offset voltage problem with analog switch ADG333A, pin-compatible substitute?

Hello,

I've got offset voltages of 0,2V with a device containing ADG333ABNZ. Do the pin compatible newer versions like ADG1434 or ADG5434 correct this?

Best regards,

Simon Brem

  • 0
    •  Analog Employees 
    on Aug 19, 2011 12:55 AM over 9 years ago

    Simon,

    you should post your full schematics with data included to show what you call 0.2V offset.

    Switches do not have offsets, so its the circuitry around them that causes something here. Help us help you with detailed information.

    Thanks,

    AnalogXPRT

  • Hello AnalogXPRT,

    thanks for your fast answer.

    I can only provide a part of the schematic:

    I think, there isn't anything special. I use 5V logic to control, which of the analog signal source (A or B) goes to selected signals (on the right side).

    The approximately +0,2V offset appears always at the selected signals, that leave the analog switch, even when both signal sources are 0V.

    Signals are DC.

    edit: I've read, that there exists a parasitic capacitance at the output of analog switches. The output leads to a SPS, which has a very high input resistance. Could that be the reason for the offset?

  • 0
    •  Analog Employees 
    on Aug 27, 2011 2:42 AM over 9 years ago

    DLR,

    what's a SPS?

    The ADP333A has a switch resistance of 45ohm, so any current draw from the selected signal side will generate a voltage drop across the switch.

    0.2V/45ohm = 4mA. So if anything connected to the switch needs the 4mA then this explains the 0.2V offset.

    You need to either select a switch with a lower resistance or make sure there is no current draw, adding an amplifier after the switch may also help

    AnalogXPRT

  • +1
    •  Analog Employees 
    on Sep 13, 2011 6:21 PM over 9 years ago

    Hi DLR,

    Offsets usually appear when the analog switches are used in high impedance environments (Mohms) and it is mainly caused by leakage currents or charge injection.

    While leakage is always present and may cause static offsets ( the switch is not toggled, just kept into one position on or off) charge injection may cause dynamic offsets( when the switch is constantly toggled) by dumping charge on the parasitic capacitance of the switch pin plus any additional capacitance of the PCB and external circuitry. The direct effect of the charge injection is voltage spikes on the switch pins, but if they are averaged out it may lead to offsets.

    You need to establish the nature of the offset in your circuit and maybe choose a different switch.

    I would recommend trying the ADG1234 which is a low leakage, low charge injection device. If the circuit operates in high impedance environments (Mohms), the ON resistance doesn't really matter as it will always be much lower in comparison with the system impedance.

    Regards,

    FEB

  • I found out that the offset has nothing to do with the analog switches. It only appears if the signals are connected to an unpowered NI-6216 DAQ. If the device is powered up or the signal is not connected, everything is allright.

    Anyway  thank you for your help!