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PSoC™ 5, 3 & 1 Forum Discussions

prbh_3338016
Level 4
Level 4
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Hi gurus,

I am using psoc5lp IDAC to generate 1u-amps to 255 u-amps source current. But as a second part of the project I want to generate 1nano-amp to 1micro amp.

I have learned that psoc5lp has Idac which has capability to output 1micro-amp/value of 8-bit.  Is there any IDAC of any psoc or current source ic which can source such small amount of current.

Or else If any of gurus can help me with the design for 1na-1ma current source meter.

Regards

P.R.B

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1 Solution

So gurus,

What can be alternative for 1na to 1ma source current production.

Do Any body know the circuitary.

View solution in original post

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8 Replies
Len_CONSULTRON
Level 9
Level 9
500 solutions authored 1000 replies posted 750 replies posted

P.R.B

I'm not aware of an external IC to achieve as low as 1nA.

The PSoC5 claims a setting as low as 125nA.

The biggest issue I see in trying to achieve a source current of 1nA is leakage.

Any IC is prone to internal circuit leakage.  I believe the PSoC5LP has a max input leakage of 2nA.  There may be other internal circuit leakages in the IDAC not specified.

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Then there is external PCB trace leakage.   At a 4mV between two neighboring traces, only 4 Mohms of parasitic resistance is needed to leak 1nA.

Len

Len
"Engineering is an Art. The Art of Compromise."
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So gurus,

What can be alternative for 1na to 1ma source current production.

Do Any body know the circuitary.

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P.R.B,

I have not found a viable 1nA precision current source generator on the market.   I'm curious in anyone else's findings.

Given the reason I indicated in my first post, this will be a challenge.   I figure at best you might find a design in the scientific community where price is generally no object to obtain the most accurate data.   Even with these types of designs at that low a resolution there are constant calibration and offset compensation cycles to achieve a stable accuracy.

With the PSoC5's abilities, it would be theoretically possible to create a 125nA to 1mA current source.  This is because the IDAC component has 3 ranges.   You could create a SW algorithm that switches between the ranges depending on the desired current source value you are targeting.  One of the downsides of this approach is that you can be as accurate as:

  • range: 0uA to 31.875uA     resolution: 125nA
  • range: 33uA to 255uA    resolution: 1uA
  • range: 256uA to 1000uA   resolution: 8uA

There may also be some non-linearities in output currents especially at the switching points.

In theory to achieve a seamless  1nA to 1mA range with 1nA resolution (if achievable) would require an IDAC with 20-bits of control.   1mA/1nA = 1,000,000 steps.  2^20 = 1,048,576 steps.

Len

Len
"Engineering is an Art. The Art of Compromise."
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P.R.B,

If you are willing to share more details about the project you are trying to achieve, myself and others may be able to steer you in more practical and (hopefully) cost-effective directions.

Len

"Engineering is an Art.

The art of compromise."

Len
"Engineering is an Art. The Art of Compromise."
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Actually we want to simulate the outputs of various gas sensors. We want to feed the simulated gas sensor output to the gas sensor modules to make the calibrations and other settings easy. As some gas sensors take 30s to 40s time .

We are making use of  gas sensors from figaro and alphasense. Some sensors have output of few na to ma.

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P.R.B,

Thank you for sharing the overall intent of the design as a gas sensor output simulator.

Let's say you can get moderate precision between settings of 1uA to 1mA.  The trick is to get precision between 1nA to 1uA.

Since you are trying to simulate the current output of gas sensors so that you can calibrate some input measuring device, what does the input of the measuring device look like?   Ie.  A input measuring device of a current output is basically a resistor to GND or VDD.   This voltage across this resistor is either amplified or measured directly using an ADC.

If you can draw the input circuit of this measuring device with ADC range and resolution, I ma be able to recommend some circuit(s) to achieve reasonable calibration results.

Len

Len
"Engineering is an Art. The Art of Compromise."
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Dear Len,

The module is ready made and outsourced. Having a close look i found the module uses transimpedance amplifier technique and make use of analog devices opams and adc. It  seem  some standard techniques.
I dont have much info regarding the constructions of modules.  We will try to develop our own modules once we are able to simulate the sensors output.

Regards,

Pratik

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Pratik,

Thanks for the further information.   However, I'm looking for the input impedance of the front-end circuit.  This will inform the design about how low of current it can take.   The input impedance is usually determined by resistor to GND (or VDD) and any active component inputs (you mentioned a trans-impedance amp [Xamp).   If you're blessed, the Xamp has input current draw of pA.  However, nA draw is not uncommon.

Next, knowing the ADC resolution is needed to know the minimum input current value that can be resolved.

Len

Len
"Engineering is an Art. The Art of Compromise."
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