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Original question:

OPA170: Single supply, negative voltage input, invert OPAMP

OPA170: Spacegrade version for negative inputs

Anthony Giordano
Prodigy 50 points
Part Number: OPA170
Other Parts Discussed in Thread: TINA-TI

Tool/software:

Hello,

I'm trying to invert a -15V rail and step it down so I can connect it to an ADC input and get a voltage measurement.

My plan is to use an opamp to invert the -15V, and I was hoping to use the opamp in a single supply mode with 3.3V(or 5V) for +VS and GND for -VS. The problem I've noticed is that a lot of opamps will experience reverse breakdown of the ESD diode when the inputs are lower than the -VS rail. Also, ideally, I do not want to use the same -15V input I'm measuring as the V- rail on the opamp.

I saw a similar questions for the op170: e2e.ti.com/.../opa170-single-supply-negative-voltage-input-invert-opamp

Would this part work for at the voltages I proposed and is there a space grade version?

Would there be sequencing issues or anything else I'd have to worry about?

If this isn't the right part for this, could you please suggest another space-grade part?

Thanks,

Anthony

  • 0
    TI__Mastermind 20882 points

    Hey Anthony, 

    You may scale a -15V rail using an inverting amplifier circuit, the following op amps are eligible for your design, depending on the space grade you would like: Operational amplifiers (op amps) product selection | TI.com

    In this example, I used the OPA4H199, using the following analog circuit design guidelines: Inverting Amplifier Circuit (Rev. C)

    I used the gain ratio of: Vo=Vin * (-RF/RIN), and assumed you would like a 1V output ~ RF = 100Ω & RIN = 1.5kΩ. Feel free to change the ratio as you would like using the formulas in the document. 

    I also made a simulation in TINA-TI, please leverage simulation models found on product pages: scaling15.TSC

    All the best,
    Caro

  • 0 in reply to Caro Walter
    Prodigy 50 points

    Hey Caro, 

    Thank you for this response and for bringing this part to my attention. I just wanted to get some clarification just in case I decide to go with a different, more readily available, TI part. 

    So I get that during normal operation the inverting and non-inverting inputs will essentially be grounds in this configuration, but the reason I don't have to worry about the -15V violating the absolute maximum rating before the opamp is powered is because it will be current limited to <10mA (15/1540= 9.7mA) and because there is no phase reversal?

    Are there other parameters I should look out for in my part selection? 

    Thanks again, 

    Anthony

  • 0 in reply to Anthony Giordano
    TI__Mastermind 30901 points

    Hey Anthony,

    Correct, although if this negative voltage on the input is going to be a sustained condition when the device is unpowered, I would limit the current further (<1mA). This is similar to how capacitors are voltage derated to extend life.

    I would also ensure that if the input voltage is present when the device is not powered, that V- is going to be a low impedance node. In this case it is ground, so that should not be an issue, but this allows a safe path for current to flow (through the ESD diodes).

    It may also be beneficial to add a small (1k) resistor in series on the IN+ node of the amplifier in the case of ground bounce to make sure the input is protected.

    Best,
    Jerry