Make a Diff op amp or just get one like ina111

Discussion in 'The Projects Forum' started by Deke, Jan 19, 2009.

  1. Deke

    Thread Starter New Member

    Jan 19, 2009

    Ok I'm a newbie to op amp circuits, but I want to make an differential op amp for improving control of a cylinder (hyd) that is connected to a linear transducer with an output of (0 to 10V).

    Currenlty I'm donig the close loop in the PLC even with a low scan rate of 3ms obviously is still not fast enough.

    So I thought why not use a differential op amp and let the op amp do the close loop control based on the transducer position (0-10V) and the command signal (0-10V)

    Now I have read a lot articles about this and viewed many circuits and also have seen chips (dip) that have all of the resistors and everything already in the chip such as the TI INA111

    The analog output signal of the command is is at 75mA and 250mA of the linear transducer.

    Oh ya one more thing I would to use a dual output DC_DC Converter to supply the op amp with the +15 & -15V from a 24v soruce.

    Any suggestions from the experts here?

  2. beenthere

    Retired Moderator

    Apr 20, 2004
    One of the best references to have concerning op amp circuits is the National Semiconductor op amp circuit collection. Follow the link - - and select the AN-31 document.

    On page 14 is the stuff you want. It shows how to make an instrumentation amp using three or four op amps. It won't be as precise as an INA111, but it may do nicely for your application.

    If all you need is the voltage difference between the command and the sensor position, the instrument amp should do fine.

    Digi-Key has the Dc-DC converter - Look in the NDH series for the 24 volt input/+/- 15 @ 100 ma output. It's $17.90.
  3. SgtWookie


    Jul 17, 2007
    Wait a minute - if you want instrumentation amplifiers, you'll be far better off buying them instead of trying to build them from individual opamps.

    Read this Instrumentation Amplifier Application Note from Intersil:

    You'll find it reasonably easy to read, and it explains quite clearly why you want purpose-built amplifiers.

    We had an engineer try to build instrumentation amplifiers for a similar control purpose; it was a disaster. He'd used 1% tolerance precision metal film resistors; as a result the CMRR was abysmal.
  4. Deke

    Thread Starter New Member

    Jan 19, 2009
    Thanks beenthere & Sgt Wookie,

    I think I will go with the special prupose op amp in leu of making one.

    But the TI INA111 specs are really good.

    Thanks for the info on the DC-DC converter, found a murata power solutions one for less at and yet another from v-infinity even less then mouser.

    Nice forum with great ppl, I'm sure I will be back when I start putting this puppy together.

  5. John Luciani

    Active Member

    Apr 3, 2007
    I would buy one (probably $4-$5 max). They are pre-trimmed for gain, offset
    and CMRR errors.

    You can get a single supply, rail-to-rail instrument amp so you won't need a DC-DC
    converter. The amp will not draw much current so you can use a linear regulator from the +24V supply to get the supply voltage. Probably $7 in parts.

    I would look at the Analog Devices instument amps.

    (* jcl *)