ADG1408

good morning
I'm going crazy because I can't understand where I'm wrong, would you be so kind to explain it to me?
what I need to do is select the gain of an amplifier.
thank you

  • Hi,

    You have connected the EN pin to -5V (VSS). It should be connected to +5V(VDD).

    I first was also fooled by the littel "+" sign left to the "5" label on the wire connected to VSS and thought you had VSS at +5V, but this little "+" sign seems a positioning sign for the "-5" label.

    So you should disconnect the EN pin from the VSS pin and instead connect it to the VDD pin.

    Regards,
    Gerd

  • It's actually like you say, I made a gross mistake. I wanted to ask you a question: in the final circuit, the S3, S5,S6, S7 and S8 entries since they are not used and better to connect them to gnd? Thank you very much for the solution.

  • If you have other signals running close to those pins and want to reduce stray coupling, you might ground them.

    Otherwise, I wouldn't do that if it isn't necessary, because there is a finite capacitance across open switches and that would increase the capacitive load of the opamp output.

    Keep in mind that if the 1M resistor is chosen, there is still the 10k resistor with the switches off-capacitance connected in parallel. This results in a high pass like behavior. Just check if this happens in your target frequency range, or if there is enough margin between your max frequeny and the onset of the high pass like behavior.

  • ok many thanks, I'll check and let you know, I think that with the working frequency set at 50Hz it shouldn't be a problem.

  • Ok, with 50Hz you don't need to waste a single thought about the few picofarads involved there.

    As with 50Hz the parasitics are negligible, you might (in case you have enough space on the PCB) route the unused S pins to a few more resistors just like your three gain setting resistors. You won't have to populate them, but it's always nice to have the possibility if you should later wish to have a finer resolution (like intermediate settings with 31.6k and 316k or a 1-2-5 per decade setting like it is used with oscilloscopes).

    Regards,
    Gerd