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TLV333: Differential amp with programmable gain and offset trim adjust

John Barrett
Prodigy 170 points
Part Number: TLV333
Other Parts Discussed in Thread: PGA308

Hello Forum, 

I am working on a design that uses a piezoelectric load cell. I need to condition the load cell O/P so that we can accurately measure the load applied using the uP ADC. 

The AFE has a few complicating factors:

1) the load cells have offset (no load applied) characteristics: min -20mV to max +2mV. 

2) The load cells have sensitivity characteristics: Min 1.6 mV/Vsupply/Nload to max 3 mV/Vsupply/Nload. (Load cell O/P is a differential voltage biased at Vsupply/2 and is always positive when load is applied except for initial offset)

Design is very cost sensitive so I want to use as few components as possible to implement the AFE conditioning circuit. I am considering using a single TLV333 opamp that has very low offset and drift characteristics in differential mode. I can add offsets easily enough to counter the load cell offsets. However my biggest problem right now is the sensitivity variation between cells (almost a factor of 2). This halves the range available when the sensitivity of a particular cell is at the minimum specified (we have to design for the highest sensitivity so that we don't saturate the amplifier). That reduces the resolution with min sensitivity load cells available in the ADC.

I've searched around but I can't find a suitable (low cost) diff amp with relatively simple programmable gain feature. Right now the gain I need is approximately in the range 45 to 90. Ideally it should be an I2C controlled device (SPI at a stretch). Even a 2 or three step gain change device would probably suffice. Ideally the device would have adjustable offset trim as well. 

We will calibrate each device in production so offset cancellation and gain setting is possible.  

The design is single rail with supply at 3.3V dc. I would really appreciate it if someone could have a look at this for me and come up with some suggestions. All comments welcomed.

Thanks and best regards

JB.

  • 0
    TI__Genius 10850 points

    Hi John,

    Thanks for your post. 

    What is the output impedance of your sensor. Typically, these sensors require an amplifier with high-input impedance like a JFET or a CMOS input. However, once you add 4 resistors for a typical difference amplifier configuration, your input impedance is limited by those resistors. 

    Please refer to section 3.1 of this application report: https://www.ti.com/lit/an/sloa033a/sloa033a.pdf 

  • 0 in reply to Tamara M Alani
    Prodigy 170 points

    Hello Tamara, 

    Thanks very much for getting back to me on this. Apologies for the multiple postings but it looked like the post wasn't submitting for me when I hit the button so I tried a few times !

    OK back to the amp. 

    I did some simulations on TINA but I didn't have a model for the sensor.  I built up a test circuit using the TLV333 configured in differential mode and from the results of this test it looks like the sensors O/P impedance is in the order of 200-300 ohms so it must have some type of integrated O/P driver. 

    I can send you further details if needs be but we are under NDAs for this design so I'm limited with what I can post here. 

    I need to try to maximise the  O/P range so that it spans 0V to Vsupply so that we can maximise the resolution of the ADC in the uP. The difficulty is the max-min range of sensitivity of the sensors (almost a factor of 2). 

    The circuit I have just about does the business. At the very low end of sensitivity levels we could adjust the range of the uP ADC to increase the resolution again. However it would be great if I could think of an easy way to adjust the gain of the circuit somehow. Right now I can't.

    As I outlined above the Ideal solution would be a low cost programmable device programmable gain and offset trim adjust capability.  I did have a search around and there are some components of this type available. One interesting option that I saw was the PGA308. This looks like it has everything that I need - the only problem with this is the cost ($1.68 in 1K from TI website). 

    So hopefully this give you a bit more info on my difficulties. I would be most grateful if you could have a look for me and come back with any suggestions on the simple circuit I have or some other suitable programmable devices etc. 

    As I said I can share additional details separately as required. 

    Thanks and best regards, 

    JB.  

  • 0 in reply to John Barrett
    Guru 140200 points

    Hi John,

    why not using a trim pot to adjust the gain?

    Kai