Using buffer as input stage

Hi,

In my design, I'm using a 12-bit ADC with SPI communication output (AD7896). Before
that, I'm using Single 16-to-1 Multiplexer (ADG707). I'm measuring 0-5V slow change
DC levels. 

My question is should I use an op amplifier with gain 1 (buffer) before the mux? Can you recommend
one? What is the most recommended configuration for the buffer (high resistance
between the minus net to the output, parallel capacitor, high input resistance…-
I need the circuit to be the most robust it can be)

    

Thank,

Idan

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  • 0
    •  Analog Employees 
    on Jan 14, 2013 12:12 AM

    Hi Idan,

    I believe you are considering buffers for isolation of the signal sources to the ADC. May I know the output impedance of your signal sources? According to the AD7896 datasheet, the AD7896 allows a maximum of 1kΩ output impedance driving the AD7896 inputs. Therefore, you just have to make sure that your ADC sees less than 1kΩ impedance at its inputs (this includes signal source impedance, Ron resistance, and PCB trace resistance). However, if you are not sure of the output impedance of your signal sources, I suggest you use a buffer AFTER the multiplexer. This simplifies your design and eliminates the need for individual buffer at each signal source. A simple voltage follower configuration of ADA4000-1 could be used for your application. The amplifier choice is still dependent on your design considerations such as bandwidth and power supply voltage.

    I hope this helps.

    Regards,

    Neil

Reply
  • 0
    •  Analog Employees 
    on Jan 14, 2013 12:12 AM

    Hi Idan,

    I believe you are considering buffers for isolation of the signal sources to the ADC. May I know the output impedance of your signal sources? According to the AD7896 datasheet, the AD7896 allows a maximum of 1kΩ output impedance driving the AD7896 inputs. Therefore, you just have to make sure that your ADC sees less than 1kΩ impedance at its inputs (this includes signal source impedance, Ron resistance, and PCB trace resistance). However, if you are not sure of the output impedance of your signal sources, I suggest you use a buffer AFTER the multiplexer. This simplifies your design and eliminates the need for individual buffer at each signal source. A simple voltage follower configuration of ADA4000-1 could be used for your application. The amplifier choice is still dependent on your design considerations such as bandwidth and power supply voltage.

    I hope this helps.

    Regards,

    Neil

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