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TLV2772 Op Amp Problem

Vinh Nguyen71
Prodigy 30 points
Other Parts Discussed in Thread: TLV2772, TLV2771

Hello All,

I'm trying to make a PCB for a charge amplifier of a piezoelectric device using the circuit shown below. However, I'm using 1x TLV2771 (for buffering the 1/2VCC) and 2x TLV2772 (I'm using 4 piezoelectric devices, and I'm using one TLV2772 for each pair). I'm using the same Rf and Cf for all sensors, but for some reason, I'm getting 2 difference output voltages from the TLV2772's. For example, one TLV2772 is showing 1.78V and 1.69V at the output pins while the other TLV2772 is showing 2.21V and 2.12V! The crazy thing is that I made this exact circuit on a breadboard, and the voltages were all the same at the TLV2772 output pins (1.69V). In fact, on my breadboard, I used no decoupling capacitors and no effort to shield wires.

Is there something about the 8-SOIC package that I need to consider when designing/fabricating my board? I checked the resistance, capacitance, and 1/2VCC values of the charge amplifier circuits, and they all appear to be roughly the same for all op amp circuits (22Mohm, 1nF, and 1.41V respectively).

 

  • 0
    TI__Guru* 93105 points

    Hello Vinh,

    Have you checked each TLV2771 and TLV2772 output with an O-scope and 10x probe to see if any of the circuit stages are oscillating? If a high frequency oscillation is present it can produce erroneous dc measurement results.

    You mention you are using a TLV2772 to produce the 1/2 Vcc. Often, engineers will set up the op amp as a unity-gain buffer having a resistive voltage divider on the input, and connect a fairly large capacitance from the op amp output to ground. Are you using it this way too? If so the large capacitive load can lead to instability due to phase margin degradation. If you didn't include the capacitance in your breadboard, then you wouldn't have seen a problem. If you did include it in your final circuit, then the oscillation can usually be remedied by adding Rnull as shown in data sheet Figure 58, which is backed up by Figure 47.

    The 8-pin SOIC is a very popular package for the TLV2771 and TLV2772. It should not present any unique usage  challenges over the other package offerings.

    Regards, Thomas

    PA - Linear Applications Engineering