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OPA369: Matching a differential MEMS microphone output to an MSP430 ADC input

Ted Mawson 3.0
Expert 1795 points
Part Number: OPA369
Other Parts Discussed in Thread: OPA320, OPA191, OPA316

I'm using an Invensense ICS-40619 MEMS Microphone which has a differential output and (in low power mode) a differential output impedance of 5.5 k Ohms and an output level approaching 0 dBV at 130 dB SPL input).  The Invensense datasheet insists that the differential outputs need to be isolated with 2.2 uF (minimum) caps.  My application will sample the ADC input to gather data that will be used to capture low frequency signals (50 - 100 Hz) and I'm thinking that gain in the range of 5 to 10 times will be needed.

I'm looking at the OPA369 datasheet and wondering if I can use it in the way shown in the circuit below, not sure if I need C1 and C2 is clearly providing frequency rolloff.  One issue is that I want all of this to run off a single rail supply of +1.8 volts.

  • 0
    TI__Expert 3350 points

    Hello Ted,

    The circuit should work if you design it with for the specification of your application. Please use the attached TINA simulation file as a starting circuit. Please use the design equations in section 5 of the attached apps note (SLOA152 "Analog Active Audio Filters) "Analog Filter Implementation: second order Filters". On page 5 please use the formula for differential inverting configuration and select the components for the gain and the corner frequency per your spec. You can download TINA simulation software for TI website. Thanks for using TI products!

    Analog Active Audio Filters_sloa152.pdf

    OPA369_Diff_interface.TSC

  • 0 in reply to Bahram Mirshab
    Expert 1795 points

    Bahram,

    Thanks for the reply.  I downloaded the circuit but I don't understand why there are the 2 voltages V1 and Vocm included - these obviously bias the inputs to be centered on the available power supply but are these actually needed in the physical circuit or are they added to make the SPICE simulation work?  If they are needed in the physical circuit, how do you propose I generate them?

    Ted

  • 0 in reply to Ted Mawson 3.0
    Expert 1795 points

    Added picture so others can see and understand the issues

  • 0 in reply to Ted Mawson 3.0
    TI__Expert 3350 points
    Hello Ted,

    When using a single supply opamp, the signal needs to be shifted (biased) to a DC level to avoid clipping. In the simulation circuit, you have the option to add a DC level to the signal source, but it would be lost pass the 10 uF caps (AC coupled path). The Vcom and V1 are place holder for biasing sources, you may need to generate the bias voltage differently (voltage divider, etc). Please use the circuit as a template and tailor it for your application.
  • 0 in reply to Bahram Mirshab
    TI__Mastermind 23050 points

    Hi Ted,

    A few things to add to what Bahram has said:

    1) If you simulate the circuit you presented you will notice the cutoff frequency does not match what is expected and there is a large amount of distortion at the output. The OPA369 is not a good choice for this application, especially if you're driving an ADC. Its output impedance is very large and this will significantly impact your response. We have many other op amps that will work better depending on what specs are important to you for this application. I have attached a revised example using an OPA320 below.

    2) You cannot apply a reference voltage to the inverting terminal, or you effectively eliminate your feedback. The proper way to bias a differential amplifier is at the reference node as shown in the attached schematic. This reference voltage needs to come from a low impedance source, such as an LDO or the output of an op amp as I have shown.

    3) 0dbV from your sensor equates to 1.414V peak. If you are using a single supply of 1.8V and thus biasing your amplifier to mid supply (900mV), you wouldn't even have the headroom to pass the signal with a gain of 1. To achieve a gain of 10, your best option would be to use a dual supply configuration of +/-15V. You may consider looking at something like the OPA191 to do this.

    OPA320_Diff_interface_ZK.TSC

    Regards,

    Zak Kaye

  • 0 in reply to Zak Kaye
    Expert 1795 points

    Zak,

    1. I appreciate your advice to use the OPA320 but I'm concerned that the OPA320 has a quiescent current of 1.7 mA where the OPA369 draws an order less than that.  I'm using a switching supply to drop the CR2032 voltage from 3 volts to 1.8 volts so there's around 675 mWHr available and at 1.7 mA the OPA320 would draw 3 mWHr which is OK but not great.   The ADC input I'm considering has a sample and hold with an input impedance of 2.2 k Ohms and doesn't the fact that there is a lot of feedback drop the effective output impedance of an Op Amp to a low value?  Is there a way to calculate the output impedance of the OPA369?  Or maybe there's another Op Amp you can suggest?

    2. I had already figured out that using an Op Amp as a voltage buffer was a great way to generate the reference voltage, I did it with a second OPA369 with a capacitor on the output to reduce noise.

    3. Tests on my mic eval board indicate that the voltage out is only around 500 mV pk to pk, it would have to be next to a rocket engine to give 1.4 volts peak so I was thinking of starting with a gain of 3 even if that meant it would clip at high sound levels - the application is to gather low level sound.

    I'm unfamiliar with TINA but am well acquainted with LT Spice.  I have loaded your circuit and run some analysis.  When I look at the plots eg AC Bode, how do I know what points in the cct are the input and output? There's no legend on the chart?

    Thanks,

    Ted

  • 0 in reply to Ted Mawson 3.0
    TI__Mastermind 23050 points
    Ted,

    1) You are correct, closed loop output impedance is open loop output impedance divided by 1 + Aol*Beta, where Beta is your feedback factor. However, the OPA369 still has an exceptionally high output impedance that rises in your frequency range of interest, which may cause stability issues when you attempt to drive the ADC. You can observe the impact of this output impedance on your response by placing a voltage controlled voltage source at the output of the op amp inside the feedback. Perhaps the OPA316 would work in your application, its quiescent current is only 400uA.

    2) The OPA369 cannot drive capacitive loads very well, so I suspect adding that capacitor will cause stability issues. The OPA316 should perform significantly better in this regard.

    In the diagram window you can go to view -> separate curves, or you can click on the icon with a question mark pointing to a sine wave and it will label whichever curve you click on next. The icon directly to the right of that one will add a legend to your plot.

    Best,
    Zak Kaye
  • 0 in reply to Zak Kaye
    Expert 1795 points
    Thanks,

    I ended up using a single-ended analog microphone (Knowles SPW0442H) with an OPA316 buffer amp; I built the front end by hand and tested, performance seems good.