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TIDA-00440: Measuring leakage current in electric machinery insulation approximately 1nA

Donjie Billy Hipolito
Prodigy 211 points
Part Number: TIDA-00440
Other Parts Discussed in Thread: LMC6082, ADS1256,

Hi Team

I have a customer that is designing to measure leakage current in electric machinery insulation.

The requirement measuring current is approximately 1nA using the devices ADS1256 ADC and precision op amps LMC6082.

I have already suggested the reference model TIDA-00440. Your response is highly appreciated. Thank you.

  • 0
    TI__Prodigy 305 points

    Hi Donjie,

        According to TIDA-00440 design, the maximum insulation resistance for measurement is 100Mohm with 500Vdc which means the minimum leakage current is 5uA.

    Since customer requires to measure 1nA leakage, this reference design can't fulfill the requirement if the rated voltage for electric machinery is 380Vac. 

    May I ask the reason why customers need such small leakage current detection for machinery? 

    BR,

    Chen

  • 0 in reply to Chen Gao
    TI__Prodigy 211 points

    Hi Chen,

    The customer is developing a megohmmeter insulation tester according to IEEE Std 43-2013. The ability to measure leakage currents ranging from 3mA down to 1nA is a requirement from my customer. The customer already developed the previous circuit I mentioned, but encountered some issues and wants to improve its performance.

  • 0 in reply to Chen Gao
    TI__Prodigy 211 points

    Hi Chen,

    The customer is developing a megohmmeter insulation tester according to IEEE Std 43-2013. The ability to measure leakage currents ranging from 3mA down to 1nA is a requirement from my customer. The customer already developed the previous circuit I mentioned, but encountered some issues and wants to improve its performance.

  • 0 in reply to Donjie Billy Hipolito
    TI__Prodigy 305 points

    Hi Donjie,

         May I know the details for the issues? I have looped the SEM T&M colleagues for better supporting. 

    BR,

    Chen

  • 0 in reply to Chen Gao
    TI__Prodigy 211 points

    Hi Chen,

    I have attached the email concern of the client regarding the leakage current measurement. I hope this helps.



    I need to develop a circuit for measuring leakage current in electric machinery insulation. The concept is to apply a high DC voltage (up to 5000V) to the insulation material and then measure the leakage current. The measurement should be conducted continuously for a duration of 1 to 10 minutes. The expected range for the measured current is between 1nA and 3mA. I'm using the ADS1256 ADC and acquiring 100 samples per second. My objective is to obtain a reading of 3V for 3mA and 1mV for 1nA. In my initial attempt, I tried using an instrumentation amplifier (two unity gain OpAmps) with multiple shunts (1k, 10k, 100k, and 1M Ohm) selected by relays. I used the LMC6082 OpAmp, powered by ±5V. This configuration allows for differential measurements with a separate analog ground plane and the return path of the high voltage (HV) source, ensuring EMI safety. In the event of insulation crack, the load becomes a short circuit, and the entire HV will be across the shunt. To protect my OpAmp, I added a 1M Ohm resistor in series with the non-inverting input of both amplifiers, which holds the high voltage and limits the current to a safe value until my system disables the HV source. However, there is an issue with this circuit. The insulation material has capacitance, so each time a shunt is selected, a transient response occurs due to the RC formed by the shunt and the capacitive load (up to 1uF), which lasts for several seconds.  

    Due to this problem, I want to explore a different circuit for this measurement. I'm considering two possibilities. First, I could use the same instrumentation amplifier but replace the multiple shunts with a single 1kOhm shunt and implement different gains (1, 10, 100, 1000) using selectable relays (although I'm unsure if the LMC6082 can achieve a DC gain of 1000). The challenge with this approach is that, from my measurement perspective, the RC formed by the capacitive load and the shunt acts as a high-pass filter. As a result, the ripple from the HV source is present in the shunt signal. Applying gain to the shunt voltage would amplify this ripple noise, making it much more difficult (or perhaps impossible) to filter the signal effectively.
     
    The second alternative is to use a transimpedance amplifier (TIA) with selectable feedback resistors controlled by relays. According to my recent research, the TIA is ideal for low current measurements, but I have no prior experience with this topology, and all the references I found online pertain to photodiode applications. In my application, two issues arise with the TIA that I'm unsure how to address:
     
    All the return current from my HV DC source will flow into the TIA. If my load (insulation material) develops a crack and becomes a short circuit, I don't know how to adequately protect the TIA.
    The ADC I'm using is the ADS1256. How can I perform a differential measurement with a TIA? If not possible, can I merge my analog ground plane with the return path of the HV? If so, won't the return path of the HV source introduce EMI?
  • +1 in reply to Donjie Billy Hipolito
    TI__Prodigy 50 points

    Hello Donjie,

    Since we followed up offline, I'm going to mark this as resolved. 

    Thanks,

    Maggie