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TINA/Spice: Unable to understand a part in the reference design provided in "Analog Engineer’s Circuit Cookbook: ADCs"

Prashant Dandriyal
Intellectual 790 points
Other Parts Discussed in Thread: TINA-TI, LMP7715, LMP7721

Tool/software: TINA-TI or Spice Models

Hello.

Recently, I was going through  "Analog Engineer’s Circuit Cookbook: ADCs" as provided among the cookbooks provided by TI. In this, I have been referring to the circuit shown on page 65.(shown below)

Within this, the guard ring shown in the left side seems very confusing. As per my observation, I see the outer ring connected to the output terminal of LMP7715 (U1) and one of the pH probe. Whereas, the inner ring is seen connected to U5. But there is no connection with the other probe of the pH sensor, which breaks the circuit (according to me). 

So, please tell me whether I am interpreting the circuit properly or not. Maybe I am unable to understand the guard ring completely. Please explain.

  • 0
    TI__Guru 62055 points

    Hello,

    Ground rings are often better visually realized by looking at an example PCB layout and it can sometimes be difficult to capture exactly what's going on in the schematic.  The LMP7721 features a really good example guard ring layout for high-Z input circuits like pH sensors.

    Since this circuit is single-supply the bias/reference input of the pH sensor is driven roughly 0.55V from the output of the U1 amplifier.  This connection would go to GND in a dual-supply circuit.  The other high-z sensor output only connects to the IN+ pin of U3.  The output of U5 drives a guard ring that surrounds the high-z sensor output.  This Guard ring looks to be guarded a second time by the bias voltage. 

    Be sure that you didn't accidentally connect the driven shield output of U5 to the high-Z sensor output as then the U1 and U5 outputs would short through the sensor and likely damage it.

    http://www.ti.com/tool/LMP7721MAEVALMF_NOPB

    http://www.ti.com/lit/ug/snou004/snou004.pdf