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.

LM7171: Need suggested method for adjusting the offset voltage of LM7171 to less than 50uV.

Part Number: LM7171
Other Parts Discussed in Thread: TINA-TI, THS4011, OPA627

Hi ,

I tried to do offset correction for the LM7171 level shifter circuit as shown below to less than 50uV.

With R5 resistor as 600 ohm, i was able to get output voltage of -411.71nV for 0V input in TINA simulation.

When it is practically tested, i found that the output voltage is not constant.

Need help to resolve the above circuit or help with any other methods for correcting offset voltage.

Thanks

Pavithra Joshi

  • Hi Pavithra, 

    The input offset voltage of the LM7171 can range from a typical value of 200uV up to 1mV. The TINA-TI model only models the device at one offset voltage and input bias current value. In order to cancel the circuit as you have drawn, you would have to "tune" each circuit individually until you reduced the offset voltage. Additionally the part is going to have offset drift of typically 35uV/degC so the offset will change as the part heats up or cools down. 

    Regards, 

  • Hi Jacob,

    Thanks for your help. 

    As youi have quoted to tune each circuit individually, can you explain how to do the same in my case.

    For your more information, my input signal is 5Vpk pk square waveform at 5MHz in steps of 60uV. The LM7171 has been selected for its higher slew rate and Gain bandwidth product. 

    Expected output is 1Vpkpk square waveform at 5MHz frequency with high amplitude of 4V and lower one as 3V.

    The input offset voltage for this opamp is up to 1mV and i need a way to correct the same. Can you please suggest a way to reduce the offset so as to achieve 60uV step of my input signal?

    Thanks

    Pavithra Joshi

  • Hi Pavithra, 

    You will need to adjust the value of R5 for each circuit until the offset is reduced. However, it is very unlikely that you will be able to meet an offset value less than 50 uV. With the part having an offset drive of 35 uV/degreeC, you will have over 50uV of offset change if the parts temperature changes even 2 degrees. If you want the output capabilities of this device with higher accuracy, I suggest you use it in a composite loop configuration of a precision amplifier. In that case the LMH7171 could act as an output drive stage for a much more accurate amplifier. 

    Regards, 

  • Hi Jacob,

    Thanks for the suggestions.I will try and get back if any help.

    Regards,

    Pavithra Joshi

  • Hi Jacob,

    I need a help regarding the composite opamp youi suggested to meet both low precision and high speed.

    I tried to use a less precision opamp OPA627 followed by the fastest opamp THS4011 as shown below.


    I find there are ringing at the output waveform.My input waveform is 6MHz squarewave with 5Vpkpk(-2.5 to +2.5V).

    Below is the output shown. I suspect that it is because of the OPA627 which has 16MHz unity gain bandwidth.

    Can you please help me in solving this issue and i wanted to know whether this cascading of OPA627 & THS4011 will help me out in getting high slew rate of THS4011 and low offset of OPA627?

    Thanks

    Pavithra Joshi

  • Hi Pavithra, 

    No, cascading them will not maintain the precision performance that the composite architecture achieves. You are likely seeing ringing because the circuit is in an attenuation and is likely de-compensating the response. I would suggest you run the THS4011 in a unity gain configuration and increase the gain of the total loop. 

    Regards,