# Current buffer SMD IC

#### Bakr.Abdelgaliel

Joined Nov 28, 2022
11
I need an SMD current buffer (low Zin<=100ohm, high Zout>=1Meg) to drive a 100Kohm electrode supplied by a very low output impedance current DAC(this DAC i cannot access and redesign).

Unfortunately, I couldn't find any in the market. Does anyone have experience with any current buffer IC to recommend? or should I design it from scratch using MOSFETs?

Here you are a simplified diagram of my circuit:

#### MrChips

Joined Oct 2, 2009
30,488
Welcome to AAC!

Why do you need a current buffer?
If the load is a fixed resistance then current into the load will be proportional to the applied voltage:

I = V / R

#### Bakr.Abdelgaliel

Joined Nov 28, 2022
11
Thanks a lot!
The problem is that the Rout of the DAC is not contestant it is changing as the RL changes, so I need a current buffer to isolate the electrode resistor from the DAC resistor, and to take all the DAC output current to the electrode.

#### BobTPH

Joined Jun 5, 2013
8,664
If the DAC has a low output impedance and the electrode is 100K, it would not interfere at all with output voltage.

Are you experiencing a problem or are you guessing that there will be a problem?

#### Bakr.Abdelgaliel

Joined Nov 28, 2022
11
Of course, i am experiencing a problem.
To make the story short, I need to measure R_elect using these iDAC and ADC. So ideally R_elect=Vadc/iDac.
But this is not achieved. So, I Replaced the R_elect with a known RL values and measured the Rout_DAC. I found it changing for each RL value.

#### crutschow

Joined Mar 14, 2008
34,047
I need a current buffer to isolate the electrode resistor from the DAC resistor, and to take all the DAC output current to the electrode.
Sounds more like you need a voltage buffer so that the DAC output voltage appears across the load.
This can be done with an op amp configured as a non-inverting voltage follower (output connected to minus input, DAC output to plus input).

#### Bakr.Abdelgaliel

Joined Nov 28, 2022
11
Sounds more like you need a voltage buffer so that the DAC output voltage appears across the load.
This can be done with an op amp configured as a non-inverting voltage follower (output connected to minus input, DAC output to plus input).
I am using a current DAC so the output is current, and the electrode should be stimulated using current.
That's why I need a current buffering

#### MrChips

Joined Oct 2, 2009
30,488
Even if the DAC provides current output you can convert it to voltage output with the use of an external op-amp.
Check the manufacturer's datasheet for proper circuit configuration.

AD420 can provide 0-20mA or 4-20mA output for resistance loads in a given range.

#### crutschow

Joined Mar 14, 2008
34,047
I need an SMD current buffer (low Zin<=100ohm, high Zout>=1Meg) to drive a 100Kohm electrode
I am using a current DAC so the output is current, and the electrode should be stimulated using current.
That's why I need a current buffering
Okay, but that doesn't tell us much.
So how much current do you expect to apply to this 100k ohm electrode?
You would need a current source with an output voltage of 100V for each mA of current.
Does the electrode resistance vary?

#### BobTPH

Joined Jun 5, 2013
8,664
There is something wrong in what you are describing. You say the DAC has a current output, i.e. it acts as a current source. But you say it has a very low output impedance. A current source would have a high output impedance, a voltage source would have a low output impedance.

What current does your DAC output full scale? What is it its compliance voltage?

I suspect that the DAC cannot put out the voltage necessary to drive its current into your 100K load.

#### crutschow

Joined Mar 14, 2008
34,047
You say the DAC has a current output, i.e. it acts as a current source. But you say it has a very low output impedance. A current source would have a high output impedance,
Many DACs have a current output, but their voltage compliance is very low (perhaps a volt or so) and are meant to drive a low impedance, such as the summing junction of an op amp, to get a voltage out.
Thus it would need to be converted to a current-source with a high output impedance and high voltage compliance to drive a high impedance load.

