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INA199: output impedance

Part Number: INA199
Other Parts Discussed in Thread: TLV2333

Hi Ti guys: 

It’s said that I can eliminate the influence of impedance by REF Potential according to the design of Ina199( 8.4.3).

I need the output impedance less than 5KΩ.

Please could you share me what’s the value of REF PIN resistance to the ground or VCC resistor?

thx

  • Hello Jack,

    The circuit in section 8.4.3 is an example of how to cancel out the high-impedance reference voltage source, which is generated by the resistor divider network. You cancel out the effects of loading the REF pin by sensing the the output voltage as a differential signal, as opposed to sensing the OUT pin with respect to ground. Thus, ADC input signal floats with any variance in the actual, real-time reference voltage.

    I am unclear on what output impedance you are referring to, but you can determine the REF input impedance by referring to Figure 22 in INA199 data sheet where internal resistor values for the differential amplifier are listed.

    Please post back if you have more questions.

    Sincerely,
    Peter Iliya
    Current Sensing Applications
  • Hey Jack,

    So you can have as much information as possible, please review this article in our FAQ pages covering the effects of reference impedance.
    e2e.ti.com/.../3602.effect-of-external-resistors-on-vref-on-cmrrgain-error

    Sincerely,
    Peter Iliya
    Current Sensing Applications
  • Dear Peter,

    This is David of Gortech Industrial, I am Jack's customer, this question was I ask to Jack and TI, we are design Kilo-watts BLDC motor driver and used TLV2333 to detect motor rush current.

    We used EMC MTF551N BLDC MCU+TLV2333 OPAmp IC to drive BLDC motor, in this type of MCU ADC channel, it suggest input impedance must  lower than 5k ohms, if it is over than 5k ohms,  the MCU ADC channel will not identify the OPAmp signal.

    I tried 300 + 15K ohms resistor of differential OPAmp circuit for TLV2333 ( gain was 50 times ) , the MTF551N MCU ADC channel can identify  the OPAmp's signal, but the TLV2333 OPAmp IC will burn out by motor rush current ( the most motor rush current  will over 38 Amps ).

    I check up the INA199 datasheet, it suggest the differential OPAmp circuit  resistor, it is better between 20k ~ 1M ohms, in these range of resistor value, ours MCU can not identify the TLV2333 OPAmp signal, so how to cancel out the high impedance between MCU and TLV2333? I check the INA199 datasheet in page 13, it shows the circuit schematic to cancel out the high impedance, but it have no mark the resistor's value, I wonder the resistor value to cancel out the high impedance between MCU and TLV2333, Thanks for your help!!

    sincerely yours,

    David Wong

    Gortech Industrial Co. Ltd.

     

  • Hello David,

    Thanks for reaching out. I’d be happy to work with you.

    I can’t find the MTF551N online, but I’m assuming the ADC needs whatever is driving it to have an impedance of less than 5 kOhm. What this means is that the closed-loop output impedance (Zout) of the TLV2333 needs to be less than 5 kOhm. The TLV2333 has an open-loop output impedance (Zo) of 2 kOhms. You can calculate Zout has:

    Zout = Zo*Acl/Aol = 2 kOhms * 50/10^7 = 10 mOhms.

    Where Acl = closed-loop gain and Aol=open-loop gain.

    Overall, the size of the feedback resistors should not affect your Zout. I am curious as to why the TLV2333 is actually burning up from in rush current. Is it because 300 Ohms is too small. I’ll need to look more into this. I’m also curious how you are driving the ADC with the TLV2333 at gain of 50V/V. This means your closed-loop BW is 7kHz, which is pretty low when driving ADCs.

    The INA199 is already a closed-loop device and it’s Zout should be fairly low, definitely less than 5 kOhms. The device also has closed loop BW of 80kHz, 30kHz, and 14kHz depending on gain. Thus, you do not need a resistor in output of INA199 to cancel Zout since Zout is already low. This is what is shown on page 13 of INA199 datasheet. Additionally the INA199 has a higher BW a compared to TLV2333 so you could sample the ADC more quickly and not lose any settling time.

    If you would like to know what the internal resistors are for your gain variant, you can refer to page 15.

    Hope this helps.
    Peter Iliya
    Current Sensing Applications