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.

ISO224: Impedance matching problem

Part Number: ISO224
Other Parts Discussed in Thread: , ADS8686S

I use ISO224 and ADS8686 to do the design, in THE ISO224EVM circuit, ISO224 after the operational amplifier to do 3 times amplification, but I do not need 3 times amplification, I hope ISO224 output directly access to the ADS8686 input,

The line length from ISO224 output to ADS8686s input is about 5cm;

1、 whether there will be impedance mismatch problem?

2、Will the acquisition accuracy be affected?

3、 Will it improve the acquisition accuracy?


  • Hello,

    The main thing that will need to be addressed is that the ISO224 outputs a differential voltage, whereas the ADS8686 inputs are single-ended.  You'll note in the ADS8686 datasheet that the VIN_nXGND pins are only meant to accept input voltages from -0.3V to 0.3V.  Therefore, an amplifier circuit, like the one shown in Figure 48 of the ISO224 datasheet will be required to convert the differential output to single-ended.

    Would you share a proposed block diagram, or schematic, of your circuit implementations?

  • Hi Zhang,

    I'm having a little trouble understanding your circuit from the description. Can you please attach a proposed schematic or block diagram for clarification? 

    The ISO224 has an input of +/-12V, an internal gain of 1/3 and an output common-mode voltage of VDD2/2.

    On the ISO224EVM, U4 provides an amplification of 3 such that the output swings similar to the input, +/-12V. This amplification is not necessary, however a differential to single-ended conversion is as Collin pointed out since the negative input of the ADS8686 is limited in swing and should be tied to the ADS8686 AGND. 

    This application note should help with the differential to single-ended conversion:

    We have several pieces of collateral to assist with ISO224 designs: SBAA317 will be very similar to the circuit you will need to design, however in order to utilize the full-scale range, AGND will be used instead of 1.65V for the differential to single-ended common mode VREF. Please note that an operational amplifier with bipolar supplies (+/-5V) will be necessary to utilize the full-scale range. 

    If you choose to do the differential to single-ended conversion without bipolar power supplies (0-5V) for the operational amplifier, a 2.5VREF should be used for the differential to single-ended common-mode. This will result in losing half of your full-scale resolution at the input of the ADC since it will be +/-5V, but will still be quite accurate.