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TLV9001: Add offset to the output signal

Tylor
Prodigy 200 points
Part Number: TLV9001
Other Parts Discussed in Thread: INA500, INA148, TLV9002, TINA-TI, INA149

Tool/software:

Hi, We are reading a signal from sensor in the field, the signal is varying from 2 - 0V dynamically. the scope is to measure amplitude and its frequency which was done. The microcontroller is able to read the amplitude and frequency but near 0 voltage(approx 50 - 100mV) if there is any oscillation the controller couldn't read it properly. The sensor signal is fed to unity gain amplifier and its output is connected to controller. We are thinking of adding 500mV offset to the signal so when the signal vary from 2-0V we will get 2.5-0.5V. we don't want to add this offset before amplifier just to avoid impact the impedance of signal. Offset at the output will be a best option. current schematic of amplifier stage

what is the best way to achieve it ?

  • 0
    Guru 316510 points

    This circuit already loads the input signal with the low-pass filter. Use 10 kΩ and 1 nF to reduce the load. Use three more 10 kΩ resistors for a difference amplifier. If you care about precision, use an integrated difference amplifier like the INA500.

    Do you already have the 0.5 V reference voltage? If you generate it with a resistor divider, you might need to buffer it with another opamp.

  • 0 in reply to Clemens Ladisch
    Prodigy 200 points

    Thanks for the inputs !!

    We are going for next version of Hardware so we have freedom to change/update the circuit. After your input we went on a search for differential amplifier and its designs. this example from INA500 datasheet matches exactly

    We also found another example with INA148 which has same configuration and output but with feedback from output and reference tied to output.

    In our case we will connect the -ve input to GND and positive to Signal Input. the difference will always between 0V-Signal Voltage. May we know which one suits best for our application?

  • 0 in reply to Tylor
    Guru 316510 points

    Both the INA500 and INA148 would work, as would many others. The INA148 has a more complex feedback network to allow a wider common-mode range, but this does not matter for you.

    What accuracy do you need?

  • 0 in reply to Clemens Ladisch
    Prodigy 200 points

    We are sampling at 4Khz 12bit rate of  3.3V full scale. 3mV accuracy is more than sufficient for our application

  • 0 in reply to Tylor
    TI__Guru 67601 points

    Below please find a discrete implementation of the circuit you describe using dual TLV9002 - see below.

    Below I have attached Tina-TI schematic for your convenience.

    Tylor TLV9002 difference amplifier G=1.TSC

  • 0 in reply to Marek Lis
    Prodigy 200 points

    Thanks for your efforts. we will check this with current hardware we have. But for next version we like to proceed with Differential amplifier. Now we would like to conclude the selection of suitable differential amplifier.

  • 0 in reply to Tylor
    TI__Guru 67601 points

    Sounds good.  Please let us know if you need further assistance.

  • 0 in reply to Marek Lis
    Prodigy 200 points

    Need one last favour, which one to choose from INA500 and INA148? What will be the primary factor and any strong reason ?

  • 0 in reply to Tylor
    TI__Guru**** 164546 points

    Tylor,

    Expect an answer on Tuesday, Monday is a US Holiday.

  • +1 in reply to Tylor
    TI__Guru 67601 points

    INA148, like OPA146 and INA149, is a high voltage difference amplifier meant to be used for very high input common-mode voltage range of up to +/-200V and for this reason it's higher price whereas what I believe you need is 3.3V single supply diff amp with input common-mode voltage within the supplies and Vin_diff from 0 to 2V.  For this reasons, a very inexpensive low voltage INA500A is a better choice.  One thing you need to be aware, however, that since output is a function of Vref, in order to measure Vin_diff input signal down to zero, you must lift the output to al least 50mV to 100mV so your microcontroller can read by applying said voltage to reference pin:: Vout = Vref + Vin_diff*Gain = 100mV + 0*1 = 100mV

    Also, please note that the maximum input voltage offset of INA500A is +/-3/5mV - see below.

  • 0 in reply to Marek Lis
    Prodigy 200 points

    Thanks for your time and detailed explanation. It gives clear idea on proceeding further with INA500A.

  • 0 in reply to Tylor
    TI__Guru 67601 points

    No problem.  Good luck!