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INA1620: Recommendation for Multiple Fixed Gain Settings

Part Number: INA1620
Other Parts Discussed in Thread: TINA-TI

In the schematic provided in the INA1620  Datasheet for Microphone preamp, there is a potentiometer provided to se the gain from between 6db and 40dB.   I have an application that requires multiple fixed gain values, currently: 0dB, +20dB and +40dB.  Not being an expert on this part, are there any considerations I should worry about in regards to stray capacitance, or other factors, when selecting either a digital potentiometer, or digital switch which would switch in different fixed resistances at that location?  Thanks!

  • Hi Robert,

    I see two issues:

    1. Digital pots can show big stray capacitances between the terminals and from each terminal to signal ground. Since they directly connect to IN-A and IN-B of INA1620 which are inverting inputs of OPAmps, this can cause stability issues by ruining the phase margin. So, very probably you have to add stabilizing phase lead capacitors from the -input to the output of each internal OPAmp.

    In any case you should look for a digital pot showing very low stray capacitances.

    2. Because there's always an unwanted internal capacitive stray coupling from the digital control lines to the analog pot section, you may suffer from digital feedthrough noise. So, you should isolate the control lines of digital pot from the digital bus of microcntroller by the help of low pass filters at the control inputs or by using opto couplers. Then it can be helpful to choose a digital pot with Schmitt trigger inputs. The supply voltage of digital pot should also be gained from the analog section.

    Since the digital pot would sit in the above circuit at the weakest point of signal path (lowest signal) and is most vulnerable against the digital feedthrough, I would use the digital pot not in the first gain stage but in the second and would preamplify the microphone signal before running it through the digital pot stage.

    Kai

  • Hello Robert,

    To add to Kai's comments, check the digital potentiometer for max current and voltage levels and be sure that amplified signal passing through stays within the specified bounds of the digital potentiometer to avoid signal clipping. 

    Best Regards, 

    Chris Featherstone

  • Thank you for the quick reply and for the insight on this.  First time on a forum like this, so glad to know I have resources like yourself watching out for me.   

    Based on potential issues with Digital Potentiometers would you feel digitally controlled switches would have the same issues?

  • Thanks, Chris!   Will do.  Trying to develop a microphone pre-amp with those preset gain settings, and just trying to figure out the cleanest way to do it from CPU control.

  • Hi Robert,

    digital controlled switches also suffer from stray capacitances and digital feedthrough, especially when you want to implement more than a couple of gain settings.

    I have run a TINA-TI simulation of the INA1620 microphone circuit and am optimistic that you can run the INA1620 stably with a digital pot like the high voltage "MCP45HV51" or similar. This digital pot shows 75pF stray capacitances from each end to signal ground and 120pF from the wiper to signal ground. Without the adding of phase lead capacitors you would see the following result:

    As you can see there are some peaks in the frequency response and ringing in the transient response (input signal is an 1mV peak 100kHz square wave), clearly indicating that the circuit has a severe stability problem.

    But when adding two 120pF phase lead capacitors (C1 and C5) the stability of the circuit heavily improves:

    robert_ina1620.TSC

    R3 simulates the digital pot and varies between 1R and 10k in decades' steps.

    It can be helpful to mute the output signal of INA1620 during changes of the digital pot settings. See this very nice appnote from the awesome Rod Elliott:

    https://sound-au.com/articles/muting.html

    Kai

  • You guys are seriously righteous!  Thanks for your insight on how I can address these types of issues going forward using TINA. I have seen the TINA app, but never worked with it.  The file you provided will provide a starting point for me on this circuit, and probably many more.   Thank you all for your support!