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LM7171: LM7171 can it be used with a single-supply?

Joel Holland
Intellectual 765 points
Part Number: LM7171
Other Parts Discussed in Thread: LM7705, TLV3541, OPA365

I suppose this is a more broad question about operating dual supply OPAMPS with a single supply. I have a signal from a current transformer of 1MHz frequency and 3V peak value. I want to invert it using the LM7171, which is 200MHZ GBW and should be able to replicate a 1MHZ input signal at unity gain.

However the output of this OPA will be feeding an ADC, and I was wondering whether I could use ground as the negative voltage supply such that the ADC never sees negative voltage. I could also have a buffer amplifier following this opamp that is only supplied with a single supply to achieve this but thought it could probably be done by a single amplifier alone.

The input signal does sometimes have negative values - but I thought that because the input voltage common mode voltage range is +/-3.2V, I don't actually need a negative supply to replicate a negative signal - correct me if I am wrong on this?

So, can I replicate a 1MHZ input signal, approximated by a triangle wave, with the LM7177 and a single +5V supply? Or is the -5V supply essential for such a technique?

Best, 

Joel

  • 0
    Guru 140200 points

    Hi Joel,

    the +/-3.2V common mode input voltage range specification of LM7171 should be read differently, that the input voltage should stay 1.8V away from the supply rails.

    Also, the recommended minimum supply voltage of LM7171 is 5.5V, not 5.0V.

    Kai

  • 0
    Guru 140200 points

    You could think about using the TLV3541, or a similar RRIO-HF-OPAmp, in combination with the LM7705:

    joel_tlv354.TSC

    Kai

  • 0
    TI__Mastermind 26825 points

    Hello Joel,

       Will the amplifier interface with a differential input ADC? If so, a FDA might be a better option for this application. FDAs have a common-mode voltage pin which makes it easy to set the common-mode of the signal to the ADC's: https://www.ti.com/lit/an/sbaa406/sbaa406.pdf?ts=1649108283627&ref_url=https%253A%252F%252Fwww.google.com%252F. 

       LM7171 won't be able to accept voltages below/above the power supply settings. The +/-3.2V specification is for power supplies set to +/-5V rather than single supply as Kai explained. Otherwise, another method would be setting a bias at the non-inverting input of the amplifier. 

    Thank you,
    Sima 

  • 0 in reply to kai klaas69
    Intellectual 765 points

    Hello Kai,

    First of all thanks for the reply, it is certainly helpful. I do actually have bipolar supplies on my board, I was just wondering about whether dual-supply OPA could be used with grounded negative supply pin.

    I should have mentioned that I am using a breadboard because I don't yet want to pay for another PCB as this is an addition to a prototype circuit. I can't seem to find these devices in through hole versions. 

    Is there anything similar to this bandwidth/method that you've proposed with through hole components?

    My ADC is a single-ended source 12-bit ADC - not differential. The max input is 3V I believe, but I also need the output to be clipped at zero volts to ensure the ADC doesn't see negative voltage. The DSP I am using is the F2837xD.

    Maybe that helps with the suitability of this LM7171 device, or alternative?

    best,
    Joel

  • 0 in reply to Sima Jalaleddine
    Intellectual 765 points

    Hello Sima, 

    No this is a single-ended, 12-bit ADC source. Not a differential. I do actually have an FDA on my PCB for a different application but for this, I just need to invert a current sense signal as I connected the current transformer in the wrong polarity... duh! :) 

    Thanks for your suggestion.

    Best,
    Joel

  • 0 in reply to Joel Holland
    Intellectual 765 points



    Here is an approximate implementation of the circuit recommended for inverting amplifiers with LM7171. It does invert the signal but obviously does not clip the output at all which I suppose is not great for the ADC.

  • 0 in reply to Joel Holland
    Guru 140200 points

    Hi Joel,

    I'm still not exactly knowing what you are trying to do. What current transformer are we talking about?

    If you want to feed the ADC input of F2837xD, you should read section 11.15.3 of "user guide". You may need a charge bucket filter at the input of ADC.

    Also, the F2837xD seems to allow under- and overvoltages at the ADC inputs to a certain degree, provided the current through the input clamps is limited to below 20mA. See the "absolute maximum ratings" in the datasheet. The resistor of charge bucket filter can help to limit this current.

    TI has special RRIO OPAmps offering fast settling times for driving ADCs. I think the OPA365 is such a candidate.

    Kai