Help with Programmable Current Source-

Thread Starter

Ta Ba

Joined Aug 25, 2019
5
Hi everyone,

i need to design a Programmable current source, it 's output current value is from 10 nA - 10 uA.
the controlled voltage are generated from a DAC and it is 0 - 40 mV.

is it possible to get this low current value? all the circuit that i have found can generate a current above 10 uA ,but none of them can generate a 10 nA.

Thanks.
 

TeeKay6

Joined Apr 20, 2019
454
Hi everyone,

i need to design a Programmable current source, it 's output current value is from 10 nA - 10 uA.
the controlled voltage are generated from a DAC and it is 0 - 40 mV.

is it possible to get this low current value? all the circuit that i have found can generate a current above 10 uA ,but none of them can generate a 10 nA.

Thanks.
Assuming that the output current flows through some load, over what range of load voltage must the current be held constant?
 

TeeKay6

Joined Apr 20, 2019
454
Or, in other words, what is the maximum value for the load resistance?
@crutschow
I don't yet know whether the load is indeed a resistance. Perhaps it is a capacitor? Or perhaps a diode? I don't believe it's possible to complete the design without knowing the allowed range of voltage. I was assuming we would also get a descriptive reply to your question as to whether the load is grounded or not. It would be desirable to know the allowed voltage (relative to a reference such as ground) at each "end" of the load as the current is varied over the required range.
 
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ci139

Joined Jul 11, 2016
797
trivia ? OUTP.error ? variation speed / waveform ? ((in case of stepped V.ctrl)) settling time / range(error) . . .
 

Thread Starter

Ta Ba

Joined Aug 25, 2019
5
Or, in other words, what is the maximum value for the load resistance?
The output is an Input for next stage,which is Transimpedance amplifier.
but the question, why should i consern about the resistance.
as i understand, the current source provide a constant current and depends only on the input voltage.

The Input voltage could be modified, but should be within the microcontroller output voltage or an external DAC.

Assuming that the output current flows through some load, over what range of load
The current flows to Transimpedance amplifier, which have a low input impedance, i do not know exactly what is the value of Rin. but i need to know how to design such vccs at first place.
 
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TeeKay6

Joined Apr 20, 2019
454
So, thus far--
We Know:
  • Programmable/controllable current source able to control a current over the range 10nA to 10uA.
  • The control is via a voltage in the range 0-40mV.
  • The controlled current flows to the input of a transimpedance amp.
We Assume (until corrected by TS):
  • The current will flow into (conventional sense, not the electrons) the transimpedance amp input. UPDATE: TS says current will flow out of TIA input.
  • That input will be at ground potential and fixed within some yet to be defined variation, e.g. ±?mV.
  • When the control voltage is 0mV, the current should be 10nA; when control voltage is 40mV, current should be 10uA.
We do not know (until answered by TS):
  • How control voltage is set.
  • What relationship exists between control voltage and controlled current; i.e. linear or some defined nonlinearity. Required accuracy/correspondence between control voltage and controlled current.
  • Stability of current vs temperature, time, relative humidity.
  • Equivalent resistance of current source. How stable must current be vs change in load voltage.
  • Preferred/allowed source(s) of power for current source circuitry.Allowed power consumption of current source circuitry.
  • If control voltage must vary repeatably, description of variation (e.g. waveform) or, if control is via steps, what is allowed settling time and error. [@ci139]
  • Which of the above questions/statements are relevant to the project TS is placing before AAC commenters.
  • On what day the universe began.
  • Whether it is healthier to eat organic or not-organic tomatoes.
Changes to the above list are invited, especially by the TS.
@Ta Ba : Despite my humor, I am intending to be serious, not flippant.

UPDATES are noted above in list.
 
