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simple question on guarding the LMP7721

Pawel Wierzba
Prodigy 30 points
Other Parts Discussed in Thread: LMP7721

In his interesting article: Design femtoampere circuits with low leakage - Part 3: Low-current design techniques Paul Grohe writes:

The LMP7721 has a unique pinout that separates the input pins (pins 1 and 8) from the power and output pins with guard pins (pins 2 and 7). These pins are connected to the guard to provide guarding all the way down to the lead frame level.

A look into the datasheet reveals something different:

The input pins should be fully guarded as shown in Figure 7.

Figure 7 shows that the guard ring is routed between pins 1 and 2 as well as between 7 and 8. This may imply that the N/C pins are in fact "do Not Connect anything here" pins, at least when one reads only the datasheet.

Can anyone clarify this issue?

And a bit more general question about the N/C pins in general. Where one can find out whether they are: "No internal Connection" or  "do Not Connect anything"?

.

 

  • 0
    TI__Guru 61796 points

    Hello Pawel,

    Pins 2 and 7 are NOT connected internally, and can be used to guard the input pins down to the leadframe level (down to the die).

    Experiments with the LMP7721 eval board showed a very slight improvement with the pins connected to guard vs. floating, so I recommend these pins be used as guard pins.

    I did not write the datasheet, and would have shown it differently. I'll look into changing the datasheet.

    Regards,

  • 0 in reply to Paul Grohe
    Intellectual 625 points

    how to use this guard pins and how to make guard ring on pcb  can please explain in detail?

    i have got 5mv noise with lmp7721 circuit explained in designing with ph probes pdf.

     

  • 0 in reply to as
    TI__Guru 61796 points

    Ism,

    5mV of noise is expected. The LMP7721 has a 17MHz bandwidth, so the noise bandwidth is high. pH probes are not fast devices, so you do not need all that bandwidth.

    Instead of a straight follower as shown in the schematic (which is simplified), replace the short between "- Input" and the Output with a parallel 100K resistor and 0.1uF cap - this will limit the BW to 15Hz, which should be plenty of bandwidth for a pH probe, and also attenuate some line noise.

    The "Guard" trace should be at a potential equal to the average of the input voltage. Since this is a follower, you can connect the guard to the output, though it is a good idea to slightly attenuate the output (to keep the overall gain slightly less than one - to prevent oscillations). This can be done with a 100ohm and 10Kohm resistor attenuator. The guard would be driven by the tap point.

    The guard should encircle all of the input traces and pins.

    See the LMP7721 demo board manual, as well as my "Femtoamp" articles in EDN, and read section 2 of the Keithley Low Level Measurements Handbook.

     http://www.ti.com/litv/pdf/snoa513b

    http://www.edn.com/design/analog/4368681/Design-femtoampere-circuits-with-low-leakage-part-one 

    http://www.edn.com/design/analog/4375459/Design-femtoampere-circuits-with-low-leakage---Part-2--Component-selection

    http://www.edn.com/design/analog/4395651/Design-femtoampere-circuits-with-low-leakage---Part-3--Low-current-design-techniques

    http://www.keithley.com/knowledgecenter/knowledgecenter_pdf/LowLevMsHandbk.pdf

    Regards,

  • 0 in reply to Paul Grohe
    Intellectual 625 points

    Thanks Paul. Can you please draw the scheme? I am confused.