This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

INA146: High-Voltage (0-24vdc) Signal Conditioning for Low-Voltage ADCs (3.3vdc)

Part Number: INA146
Other Parts Discussed in Thread: LM7705

Hi everyone,

I'm trying to seek a simple method to condition high voltage signal (0-24vdc) for low voltage ADC (0-3.3v).

Specifially, I'll like to scale input  signal of 0-10vdc linearly to 0-3.3vdc, and clip any input signal above 10vdc at 3.3vdc output.

We have tried this with a INA146 using V-=0v, V+=4.4v, total gain=0.33. However, one key issue was that at Vin=0, Vout is clipped at min of 0.15v instead of 0v. Is there a good way to solve this?

In addition, the input impedance of INA146 at 100k ohm is too low and may affect the measured system. How can I increase input impedance to >1Mohm?

Thank you so much for your support.

Samuel

  • Hi Samuel,

    With regards to the output swing, no op amp can swing all the way to the rail. Even devices we call rail-to-rail have limitations. The closest you can get is limited by the saturation characteristics of input and output stages. If you want to swing to 0V on the output, you will need a negative supply rail. One of the simplest ways to solve this problem is to use a negative bias generator like the LM7705. It takes a 3V to 5.25V input and produces a -232mV output.

    Regarding the input impedance, the best solution would be to buffer both inputs to effectively build an instrumentation amplifier. You could place large resistors in series with the inputs, but this will significantly degrade your CMRR performance.

    Do you need a difference amplifier? Is this a differential signal you are trying to condition? If not, then there is a much simpler solution using an op amp.
  • Samuel

    We haven't heard back from you so we assume this resolved your issue. If not, post a reply below, or create a new thread if this one has timed-out.

    Thanks
    Dennis