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Thread Starter

Ta Ba

Joined Aug 25, 2019
5
@crutschow
I don't yet know whether the load is indeed a resistance. Perhaps it is a capacitor? Or perhaps a diode? I don't believe it's possible to complete the design without knowing the allowed range of voltage. I was assuming we would also get a descriptive reply to your question as to whether the load is grounded or not. It would be desirable to know the allowed voltage (relative to a reference such as ground) at each "end" of the load as the current is varied over the required range.
well, the next stage is Transimpedance, i am not sure if there will be a voltage limit (or it is just the critical value of the electronics parts).
 

TeeKay6

Joined Apr 20, 2019
454
well, the next stage is Transimpedance, i am not sure if there will be a voltage limit (or it is just the critical value of the electronics parts).
@Ta Ba
Is it your intent that the output of the transimpedance amplifier become more positive or more negative as the input current increases?
Do you have in mind a desired voltage range of the amplifier output as the input current varies over its full range? (This is relevant only if you wish to discuss the transimpedance amplifier here.)
 

Thread Starter

Ta Ba

Joined Aug 25, 2019
5
@Ta Ba
Is it your intent that the output of the transimpedance amplifier become more positive or more negative as the input current increases?
Do you have in mind a desired voltage range of the amplifier output as the input current varies over its full range? (This is relevant only if you wish to discuss the transimpedance amplifier here.)
The TI should has just a positive voltage output and it increase when current increases.
it's about uni project and my concern is just VCCS ,the Transimpedance is already designed and it has a 100.000 gain.

Thanks alot
 

TeeKay6

Joined Apr 20, 2019
454
The TI should has just a positive voltage output and it increase when current increases.
it's about uni project and my concern is just VCCS ,the Transimpedance is already designed and it has a 100.000 gain.

Thanks alot
@Ta Ba
If your TIA has an output that becomes more positive as the input current increases--this is the simplest, most common, configuration--, then the current source at the input must cause (conventional) current flow out of the input of the TIA; i.e. a negative current. We do then need to know what is the input voltage of the TIA relative to circuit ground. (It could be 0V or some + or - value.) The current source must be of a "sinking" type (rather than a "sourcing" type). Either the TIA input must have a positive voltage offset or the sinking current source must have a negative voltage offset (or both) in order to draw current out of the TIA input.
 

TeeKay6

Joined Apr 20, 2019
454
@Ta Ba
If your TIA has an output that becomes more positive as the input current increases--this is the simplest, most common, configuration--, then the current source at the input must cause (conventional) current flow out of the input of the TIA; i.e. a negative current. We do then need to know what is the input voltage of the TIA relative to circuit ground. (It could be 0V or some + or - value.) The current source must be of a "sinking" type (rather than a "sourcing" type). Either the TIA input must have a positive voltage offset or the sinking current source must have a negative voltage offset (or both) in order to draw current out of the TIA input.
@Ta Ba
You have not yet answered the question by @crutschow as to why the DAC output is so low. Nor have you offered much more info on the items in the lists I made earlier. One other topic we have not discussed is whether you will build or merely design the current source. If you will build it, do you intend to solder components together or use some patchboard system? If you will build it, do you have access to appropriate test equipment (e.g. power supply, VOM, etc)?
 

ci139

Joined Jul 11, 2016
797
10 nA - 10 uA
DAC output voltage so small?
that pretty much suggest it's inside particular integrated circuit block = "i'm out" :eek:

? can you provide a discrete component level schematic (of the next-stage input) that you feed the (10n to 10µ A) output to ((as Fig.3 in http://www.hrpub.org/download/20171030/UJES1-14610182.pdf))
+ also answering the list by TK as much and exactly you can / ? are allowed to ? would be essential /// as is also "what everything you can use for the design" ( mos/j-fets bi-polars only/mixed)
 
