• State Resolved
  • Locked Locked
  • Replies 1 reply
  • Subscribers 49 subscribers
  • Views 1791 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

OPA227 floating voltage measurement

Pol REYMOND
Intellectual 1300 points
Other Parts Discussed in Thread: CSD17585F5, OPA2277, OPA197, OPA277, OPA191, OPA192, INA149
Hello,
The purpose of our system is to measure a 80VAC signal generated by an independent power supply (Totally different ground) ,  and convert it into a -/+ 10 Vac signal  suitable to be analyzed .
This power supply is a Sinus 80VAC MAX  , 400 HZ , but why have no idea about the common mode .
We wanted to try a classic difference amplifier with a Gain of 0.125 , powered by an isolated DC/DC converter but , we don’t know if the voltage on the input pins are acceptable because of common mode .
Beside we need a 1Mohm input impedance ( at least ) .

We tried this circuit but we had some oscillations noises because of the too large feedback resistance. And the internal features of this OP AMP
Do you have a better solution or another kind of OPAMP more suitable to do the JOB
Thx

Regards 

CSD17585F5

  • 0
    TI__Guru 62035 points
    Hi Pol,

    Regarding the common mode voltage, this depends on how the isolated supplies relate in voltage potential to the 80VAC signal. In order for this circuit to function the isolated supplies for the difference amplifier must have a defined voltage potential relationship relative to the potential of the 80VAC input signal. Is there a connection between the GND of the OPA2277 circuit and the GND of the 80VAC signal?

    The OPA277 is a good choice for this circuit. By adding a feedback capacitor in parallel with the feedback and reference resistances (R3/R4) you can maintain a balanced difference amplifier while stabilizing the circuit. The closed loop BW will be limited to 1/(2*pi*RF*CF). Other op amp options are the OPA191, OPA192, or OPA197.

    The INA149 could work without the isolated supplies if the common-mode voltage of the 80VAC signal is expected to be within +/-275V of the GND of the difference amplifier.