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INA121 Data sheet has multiple figures with no writeups

Mark Fontz
Prodigy 135 points
Other Parts Discussed in Thread: INA121, INA111

I'm interested in using the INA121 in a capacitive bridge transducer circuit but there is absolutely nothing written about the circuit except the circuit diagram in FIGURE 11. I'm using 40KHz for Vac with matching 100K ohm resistors with the values of the capacitors around 0.8pF. I'm also power the in-amp with +/-5V. Problem is with the output of the transducers and the gain that I'd like to use. Are there app notes or something that I could refer to with this circuit. I've worked with in-amps before and I'm aware of the problems but this circuit just doesn't give me the gain I would expect.

Thanks, Mark

  • 0
    TI__Genius 12335 points

    Hi Mark,

    I'm not aware of any app notes on capacitive bridges at the moment but I did have a couple of thoughts on what could be causing you to observe less-than-expected gain. The common-mode input capacitance of the INA121 is 12pF typical, which will dominate the impedance of your active elements if they are indeed .8pF. Because the capacitive element forms a low-pass filter with the resistors, the additional capacitance may begin to roll off the output signal earlier than expected. Also, any parasitic capacitance in the PCB layout or connections from your transducer to the input of the amplifier will contribute additional error to this measurement. Lastly, if the INA121 is configured for very high gain, 40kHz may be outside of the flat portion of the amplifier's transfer function.

  • 0 in reply to John Caldwell
    Prodigy 135 points

    Hi John,

    Thanks for the reply on the INA 121. The 40kHz excitation can be changed as I've noted the curves on the Gain/Freq plots. The problem may well be the common-mode input capacitance of the INA121 dominating the circuit which I did miss. The two 0.8pF capacitors are in a MEMS circuit and the actual movement is much less in the femto range. Do you have and suggestions on other components that would work better in the circuit.

    thanks, Mark

  • 0 in reply to Mark Fontz
    TI__Genius 12335 points

    Mark,

    The INA111 has a wider bandwidth and a common-mode input capacitance of 3pF which is an improvement over the INA121. However, this low of a capacitance can easily be exceed by parasitics in the system, especially if there is a cable connecting the MEMS device to the input. You might be able to use common-mode driven power supplies as recommended in this Burr-Brown app note to eliminate the effects of the input capacitances of the amplifier itself: http://www.ti.com/lit/an/sboa014/sboa014.pdf

    I'm thinking that this approach would eliminate any voltage drop across the common-mode input capacitances of the amplifier, greatly increasing the input impedance of the part at high frequencies.

    If you give this a shot please let me know how well it works!

  • 0 in reply to John Caldwell
    Prodigy 135 points

    John,

    Thanks for the advice, the connection to the MEMS device is directly on the PCB but I'll look at both the INA111 and the app note to see if I can use these in the system.

    Thanks.  Mark

  • 0 in reply to John Caldwell
    Prodigy 135 points

    John,

    Looking at our vendors, it looks like there's a long lead time on the INA111, can you help with 1 sample?

    Mark

  • 0 in reply to Mark Fontz
    TI__Genius 12335 points

    Mark,

    Not a problem, I sent a friend request on here and if you accept it we can take the shipping details offline.