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Thread Starter

Ta Ba

Joined Aug 25, 2019
5
So, thus far--
We Know:
  • Programmable/controllable current source able to control a current over the range 10nA to 10uA.
  • The control is via a voltage in the range 0-40mV.
  • The controlled current flows to the input of a transimpedance amp.
We Assume (until corrected by TS):
  • The current will flow into (conventional sense, not the electrons) the transimpedance amp input. UPDATE: TS says current will flow out of TIA input.
  • That input will be at ground potential and fixed within some yet to be defined variation, e.g. ±?mV.
  • When the control voltage is 0mV, the current should be 10nA; when control voltage is 40mV, current should be 10uA.
We do not know (until answered by TS):
  • How control voltage is set.
  • What relationship exists between control voltage and controlled current; i.e. linear or some defined nonlinearity. Required accuracy/correspondence between control voltage and controlled current.
  • Stability of current vs temperature, time, relative humidity.
  • Equivalent resistance of current source. How stable must current be vs change in load voltage.
  • Preferred/allowed source(s) of power for current source circuitry.Allowed power consumption of current source circuitry.
  • If control voltage must vary repeatably, description of variation (e.g. waveform) or, if control is via steps, what is allowed settling time and error. [@ci139]
  • Which of the above questions/statements are relevant to the project TS is placing before AAC commenters.
  • On what day the universe began.
  • Whether it is healthier to eat organic or not-organic tomatoes.
Changes to the above list are invited, especially by the TS.
@Ta Ba : Despite my humor, I am intending to be serious, not flippant.

UPDATES are noted above in list.
  • The control voltage is set by DAC from a microcontroller or an external DAC controlled by a mcu.
  • When DAC=0 v => the current should be null, I=0. when DAC=20 mv => the current I=10 nA.
  • for every +2 uV the current increases by 1 nA until its maximal alowed current when Vin =40 mV and I=10 uA.
  • Settling Time is 1 us. the Signal from DAC has a constant period of 10 us and the duty cycle varies from 1/(10us +40us) to 1/(10 us+1s).

that pretty much suggest it's inside particular integrated circuit block = "i'm out" :eek:
well it is not my work, for the control signal another student should generate the signal based on measured parameter.
but theoretically, we can make the voltage higher (like instead of (from 20mv-40mv) we can make it (from 2v to 4v) by the DAC or just amplify that signal to meet our goal.

@Ta Ba
You have not yet answered the question by @crutschow as to why the DAC output is so low. Nor have you offered much more info on the items in the lists I made earlier. One other topic we have not discussed is whether you will build or merely design the current source. If you will build it, do you intend to solder components together or use some patchboard system? If you will build it, do you have access to appropriate test equipment (e.g. power supply, VOM, etc)?
excuse me being late.
i have answered some question but a bout the stability: should i consern about that after or before the Design? i mean when i have designed Circuit, then i can mesaure and check its stability against chang in teperature and change in Output power.
i want to make the it on Board, two layer Board and yes in the lab we have appropriate equipment, but i need at first to have a design im my Hand to take access in the lab.
 
Last edited by a moderator:

TeeKay6

Joined Apr 20, 2019
454
  • The control voltage is set by DAC from a microcontroller or an external DAC controlled by a mcu.
  • When DAC=0 v => the current should be null, I=0. when DAC=20 mv => the current I=10 nA.
  • for every +2 uV the current increases by 1 nA until its maximal alowed current when Vin =40 mV and I=10 uA.
  • Settling Time is 1 us. the Signal from DAC has a constant period of 10 us and the duty cycle varies from 1/(10us +40us) to 1/(10 us+1s).


well it is not my work, for the control signal another student should generate the signal based on measured parameter.
but theoretically, we can make the voltage higher (like instead of (from 20mv-40mv) we can make it (from 2v to 4v) by the DAC or just amplify that signal to meet our goal.



