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INA149: INA149 for high side voltage measurement

Brendan Hayes-Oberst
Prodigy 60 points
Part Number: INA149
Other Parts Discussed in Thread: OPA186, , INA143, OPA333

Hello, 

I need to measure a battery voltage (between 0 and 5 V) as shown in the inputs to the INA149 on the top of the attached image. This battery negative or positive can be connected to 12V at anytime or be open. This is done via relays not shown. The output of the INA149 then needs to be fed into a 24 bit ADC for measurement. Max input to this ADC is 3.0V. Therefore, I trimmed the voltage down by 1/2 through the OPA186. Would this be a highly precise way of measuring this battery's voltage or could another solution be better? Would a current sense amplifier be good for this application? I want to measure the battery voltage well under 1mV of error. 

Thank You,

Brendan 

  • 0
    TI__Mastermind 25488 points

    Brendan,

    1.   A voltage divider is a common approach to this issue.  It will have a gain error and drift associated with the tolerance of the divider.  If you are doing a calibration then the gain error can be eliminated.  A tolerance of 0.1% and drift of 20ppm/C is a common choice to minimize the error while still maintaining reasonable cost.  If you are not calibrating the error could be as large as 0.2% for this case.
    2. One alternative that will give you better accuracy is to use an INA with attenuation.  In this case case the INA internal resistors are trimmed and also have matching temperature coefficient.   This means that the gain error and drift will be much smaller than the discrete voltage divider approach.  The INA143 is an example of a device that has a gain of 0.1 option.  This is smaller than your target gain but if you are connecting to a 24 bit ADC, you will get good accuracy without using the full input signal range.  Furthermore many 24 bit ADC have internal PGA, so you can gain the signal up if needed.  See example schematic below
    3. The OPA186 is optimized for a wider supply.  If you are driving a an ADC with a 3V input you should consider using an op amp with a 3V supply option that  is optimized for lower supply voltages.  In the example below I show the INA143 with OPA333.  A filter is placed between the two devices.  The filter is used to limit the bandwidth but will also provide some protection between the INA143 and OPA333.  Always keep in mind that a device with high supply voltages can output a voltage equal to the supply.  This can damage the low voltage devices.  So, for example, the INA143 could have a transient output of +/-15V during start up or if the input floats.  The 10k resistor between the two devices should provide some protection.
    4.  One other option would be to place the OPA333 in gain.  You could do this if your 24 bit ADC doesn't have an integrated PGA.
    5. Below is the schematic, and TINA source file.  I hope this helps.

    Art Kay

    gain_p1_3p3out.TSC

  • 0 in reply to Art Kay
    Prodigy 60 points

    Hi Art, 

    Regarding solution 1, I have provided a picture below and basically, when the relays are totally open, there is no reference from the battery to the system. Therefore, adding a voltage divider to ground will set the negative side of the battery to gnd - batV/2. I could then feed this into an op amp but would have to prevent it from inputting negative voltages into my ADC. For this reason, and because I want a very high impedance solution, I have distanced myself from this solution.

    Regarding the rest of the post, I do not think the INA 143 would work because it is not a high common mode op amp. My battery needs to be entirely open, or have the negative or positive side connected to 12V at anytime. Therefore, at some points my battery positive will be up to 17V relative to system ground, above the positive supply rail I have available. 

    I chose the INA149 because it can have an input put into it that is above the positive rail. It is also high impedance. Please let me know what you think. I was also thinking about a current sense amplifier because some of them have high common mode input as well. 

    Thank You,

    Brendan 

  • 0 in reply to Brendan Hayes-Oberst
    TI__Mastermind 25488 points

    Brendan,

    Your original INA149 will work well, but I suggest you use an amplifier optimized for 3V operation to drive the 24 bit converter.  You are correct that the INA149 will give you a large common mode range.  The voltage divider at the output can be selected to assure that the input voltage to the 3V amplifier is less than or equal to 3V.  In the example below I show a divider that will output 3V for a 15V input.  You can then add gain to the 3V amplifier so that your 5V signal gives a larger output voltage.  In this example I gave the OPA333 a gain of 2 so that the output is 2V when the INA input is 5V.  Note that this circuit will never give an output larger than 3V for any input so this will keep your ADC input is a safe range.  Keep in mind that the voltage divider will introduce some gain error so choose precision resistors for the divider and amplifier gain.  Note that you may also choose a buffer configuration for the amplifier as this will not introduce any significant gain error.   

    ina149-div.TSC

    Best regards,

    Art

  • 0 in reply to Art Kay
    Prodigy 60 points

    Hi Art, 

    Thanks for the advice. I will use a 3V op amp for driving ADC. One other thing I am unsure of I will mention here. What if the user plugs in the battery backwards? It would be nice to still be able to read that voltage and for it to not hurt the system. In the current configuration, I believe the INA149 would output a negative voltage and this would then go into the OPAx333, correct? This would then hurt the OPA333. I am looking for a design way around this. 

    Thank You,

    Brendan 

  • 0 in reply to Brendan Hayes-Oberst
    Prodigy 60 points

    Hi Art, 

    I could just add an offset to RefA and RegB and shift my output. Is there a way to make this op amp higher impedance? I think it is around 800kohm right now. If it was 10Mohm or higher would be great. Could another op amp work that has higher impedance? 

    Thank You,

    Brendan 

  • 0 in reply to Brendan Hayes-Oberst
    TI__Mastermind 25488 points

    Brendan,

    Yes, you can add an offset to RefA and RefB to shift the output.  However, this offset needs to be from an amplifier, reference, or another low impedance source.  In other words, you can't just use a voltage divider as the impedance of the voltage divider will interact with the input impedance of RefA and RefB.  You can use a voltage divider to buffer the reference.  See https://www.electronicproducts.com/selecting-the-right-op-amp/ for more info on this.

    Increasing the input impedance is difficult for this case.  You cannot just add external resistors in series with the input as this will impact the circuit gain.   You could use a simple op amp buffer but in this case your common mode is limited by the buffer amplifier so the main feature of the INA149 (wide common mode) is no longer valid. 

    Can you draw a diagram showing how the INA would connect to your battery?  Can you provide source impedance values?  If I understand your source configuration better I may be able to provide a better recommendation and also can estimate loading errors from source impedance.

    Best regards,

    Art

  • 0 in reply to Art Kay
    Prodigy 60 points

    Hi Art, 

    Would I be able to share it on a forum that is not open to the public? I will type out more information in the meantime while waiting to hear back. 

    Thank You,

    Brendan 

  • +1 in reply to Brendan Hayes-Oberst
    TI__Mastermind 25488 points

    Brendan,

    Send me an E2E friend request.  I will send you may email in a private message and we can take this offline.  I will close this thread. 

    Art

  • 0 in reply to Art Kay
    Prodigy 60 points

    Sent. Look forward to speaking with you.