I'm having a problem with the ADA4899 Disable function.

I'm using the ADA4899-1 in the CP-8-2 package.  When the DISABLE input is floated what does the output, pin 7, of the amplifier do?

I have a circuit with two amplifiers and want to mux them together.  The inputs to each amp are opposite polarity and I want to use the DISABLE to select the proper polarity for followup circuits.  The circuit runs off +/- 5 volts and each amplifier is configured as a unity gain buffer (pin 2 connected to pin 3).   Each amp by itself works fine when enabled.  When an amplifier is disabled (DISABLE input brought to ground) the outputs don't TRI-State as I thought they should.  Anyway when I connect the outputs of the two amps together the disabled amp is messing with the enabled amp.  Should I use a 10K feed back resistor rather then a short between pins 2 and 7?

Thanks

John

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  • Charly,

    I'm using a couple more of the the ADA4899 to buffer the input to an AD7322 A/D where I'm letting the DISABLE signal float.  The BW appeals to me and reduced # of different components on the PCB.

    So you think a 499 ohm in the feedback loop would accomplish the same thing as an 8.2 K in the input but wiht lower noise?  I'll try that on the next rev of the circuit board - right now its much easier to change the input resistor then the feedback one as the current PCB layout makes that difficult and messy.

    Thanks

    John

Reply
  • Charly,

    I'm using a couple more of the the ADA4899 to buffer the input to an AD7322 A/D where I'm letting the DISABLE signal float.  The BW appeals to me and reduced # of different components on the PCB.

    So you think a 499 ohm in the feedback loop would accomplish the same thing as an 8.2 K in the input but wiht lower noise?  I'll try that on the next rev of the circuit board - right now its much easier to change the input resistor then the feedback one as the current PCB layout makes that difficult and messy.

    Thanks

    John

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