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TLV1814-Q1: Designing a comparator circuit with inputs susceptible to considerable noise

Ethan Pang
Prodigy 30 points

Part Number: TLV1814-Q1
Other Parts Discussed in Thread: TLV1814,

Tool/software:

Hello TI forum,

The task at hand is to buffer a Hall effect sensor output signal. The sensor cannot reliably source enough current on its output channel, so a comparator seems like a good solution - it will provide any necessary current sourcing capability, and it should have the added benefit of not nearly as much noise on the output. The Hall sensor output switches digitally between 0V and 5V, and the comparator would be powered off of 5V. 

The concern is that, since the Hall effect sensor will be transmitting in a noisy environment with plenty of EMI, the Hall sensor input may swing beyond 5V and below 0V. As an example comparator, the TLV1814-Q1 has a common-mode voltage range of just ±0.2V compared to the supply voltage, which leaves almost no margin for input signal fluctuation in the case of this application.

Are there any ways to mitigate or eliminate the risk of violating the CMVR in a noisy environment for comparators? Or is there a line of similar comparators with more acceptable CMVR tolerance?

  • 0
    TI__Mastermind 26720 points

    Ethan

    thanks for your post

    From your description, I assume the Hall effect sensor you are referring to has a digital output. Can you share details or waveforms of what we can expect at the sensor output? It sounds like the output is switching but I’m not sure as I’m not familiar with these sensors. Also, please specify what supply voltages are available in the system. 

    thanks,
    Chuck

  • 0
    Guru 321080 points

    The comparator has internal clamping diodes to the power rails. They are intended only for ESD events, so it would make sense to add external diodes in parallel. With a series resistor between the diode and the input pin, almost all the current will flow through the external diode.

    To remove high-frequency noise, use a low-pass filter.

  • 0
    TI__Genius 13225 points

    Hi Ethan,

    As Clemens said, the TLV1814 has internal diodes to the rails at the inputs that become forward biased should the input voltage exceed the rails. We have an application's note about the different comparator input types here: Comparator Input Types (ti.com). There is an ESD input structure called the "fail-safe" inputs. These inputs do not have an ESD diode to the upper rail, and the inputs are protected by snapback diodes connected from the input to the device VEE. This makes it so that the inputs can go up to a certain voltage independent of supply.

    The TLV902x4 has fail-safe inputs and is made for noisy environments. As you can see, the inputs can swing from -0.2V to 5.7V independent of the rail:

    The common mode voltage range is still bound by VCC + 0.2V, so this means your DC reference voltage would need to remain within the 5V rail.

  • 0 in reply to Chuck Sins
    Prodigy 30 points

    Yes, they are digital signals. There are in fact three Hall sensors, which together indicate the position of the rotor of a motor. They are identical in build so the circuit being worked on will be the same between them. The Hall sensors should be switching at a maximum of 2kHz. In case it wasn't clear, the signal switches between 5V high and 0V low.

    The supply voltages available in the system are 14V, 5V, and 3V3.

    Thanks for your attention,

    Ethan

  • 0 in reply to Clemens Ladisch
    Prodigy 30 points

    Thank you for the suggestion on the topology. Is the following an accurate interpretation of your suggestion?

    My concern with this setup is that the diodes can only clamp within a margin of their voltage drop. i.e., in this case the diodes would only clamp once the input line reaches 5V + whatever their forward voltage is. Since the TLV1814-Q1 (and a few other similar comparators I've looked at) have an input voltage range of ±0.2V outside the power rails or something similar, sourcing a diode with a Vf that low may present issues. Any suggestions on this issue are appreciated.

    As an adjacent question, is it stated or inferrable how the CMVR rating translates to transient voltage tolerance? It isn't expected that voltages beyond the CMVR would be present for too long, so if the component has some tolerance in that dimension it would be great.

    Again, thanks for your time.

  • 0 in reply to HO SIU
    Prodigy 30 points

    Thanks for your response, I want to clarify and confirm one thing however - 

    HO SIU said:
    The common mode voltage range is still bound by VCC + 0.2V, so this means your DC reference voltage would need to remain within the 5V rail.

    In other words, although the inputs can swing from -0.2 to 5.7V under some conditions, when I power this comparator with 5V I would still need both inputs to obey the CMVR and stay within 5.2V? Or am I interpreting this incorrectly?

  • +1 in reply to Ethan Pang
    Guru 321080 points

    The main purpose of the diodes is to prevent damage (the current through the internal clamping diodes must not exceed 10 mA).

    But when R2 is large enough, the forward voltage of the internal diodes will be very small. The characteristics of the internal diodes are not specified, but if we assume that, when the external diodes drop 0.6 V, then we want no more than 0.1 mA when R2 drops 0.4 V, then R2 must be at least 4 kΩ. With Schottky diodes, the current will be much smaller.

  • 0 in reply to Ethan Pang
    TI__Genius 13225 points

    Hi Ethan,

    What you've stated is generally the case, where we recommend using the device with both inputs within the input common mode range to ensure the device behaves per datasheet specifications.

    However, some comparators have the feature where they can still output the correct output as long as one of the two inputs are within the input common mode range. This is from the TLV902x datasheet:

     

    This means that as long as one input is within the input common mode range, the output will be correct. This means that with a 5V power supply, as long as you maintain one input within -0.2V and 5.2V, the other input can swing from -0.2V to 5.7V and the output will still be correct. In this case, you could set your IN- to 2.5V as in your image and the hall output can swing from -0.2V to 5.7V without damage to the comparator.

    Please note the last paragraph of the screenshot, where the comparator may operate at degraded specs in propagation delay and input bias current under this condition.