Dear Sir,
I would like to use XTR111 to obtain 0 to 20mA/4 to 20mA current output for 0 to 5V input. Attached herewith is the schematic along with PMOS (CPH3351) and BJT(BC557b) datasheets for reference.
The problem is, when I connect different load resistances in the range of 10 ohm to 1.1K ohm for the same input voltage of 5V, the output current changes from 3.2mA for 10Ohm to 19.3mA for 1.1KOhm. For a load of 250 Ohms current is 17.6mA at 5V. When I remove R load and directly connect ammeter at the output I get 19.8mA at 5V.
I tried simulating the circuit on TINA but couldn’t find the PMOS part number. Can you guide me how to add a particular part in its library? While selecting the PMOS I took following parameters into consideration: Id=1.8A, Vdss=-60V, Vgss=+/-20V, Cinput=262pF. Do you think there is problem with PMOS or BJT part number selection(Attached datasheets)?
I have also tried the following experiments to troubleshoot :
1) I changed the transistor base resistance(R3) from 15 ohm to 2.2 Ohms; but there was no change in the result.
2) I changed the Rset resistor from 2.5KOhm to 2KOhm to check if this problem persists for other configuration i.e 0 to 25mA output for 0 to 5V input.
For 5V input, 250Ohm load, output current was 22.6mA.
For 5V input, No load(Only Ammeter), output current was 24.96mA.
Please guide me as to how I could resolve this problem.
Thank you,
Regards,
Komal Marathe
Niyantran systems