austria Microsystem : AS8710
Which has 8 dedicated individual channel...you can control this device through SPI port...integrate it with a PIC mid range controller....

 

[Roff]

Can you get any Linear Tech parts?

 

[Roff]

What exactly you wanted to control (load) ? If you wanted to adjust current digitally, take a look at this constant current driver from Austria Microsystem : AS8710
Which has 8 dedicated individual channel...you can control this device through SPI port...integrate it with a PIC mid range controller....

AS8710 does not show up on Austria Microsystems web site.

 

[Rajaneesh]

AS8710 does not show up on Austria Microsystems web site.

May be under some modification.... I have used this chip an year back....

Or try Allegro constant current driver...

 

[abicash]

Can you get any Linear Tech parts?

Yes I can get some LT10xx parts.(or let me know of some part and i can check)
I also looked up OP777,727,747 checking whether i can get them.
AD620 is the one i can get easily and seems AD is well stocked up.
I can use a dual supply if OP07 has to be used.

Its just that, the specific part LT1677 i am not able to get

@Rajneesh

I looked up Austria Microsystems and found THIS page for LED drivers.Can you point out which ones the closest?

Thanks for the support

 

[abicash]

May be under some modification.... I have used this chip an year back....

Or try Allegro constant current driver...

For current driver IC's the max current is in the range of 300-400mA.I need up to 1A current,so maybe these devices may not be the right part

 

[Rajaneesh]

Yes I can get some LT10xx parts.(or let me know of some part and i can check)
I also looked up OP777,727,747 checking whether i can get them.
AD620 is the one i can get easily and seems AD is well stocked up.
I can use a dual supply if OP07 has to be used.

Its just that, the specific part LT1677 i am not able to get

@Rajneesh

I looked up Austria Microsystems and found THIS page for LED drivers.Can you point out which ones the closest?

Thanks for the support

Hi Abi...

AS1109 can support 100mA per channel in constant current mode... Im clue less what is exactly you are going to drive with this.... This is specially designed for constant current application (Ex: to get a stable & uniform luminance from the LED's in various application like vehicle clusters)...

Also for instrument amplifier application.....

If you are going to drive drive more than the max channel current, this will sink....

 

[abicash]

Hi Abi...

AS1109 can support 100mA per channel in constant current mode... Im clue less what is exactly you are going to drive with this....

Also for instrument amplifier application.....

If you are going to drive drive more than the max channel current, this will sink....

Hi Rajaneesh please look up post#28 .Here i have more or less explained what i am doing.
Regarding sinking,can the outputs be paralleled to sink 1A current?

What do you mean by "instrument amplifier application"?

Thanks and regards

 

[abicash]

Hello Ron

I have another opamp from intersil available immediately

ICL7650S (chopper stabilised)

Please advise

 

[Roff]

Hello Ron

I have another opamp from intersil available immediately

ICL7650S (chopper stabilised)

Please advise

That should work for your current sink op amp. Intersil doesn't appear to have a spice model for it.
You can also use it for the output level shifting, if you build a . If you make the resistors large enough, you shouldn't need input buffer amplifiers. However, they will probably have to be very closely matched to get the accuracy you need. You should be able to do the calculations.
You can also buy differential amplifiers with the resistors internal to the package, but given your difficulty in getting parts, I don't know if that is an option.
If you make a current source instead of a current sink, the load resistor can connect to ground, which makes your output buffer a simple voltage follower (or nothing, if your A/D input resistance is reasonably high). However, the current source (in my implementation) would require two op amps with rail-to-rail input capability, and power p-channel MOSFET.

 

[abicash]

You should be able to do the calculations.

How about 47k as each resistor?

You can also buy differential amplifiers with the resistors internal to the package, but given your difficulty in getting parts, I don't know if that is an option.

What could be these parts?

If you make a current source instead of a current sink, the load resistor can connect to ground, which makes your output buffer a simple voltage follower (or nothing, if your A/D input resistance is reasonably high). However, the current source (in my implementation) would require two op amps with rail-to-rail input capability, and power p-channel MOSFET.

Can you share some schematic on this?

Anyways i am getting the intersil part and some OP07 (which i will try removing offsets with a multiturn preset)

I will try building this today and post the outcome.
Thanks for the support Much appreciated

 

[abicash]

I can get these parts

see attached

 

[abicash]

If you make a current source instead of a current sink, the load resistor can connect to ground, which makes your output buffer a simple voltage follower (or nothing, if your A/D input resistance is reasonably high). However, the current source (in my implementation) would require two op amps with rail-to-rail input capability, and power p-channel MOSFET.

Something like this?

NOT ABLE TO ATTACH IMAGE!!!**broken link removed**

 

[abicash]

Ok I dont understand why i am not able to attach an image here

Anyways,I replaced the LM358 with an OP07 (with +12v ,-12v supplies)
I stuck in a 20k multiturn preset between pins 1 & 8 (as in the data sheet)

I now connect pins 2 & 3 to ground and try varying the preset ,but it remains in either +ve saturation or -ve saturation . Is this because its used in open loop mode?
I also tried putting a feedback resistor with a finite gain of 100 (or so) ,still i cannot adjust the o/p to zero volts.

Am i doing something wrong here?