• State
  • Locked Locked
  • Replies 5 replies
  • Subscribers 47 subscribers
  • Views 2734 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

LM324 I to V converter

Sebastian Scheiff
Prodigy 70 points
Other Parts Discussed in Thread: LM324, LM358

Hi all,

I am working on a project using the LM324 op-amp to convert 4 0-20mA channels to 4 0-3.3Vdc signals. I am using the standard Current to Voltage OP-Amp configuration. A 165ohm resistor between IN- and OUT with IN+ grounded. Single ended supply of +5Vdc.

The problem that I am experienceing is that the voltage output is nowhere near what it should be. I am getting 0.52Vdc between OUT and GND for an input signal of 20mA. The voltage drop across the resistor is giving me the correct Voltage, but the Voltage between OUT and GND is not. It sounds like a reference problem, but I have no idea what it could be. The problem is on all 4 channels and the signal does change when I vary the input current.

Any help would be greatly appreciated. I am sure it is something simple.

Regards,

Sebastian

  • 0
    TI__Guru**** 166486 points

    Sebastian,

    I see three issues.
    The circuit is inverting, 0-20mA = 0V to -3.3V.
    VOUT can't go negative do to VCC- being at ground.
    VOUT cannot sink more than about 50uA unless VOL is greater than 600mV. (LM324,LM358 only)

  • 0 in reply to Ron Michallick
    Prodigy 70 points

    That makes more sense now. Any suggestions on a non-inverting design. Preferably not with a second op-amp to invert the signal again. Any suggestions on an op-amp that is better suited?

    Thanks again for the help.

  • 0 in reply to Sebastian Scheiff
    TI__Guru**** 166486 points

     Sebastian,

    You could use a 16.5 ohm resistor to ground then a standard non-inverting amplifier set to a gain of ten. (other resistors and gains are also possible)

    How fast does your 0 to 20mA signal change? This will set the bandwidth and slew rate needed.

  • 0 in reply to Ron Michallick
    Prodigy 70 points

    Hi Ron,

    thanks for the response.

    I was thinking about using a differential voltage op-amp to measure the voltage across the shunt resistor. This would reduce errors due to bad solder connections or other poor connections.

    The sampling rate is not  particularly high, about 100mS. The op-amp needs to have a fairly large operating voltage (24Vdc or above) given that the Voltage coming from the transducers can be that high and the input voltage to the op-amp can not exceed the rail voltage.I am designing this to work with a variety of pressure transducers that sometimes require 12Vdc and other may require 24Vdc. The transducers would be powered through my board and hence I would have the op-amp powered by whatever supply is selected. The MCU has a voltage regulator with a wide input voltage.

    The MCU is at 3.3Vdc and the output signal will be scaled not to exceed that Voltage. However, do you think I should include a Zener clamping diode to protect the MCU should someone run more than 20mA into the system? I would loose some range/resolution but might be worth while.

    Thanks again for your input and help.

    Sebastian

  • 0 in reply to Sebastian Scheiff
    TI__Guru**** 166486 points

    Sebastian,

    Some facts to keep in mind.

    The LM324 can tolerate 24V on the inputs, but the shunt resistor will keep the voltage on the inputs low.
    With a 5V supply the VOH of the LM324 is about 3.5V.