LTC2063 / LTC2064 Bias Current in LTspice

Good Morning-

I'm planning a design to measure small DC currents (~nA) across a shunt, apply some gain with a series of zero-drift amplifiers, and read the voltage output with a DMM. The goal is to reduce the burden voltage introduced in a typical DMM when measuring current. The LTC2063 / 2064 look to be well-suited to the job thanks to low input bias current and low offset voltage. However, when running a DC sweep in LTspice I get some very significant error introduced by the bias current of the amplifier, which looks to be way above the specified maximum at ~120pA. Even when driving the input from an ideal voltage source, the bias current doesn't change. The input voltage is centered at midrail, right in the middle of the common mode input range. Am I missing something here, or is this SPICE model inaccurate?

Thanks,

-Erik

 LTC2063_BiasError.asc



Edit: Formatting
[edited by: EHaag at 2:36 PM (GMT -5) on 22 Dec 2020]

Top Replies

    •  Analog Employees 
    Jan 4, 2021 +1 suggested

    Hi ehaag,

    The LTC2063 input bias current is modeled to be 124pA in LTspice. I understand that this value is the maximum at the highest operating temperature and not the typical you'll measure at room temperature…

  • 0
    •  Analog Employees 
    on Jan 4, 2021 4:12 AM 1 month ago

    Hi ehaag,

    The LTC2063 input bias current is modeled to be 124pA in LTspice. I understand that this value is the maximum at the highest operating temperature and not the typical you'll measure at room temperature. The model was apparently done so you'll see the worst case behavior over temperature.

    If you wanted, you could add input current sources to counter this effect if you're only concerned with typical room temperature. I've done that here by adding I2, and I3 for U1 so that "out1" is exactly at Vcc/2 (no input bias current error).

    Hope this helps.

    LTC2063_BiasError edited 1_3_21.asc

    Regards,

    Hooman