Re: hi.in ADI Water Analysis Solution for pH Meters.pdf there is a block in the name of programmable ac current in conductivity meter . i can not understand why there is not a single current source for that

Hi Azita,

the problem in your circuit seems to be the value of R3.  you can come up with a 1uA by replacing R3 with 12.5k value.

you also need to lower the value of your Rload(R6) to atleast 2k.

You can use the following equation in getting the desired current(I) value base on the circuit you provided:

I = Vref*R2/(R1*R3)

where R2 = R4 and  R1=R5

          Vref = input voltage to the Howland current pump

          and I = desired current output

please let us know if this helps.

Regards,

Erbe