excuse me being late.
i have answered some question but a bout the stability: should i consern about that after or before the Design? i mean when i have designed Circuit, then i can mesaure and check its stability against chang in teperature and change in Output power.
i want to make the it on Board, two layer Board and yes in the lab we have appropriate equipment, but i need at first to have a design im my Hand to take access in the lab.
@Ta Ba
Thanks for the additional info; I will update my specs lists. The settling time is rather more complex than a single value; it might depend on the size of the change or even the level of the output.
The problem with the low DAC voltage is that noise and thermal stability become quite troublesome at such low voltages. If possible, I certainly recommend that the DAC output be raised. Yes, a full-scale value of 4V would be much safer than 40mV.
If you were asked to design a car, would you wait until the design was complete before you decided what top speed (or hundreds of other specs) was required? Don't you think that it would be foolish to take that approach? The situation is the same for any project. If you don't know where you are headed, you will never reach the goal. That is the purpose in making the list of specs. We want to know early during design what is required. A requirement that turns up late in the design process could easily invalidate all the work done up to that point. Meaning, at best, you would be fired from your job as designer, if not sued for incompetence. It's far better to do your planning at the beginning, not after you finish. As the design progresses, you continually review it against the list of specs. You may discover that some spec is not achievable (or not achievable for an acceptable cost, or for an acceptable development/design time) and a change needs to be negotiated.

So, the first goal is to define as many specs as possible. And very soon, it should be you, not I, who maintains the lists.
 

ci139

Joined Jul 11, 2016
797

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crutschow

Joined Mar 14, 2008
23,522
i have answered some question but a bout the stability: should i consern about that after or before the Design? i mean when i have designed Circuit, then i can mesaure and check its stability against chang in teperature and change in Output power.
You always consider that during the design, as TeeKay6 stated.
Otherwise you have to do a redesign if your stability measurements shows it's not sufficient.
 

TeeKay6

Joined Apr 20, 2019
454
  • The control voltage is set by DAC from a microcontroller or an external DAC controlled by a mcu.
  • When DAC=0 v => the current should be null, I=0. when DAC=20 mv => the current I=10 nA.
  • for every +2 uV the current increases by 1 nA until its maximal alowed current when Vin =40 mV and I=10 uA.
  • Settling Time is 1 us. the Signal from DAC has a constant period of 10 us and the duty cycle varies from 1/(10us +40us) to 1/(10 us+1s).


well it is not my work, for the control signal another student should generate the signal based on measured parameter.
but theoretically, we can make the voltage higher (like instead of (from 20mv-40mv) we can make it (from 2v to 4v) by the DAC or just amplify that signal to meet our goal.



excuse me being late.
i have answered some question but a bout the stability: should i consern about that after or before the Design? i mean when i have designed Circuit, then i can mesaure and check its stability against chang in teperature and change in Output power.
i want to make the it on Board, two layer Board and yes in the lab we have appropriate equipment, but i need at first to have a design im my Hand to take access in the lab.
@Ta Ba
Responding to your post#15:
You say "DAC" but you describe a "PWM" signal. How much voltage amplitude ripple is on the "DAC" signal at its output (that will drive the current source) = 0V? =0.1mV? =1mV? That is, a PWM signal is a series of discrete pulses that must be averaged to convert the duty cycle to a voltage. I am asking, if the duty cycle remains constant (e.g. at 10%), how much ripple (not removed by the averaging circuitry) will be present in the voltage presented to the input of your current source?

You say duty cycle ranges from 1/(10us +40us) to 1/(10 us+1s); that is, from 20000 to 0.999990. That is not possible; duty cycle must be in the range 0 - 100% (or 0.000 to 1.000 as a ratio). What are the actual duty cycle limits?

You say "DAC" period, actually PWM period, is 10us, fixed. This PWM signal has a resolution of 10us/(1000/2)=10us/500=20ns. That's a pretty short interval for a PWM, meaning that each "step" increase in duty cycle corresponds to an interval of 20ns. Can the MCU you are using actually produce such a fast PWM signal?

To design a current source it will be necessary to know how rapidly the "DAC" signal can change. Theoretically, from the info you have provided thus far, that signal could change from 0V to full-scale in 10us (one PWM period). That's pretty fast. Realistically, what is the fastest change expected in the real signal? For example, it might be 1mV/sec, or 400mv/sec, or ???
 
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