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  • TI Thinks Resolved

TINA/Spice/OPA189: SIMULATION, HANDHELD "Meter"

Prodigy 230 points

Replies: 8

Views: 887

Part Number: OPA189

Tool/software: TINA-TI or Spice Models

Hi Guys!

First of all i would like to share my conpect with you.
I use ADS1262 for a long time, it is perfect for us , but now i need to measure voltage more than "+/- 2,5V".
A found schematic what is looks like what i want to do, i attached the file.


I except from this schematic that the V+ is always VCC/2 and V- is swing within 0-5V more or less.
But if i made the simulation i did not get the same results.

I Would like to measure +/-24V , couse what if someone switch the plugs(5V) in this case it should be for example not +5V, -5V.


Maybe i missed something or ?
I hope some of you guys can help me.
The biggest problem with our design is the limited space , i dont have space for negative supply for the OPA and ADS1262.


Regards
Balazs.

ps.: this picture came from the TIDA-00879.OPA189FORADS1262.TSC

  • Hi Balazs,

    It is not clear which source is being swept in your TINA circuit and produced the above voltage sweeps. Where is the +/-24 V being connected? Is the +/- 24 V referenced to the same ground as U1, U2? If not, what is the intended common-mode voltage?

    By the way, it usually isn't a good idea to add a 1 uF load capacitance to the output of a unity-gain buffer. It invites stability problems. It might be okay to connect it on the other side of the 4.7 Ohm series output resistor.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • In reply to Thomas Kuehl:

    Hi Thomas,

    I attached the TINA file where i made my simulation. You can check it. it is at the end of my prev. post.

    I will also answer for your question a bit later.

    Balazs
  • In reply to Thomas Kuehl:

    Thomas Kuehl
    It is not clear which source is being swept in your TINA circuit

    The VG1 is running from -24 to +24V. I exacpt that the U2 is sweeping from ~+5V to ~0V. but is running from ~2,5 to ~0V.

    Thomas Kuehl
    Where is the +/-24 V being connected? Is the +/- 24 V referenced to the same ground as U1, U2?



    The VG1 is not connected to the ground. As you can see the VG1 is connectoed to the Input+ and Input-.

    Thomas Kuehl
    If not, what is the intended common-mode voltage?

    The common-mode voltage is AVCC/2, U1 made this. 

    To the first picture is want to copy this section from the TI Guidline: 
    "The AFE subsystem measures voltage by applying the external device-under-test (DUT) voltage between the voltage+ (red) and voltage– (black) terminals. These inputs terminals support both positive and negative voltage polarities". 

    Well i want to simulate this postive and negative voltage polarities.
    I hope you can see where i stucked.

    Balazs 

  • In reply to balazs buglyo:

    Hi Balasz,

    Okay, thanks for the additonal information. We'll take a look at the OPA189 configuations and see what we find.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • In reply to balazs buglyo:

    Hi Balazs,

    you have taken the wrong OPAmp for the simulation: The OPA189 is no input rail to rail OPAmp. If you increase the supply voltage for the both OPA189 to 10V the simulation will work.

    Kai

    P.S. Sorry, edited "no output rail to rail" into "no input rail to rail".

  • In reply to kai klaas69:

    Hi Kai!

    Thank you for your answer. Jeah it is working well right now.

    Br still have the question. Is this Input simulation good? I mean the voltage generator. Or should i use some kind of VCVS ?
  • In reply to balazs buglyo:

    Hi Balazs,

    the voltage generator you have used should be ok. But as Thomas already mentioned: Remove C1 in your simulation!

    Kai
  • In reply to kai klaas69:

    Hi Balazs,

    Alternately, you could use precision op amps that are specified as having rail-to-rail input and output ranges. The OPA196 is an example of such an amplifier. Then, the +5 V supply could be used as you originally planned. You can find my TINA simulation file and an image of the results below.  I redrew the circuit and used a 1 Hz triangle wave for the +/-24 V input.

    Regards, Thomas

    Precision Amplifiers Applications Engineering

    OPA189FORADS1262_01.TSC

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