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Problem with LMP7721 for Charge Amplifier

CJ Liu
Prodigy 30 points
Other Parts Discussed in Thread: LMP7721

Hi,

I use the LMP7721 to build a charge amplifier. Two 560K resistors are used to divide the 3V single supply. So I get the 1.5V as the commen mode voltage and have it connected to pin 'V+'. Feedback Rf of 1G ohm and Cf of 10 pF are connected in parallel between pin 'V-' and pin 'Vout'. Pin 2 and Pin 7 are used to build the Guard. Guard trace is connected to pin 'V+'. The charge amplifier design refers to Figure 6 of OPA129 Datasheet.

The problem I encountered was the potential of pin 'V-' is 0.9V, not equivalent to that of pin 'V+' 1.5V. This may cause LMP7721 saturated, and thus we can't get the right signal output.

Happy to hear your advice.

Thanks,

Eric

  • 0
    TI__Mastermind 49976 points

    Hello CJ,

    it sounds like you have some of the pin names confused. "V+" and "V-" are the supply pins.

    I will assume you mean that "V+" equals the positive input (+IN, pin 1) and "V-" equals the non-inverting input (-IN, pin 8). If this is not the case, then you have the wrong pins connected!

    Just to verify:

    Pin 1 (IN+) should be connected to the 560K divider resistor tap.

    Pins 2 & 7 should be tied to Pin 1 (+IN) as a guard.

    Pin 3 (V-) should be grounded

    Pin 4 (OUT) is the output

    Pin 6 (V+) should be tied to the 3V supply voltage

    Pin 8 (-IN) should be connected to your sensor and the feedback R & C back to the output (pin 4).

    What is your sensor impedance? It could be loading down the summing node.

    What if you remove the sensor? Does the output go to 1.5V?

    I assume you have properly cleaned the board. The board needs to be scrubbed with clean alcohol and dried to remove all traces of flux and grease/oils. Burnt flux is conductive. Be sure to get between the pins and under the body.

    I also recommend placing a 0.1uF or larger cap from +IN (resistor tap) to ground to filter the resistor noise and supply noise. The "+IN" node is also the "guard", and should surround all the input circuitry.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 30 points

    Hi Paul,

    Thank you. 

    I used the wrong pin names. 'V+' and 'V-' I wrote should be 'IN+' and 'IN-'.

    Pin 1 (IN+) was connected to the 560K divider resistor tap.

    Pins 2 & 7 were tied to Pin 1 (+IN) as a guard.

    Pin 3 (V-) was grounded

    Pin 4 (OUT) was the output

    Pin 6 (V+) was tied to the 3V supply voltage

    Pin 8 (-IN) was connected to your sensor and the feedback R & C back to the output (pin 4).

    I used the piezoelectric sensors with giga ohm impedance/open circuit.

    I power up the 7721 PCB without connecting the sensor. "IN+" is 1.5V, "Vout" is 1.5V. But "IN-" is 0.9V. I've tested two 7721 PCBs. Both showed the same problem.

    I will try the 0.1uF or larger filter cap. Yes, "IN+" tied to Pin 2 and 7 are used for guarding. Guard trace surrounds the input pad.

    Thanks.

    Chaojun

  • 0 in reply to CJ Liu
    TI__Mastermind 49976 points

    Hello CJ,

    You are probably measuring 0.9V on -IN because of the loading by the meter impedance (meter is probably around 500 Megohm). I would not worry about it...You cannot simply measure the -IN node without affecting the readings - the impedance is too high.

    If you are seeing 1.5V on the output with no sensor, then the circuit is working fine.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 30 points

    Hi Paul,

    Thank you for your advice. The circuit did work well with 100 pF feedback capacitance. I got the supposed output after adding excitation to the sensor. The amplitude of the output signal is less than 0.1 V.

    However, I got another problem. The output signal is proportional to 1/Cf. But when I changed the feedback capacitor to 10 pF,  the amplitude of the signal is not amplified by 10 times. The output signal is disordered and it seems that the op amp was working in an unstable mode. What's the possible problem?

    Best regards,

    Chaojun