This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TLV333: TLV 333 output Impedance

Part Number: TLV333
Other Parts Discussed in Thread: INA199, TLV2333, TEST

Hi Ti guys:

I have used tlv333 to read motor current by differential detection,
but the maximum input impedance in MCU input port is less than 5KΩ,
 
It’s said that I can eliminate the influence of impedance by REF Potential according to the design of Ina199.Please could you share me what’s the value of REF PIN resistance to the ground or VCC resistor?
 
In my actual application, the supply voltage is 5V, the magnification is 50 times

  • Hi Jack,

    The INA199 datasheet in Figure 22 indicates that the internal resistor from the REF pin to the input of the difference amplifier is 1 MΩ. Driving the REF pin with an op amp connected in a low voltage gain such as a buffer (+1 V/V) assures that the closed-loop output impedance will be very low over much of the op amp's usable frequency range.

    You mention using the TLV333 to drive the INA199 REF pin. There isn't a graph of open-loop output impedance (Zo) in the TLV333 datasheet, but I do know that Zo is about 10 Ω at 1 kHz. The TLV333 has about 52 dB, or 4000 V/V, of open-loop gain (Aol) at 1 kHz (see Fig. 2 in datasheet). If the TLV333 is connected as a unity-gain buffer the closed-loop output impedance (Zcl) is about equal to Zo/Aol, or Zcl = 10 Ω/4000 = 2.5 mΩ. That resistance is added to the 1 MΩ, and the gain/CMRR error introduced by it is very small.

    One thing to be aware of is that both the INA199 and TLV333 use internal switching to minimize their voltage offset and offset drift. Their internal clocks are likely operating at different frequencies. There is the possibility that some low-level beat notes could be produced in the circuit. If you do decide to use both devices in the circuit it would be prudent to check the circuit for the presence of beat notes with a spectrum analyzer.

    Regards, Thomas
    Precision Amplifiers Applications Engineering
  • Dear Thomas,

    This is David of Gortech, I am customer of WTJack.

    I used TLV2333 OPAmp in my circuit board, and the resistor are 15k ohms and 300 ohms ( gain is 50 times ), it can trigger MCU AD channel to work, but when we test the circuit board, the 300 ohms resistor and TLV2333 is very easy burn out.

    ps: 1. the MCU supplier's suggestion, the impedence of AD channel must under 5k ohms, in case the MCU AD channel can not identify OPAmp's signal.

          2. ours circuit board is use in BLDC motor driver, ii's KW grade, so the current is very large.

    According Jack's suggestion, it should use 1M ohms and 20K ohms resistor to drive TLV2333 to work, in case the OPAmp burn out by rush current, I check up the INA199's datasheet in page 13, it suggest use two resistor and connect to Vcc and REF pin, I wonder how many value of these two resistor? according your feedback and suggstion, I will run actual test after I confirm the answer from your side, Thanks for yours help.

    簡報1.pptx

  • Hello David,

    "I used TLV2333 OPAmp in my circuit board, and the resistor are 15k ohms and 300 ohms ( gain is 50 times ), it can trigger MCU AD channel to work, but when we test the circuit board, the 300 ohms resistor and TLV2333 is very easy burn out."

    That is a not something (burn out) we would expect to occur with the TLV2333 under normal operating conditions. For the 300 Ohm resistor and TLV2333 to burn out there would have to be a high current flow into the TLV2333 inverting input. I am not sure why you would have in-rush current issues. There shouldn't be any significant current beyond input bias current flowing to the input.

    The only time that sort of damage occurs is if a high-voltage, electrical-overstress (EOS) condition is presented to the device. That can happen if an input voltage high enough to turn on the normally off ESD cells that connect to the supply rails get turned on. Increasing the input resistance from 300 Ohms, to 20 kohms, surely will limit the input current to safe levels.

    It is best I don't provide suggestions for the INA199 resistors because the product is fully supported by TI's Current Sensing Products group. They have an E2E forum where you can make inquiries and receive direct support for the product.

    Regards, Thomas
    Precision Amplifiers Applications Engineering