#### BobTPH

Joined Jun 5, 2013
8,664
Yep. That is what I think. I was just trying to point out why it is not working via my post.

#### crutschow

Joined Mar 14, 2008
34,047
Below is the LTspice simulation of an example current input to constant-current output circuit using only an op amp and a P-MOSFET, along with a high voltage supply.
It supplies a current into the 100kΩ electrode load exactly equal (give or take the op amp input bias current) to the DAC output current.
The maximum current is limited by the maximum MOSFET and supply voltage you use (here with a 200V MOSFET and a -150V supply).
The op amp can be just about any general purpose op amp that comes in an SMD package, as well as the MOSFET.

Last edited:

#### Bakr.Abdelgaliel

Joined Nov 28, 2022
11
Below is the LTspice simulation of an example current input to constant-current output circuit using only an op amp and a P-MOSFET, along with a high voltage supply.
It supplies a current into the 100kΩ electrode load exactly equal (give or take the op amp input bias current) to the DAC output current.
The maximum current is limited by the maximum MOSFET and supply voltage you use (here with a 200V MOSFET and a -150V supply).
The op amp can be just about any general purpose op amp that comes in an SMD package, as well as the MOSFET.

View attachment 281678
@
Thanks, a lot crutschow for bringing us back to my first question, and thanks a lot for your design which is so helpful.
Now, I can say my main question was "I need an SMD current buffer" something like this opamp and PMOs in one SMD IC. Such that I need to use 16 of them for 16 different channels.

#### crutschow

Joined Mar 14, 2008
34,047
"I need an SMD current buffer" something like this opamp and PMOs in one SMD IC.
You are unlikely to find that in one SMD IC since that particular function is seldom used.
But there are high voltage op amps that may do what you want.

What's the maximum current (and voltage) you need for the electrode?

Last edited:

#### Irving

Joined Jan 30, 2016
3,795
I am using a current DAC so the output is current, and the electrode should be stimulated using current.
That's why I need a current buffering
Over the last 10y I've used many commercial stimulators and built a few of my own, primarily for Functional Electrical Stimulation (FES). My current goto device is a HasoMed 8ch unit. What current/voltage parameters are you aiming for, and why are you using 100k as the load? My experience says its significantly lower than that most of the time.

#### Bakr.Abdelgaliel

Joined Nov 28, 2022
11
You are unlikely to find that in one SMD IC since that particular function is seldom used.
But there are high voltage op amps that may do what you want.

What's the maximum current (and voltage) you need for the electrode?
Istim(max)=10uA, Relect(max)=200Kohm

#### Bakr.Abdelgaliel

Joined Nov 28, 2022
11
Over the last 10y I've used many commercial stimulators and built a few of my own, primarily for Functional Electrical Stimulation (FES). My current goto device is a HasoMed 8ch unit. What current/voltage parameters are you aiming for, and why are you using 100k as the load? My experience says its significantly lower than that most of the time.
I am stimulating a MEA, in which the diameter of the electrode is 10um, that's why it's impedance is too high

#### crutschow

Joined Mar 14, 2008
34,047
Istim(max)=10uA, Relect(max)=200Kohm
So you only need a maximum output of 2V?

My mistake was not originally looking at the RHS2116 chip you referenced.
It states it "Stimulators source and sink currents ranging from 10 nA to 2.55 mA over a 14 V range"
so I fail to see why that will not do what you need(?).

#### Bakr.Abdelgaliel

Joined Nov 28, 2022
11
So you only need a maximum output of 2V?

My mistake was not originally looking at the RHS2116 chip you referenced.
It states it "Stimulators source and sink currents ranging from 10 nA to 2.55 mA over a 14 V range"
so I fail to see why that will not do what you need(?).
The main problem is not that the stimulator is not stimulating the electrode.
the problem is the stimulator is stimulating the electrode but with a different current from what I am asking it to generate!
due to an output impedance that appears at the stimulator's output. as I showed previously: