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MSP430FR2355: PGA output oscillates at high gain

Sean Quilliam
Prodigy 125 points
Part Number: MSP430FR2355


I have two of the SAC modules configured in the PGA stage, non-inverting. 

The one stages feeds into the other allowing me to have a gain > 33.  I require a max gain of ~100. 

The issue that I have is that the output stage of the cascaded PGAs oscillates when I start to increase the gain.  There is oscillation when the one stage is set to a gain of 1 and the other is increased, when the one stage is set to 1 and the other max (33) the oscillation is starting to overtake my signal level of around 700mV @ ~40kHz.  This is not due to A2D sampling as there is no A2D enabled yet.

I have reduced the output and speed of the gain stages (SAC3OA |= OAPM_1; //Select Low Speed and Low Power mode) this makes a difference but doesn't solve my issue. 

My design is on a custom PCB so little noise is entering the system.  Ideally what I need to do is place a capacitor in the feedback loop of the PGA stage but this doesn't seem possible??

Is there anyway that I am able to reduce the high frequency oscillation (~40kHz)?

  • 0
    TI__Expert 7365 points

    Hi Sean,

    Do you want to distribute the gains across the 2 cascades PGAs? It is possible that the high frequency oscillation only occurs at the high gain setting. One example could be using the 5*5 gains in the non-inverting mode for 33 and using 9*9 or 9*17 for the near 100x gain?

    Have you also checked to make sure that for the gain you are setting, your signal frequency is within the module's GBW (Gain Bandwidth). The datasheet lists the typical value as 1 MHz when OAPM = 1. This is in Table 5-25. SAC, OA, section 5.12.11 at www.ti.com/.../msp430fr2355

    Srinivas

  • 0 in reply to Srinivas Lingam
    Prodigy 125 points

    Hi Srinivas

    Many thanks for you suggestion. 

    I have since done some more investigating and have found that the 40kHz instability is coming from another part of my circuit that feeds back into the primary stage before getting to the PGA stages.  It was a bit odd that an opamp of GBW of around 1MHz would oscillate at such a low frequency. 

    Now that I have isolated the issue with the 40kHz I still have the issue of the higher frequency noise that gets amplified due to the increase gain of the PGA.  I do put the output of this through a 4 pole AA filter however I still don't like the idea that the noise is getting amplified....  It would be great if I could just get a ~47pF cap in the PGA feedback loop to reduce the high frequency being amplified...

    Regards,

    Sean

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