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LMP7721 gaurd connection in trans-impedence amplifier with offset.

Anita Gupta
Prodigy 10 points
Other Parts Discussed in Thread: LMP7721

I am using evaluation board for LMP7721 with single 5V supply in inverting transimpedence amplifier with offset. In this case the non-inverting terminal and gaurd would be at 2.5 V. This amplifier would be for ionization chamber with output with respect to ground (no gaurd connection). My doubt is whether to use

1) A BNC connection at input of evaluation board or

2) A traixal connection at the input of evaluation board and leave gaurd connection open at the ionization chamber.

or there is any other method to do the connections.

  • 0
    TI__Mastermind 49966 points
    Hello Anita,

    The input footprint was designed to accept either a triax or standard BNC connector (since PCB mount triax connectors are VERY, VERY expensive).

    We chose not to mount the connector to allow the user to decide which version to use - or just wire directly (and keep the EVM cost below $100!).

    Ideally you should guard up to the point where the input meets to the source. Guarding also reduces the effect of the cabling capacitance - so that can increase your response time by decreasing the effective input capacitance.

    You can use the standard BNC and cable - but then you get about 100pF per meter added in parallel with your sensor to ground. This will change your decay times.

    How far is the chamber? Is it within the same enclosure - or remote on a cable?

    Ideally - the amplifier is placed *at* the chamber output with minimal input connections. If you have to run three conductors to the chamber- it might as well be V+, amplifier output signal and ground. You don't need expensive triax cable and connectors to do that.

    Regards,