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LOG114: LOG114 not working below 1uA

PRAVIN KHISTI
Prodigy 130 points
Part Number: LOG114

Hi,

We are using LOG114 for measuring current ranging from 100pA to 8mA.

Our Iref is 1 uA. It is generated from a stable voltage reference diode and series resistance.

Output Vlog out is perfect when input is from 1uA to 8mA. When input is below 1uA out put is not

as expected. We tried the solutions like cleaning PCBs with IP, covering LOG114 in the aluminium shield. Using Battery 

as power supply. Power supply we are using to LOG114 is 0 to 5V.

We matched out PCB layout as per LOG114EVM.

Please help me out on this.

 

  • 0
    Prodigy 130 points

    Hi,

    Precision Amplifiers TI team.

    Please help me on this issue of LOG114

  • 0 in reply to PRAVIN KHISTI
    TI__Guru* 93105 points
    Hi Pravin,

    We need more information regarding what "When input is below 1uA output is not as expected" means. If you could provide an image of your LOG 114 circuit showing all connections, input source details and the output load would be very helpful.

    Make sure that the water/solvent you are using is specifically intended for the solder flux type used for the component soldering. We strongly suggest using an ultrasonic cleaner vs. just brushing the board with brush. We go so far as to run our LOG114 boards through the ultrasonic cleaner twice; it can make a big difference in the very low current performance accuracy.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • 0 in reply to Thomas Kuehl
    Prodigy 130 points

    Hi Thomas,

    Thanks for the reply. I have attached the schematic we are using.

    As in the circuit diagram, Iref is 500 nA (0.5V/1Mohm) and Iin is give from YOKOGAWA 7651 a DC voltage/Current source.

    When Iin applied is 500nA, Vlog expected is 2V but it shows 1.8V.

    Also one thing to point out, at I2 we observed a square wave signal of amplitude 1V and approx 3 MHz. Is it normal ?

  • 0 in reply to PRAVIN KHISTI
    TI__Guru* 93105 points
    Hello Pravin,

    Also one thing to point out, at I2 we observed a square wave signal of amplitude 1V and approx 3 MHz. Is it normal ? No!

    Is the underside PowerPad soldered to a PCB trace connected to V- (GND)?

    I am going over the rest of your LOG114 circuit. I'll get back to you.

    Regards, Thomas
    Precision
  • 0 in reply to Thomas Kuehl
    Prodigy 130 points
    Hi Thomas

    We are using 0-5V supply and underside Power pad is soldered to PCB trace connected to 0V.