need to read low current input 10uA-30uA

[sushant_21]

HI,

this is sushant, i had designed a circuit for client to read analog input, using ADS1118 IC, the circuit was design to read 4-20mA,0-5V and 0-200mV.
Now the requirement has been changed and new input is added which is 10uA-30uA.
i had designed circuit as 4-20mA/0-5V to Channel 0 of ADC and 0-200mA to Channel 1 of ADC.
ADC has Programmable gain amplifier, which helps me to adjust Full Scale range to ±0.256 V , that gives me .256/32768, as 1 digit = 7.81uA, but if i use this, there will be very less Digital Value.

i have added our input schema, please kindly lemme now the improvement or guide me in this.
View attachment ADC_input485.pdf

Regards
Sushant

 

[barry]

Well, if your resolution is 7.81 uA and you want, say, a resolution of 1uA, you're going to have to add an amplifier with a gain of 7.81. That's the easy part. Now you need to make it low noise, low-drift, etc. Without more information that's not much else to say. (for instance, if you're measuring a slow moving signal, you can do some averaging in your software, or put a big filter on the input.)

 

[KlausST]

Hi,

which helps me to adjust Full Scale range to ±0.256 V , that gives me .256/32768, as 1 digit = 7.81uA,

It's not 7.81uA, but it is 7.81uV.
--> you measure volts.

Now let's assume you want to measure the 20uA ..30uA with a shunt of 1kOhms, then the resulting voltage is 20..30mV.
With your given resolution of 7.81uV you get a reading of about 3800 LSB, which means about 12 bit.
Not that bad.

Klaus

 

[sushant_21]

Hi,


It's not 7.81uA, but it is 7.81uV.
--> you measure volts.

Now let's assume you want to measure the 20uA ..30uA with a shunt of 1kOhms, then the resulting voltage is 20..30mV.
With your given resolution of 7.81uV you get a reading of about 3800 LSB, which means about 12 bit.
Not that bad.

Klaus

Thanks for your time and thoughts @KlausST

It's not 7.81uA, but it is 7.81uV.

Sorry for mistake, i was thinking about current hence wrote mA.

Yeah i got the idea of shunt resistor, but i will have to use ADC as differential ended in ADC. Sad part is i left other ADS1118 pins floating, and they are very tiny.
Thanks for help i have to try this out anyhow and ADD the part in next version.

- - - Updated - - -

Well, if your resolution is 7.81 uA and you want, say, a resolution of 1uA, you're going to have to add an amplifier with a gain of 7.81. That's the easy part. Now you need to make it low noise, low-drift, etc. Without more information that's not much else to say. (for instance, if you're measuring a slow moving signal, you can do some averaging in your software, or put a big filter on the input.)

Thanks for your time and thoughts @barry

its alcohol detecting sensor al-03s. but like Klaus Said, a shunt resistor will do, i also got this part after some searching as against C to V amplifier.

 

[KlausST]

Hi,

Sad part is i left other ADS1118 pins floating, and they are very tiny.

For the future: Mind - never let any (unused) inputs floating, not analog inputs and not digital inputs.

but i will have to use ADC as differential ended in ADC

Why "differential"? You didn´t mention this before.

Klaus

 

[sushant_21]

Hi,


For the future: Mind - never let any (unused) inputs floating, not analog inputs and not digital inputs.


Why "differential"? You didn´t mention this before.

Klaus

i was reading about Current sense Amplifier, as in image, so i thought the Rs to be connected between IN+ and IN- as differential ADC.

But again i stuck with one solution, your question WHY made me again think, and i came up with this, if i replace the SMBJ diode in my circuit with 1Kohm as you suggested at AIN1 it will work.

I think i got the solution, correct me if i am wrong.

 

[KlausST]

Hi,

I don´t think the usual current sense amplifier gives a benefit. They are for relatively high current and shunt_values in the milliohms range.

*****

if i replace the SMBJ diode in my circuit with 1Kohm

... or you add the resistor on top of C24...

Klaus