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LOG112: Incorporate guard traces for the log112

Qi Liu
Intellectual 2980 points
Part Number: LOG112
Other Parts Discussed in Thread: LOG114, LOG114EVM

Hi, support team

My customer has the questions as follow:

I'm trying to design a circuit involving the LOG112.
I'd like to set up two photodiodes that perform an absorbance measurement, as in figure 10 of the datasheet, but I'm having trouble envisioning how guard traces would fit into the circuit.
Is there a reference design I can look at that provides some insight into how guard traces would be incorporated for two photodiodes?

Thanks so much.

Best regards,
Yuki

  • +1
    TI__Guru 54416 points

    Hi Qi,

    Unfortunately, there is no immediate PCB reference design specific for the LOG112. The LOG114 is another log ratio amplifier offering an evaluation board (LOG114EVM).   The LOG114 is a higher speed logarithmic amplifier capable of measuring over 8 decades.  Figure 10 on p. 18 of the LOG114 datasheet shows another circuit example to measure absorbance of two matched photodiodes.

    In the board layout, ideally, a complete guard ring is implemented with traces that encircle the complete signal path of each input diode current path to the high impedance inputs of the log amplifier to prevent leakage, when performing low input current measurements.  The guard ring must be biased at a potential close to the input path voltage (in this case the reference voltage, Com voltage), and is typically driven with a low impedance source. Another factor to consider when measuring small currents is to ensure that the PCB board is completely clean and free of flux residue and contaminants.  On our evaluation boards we use a deionized water, ultrasonic bath clean for 15 minutes to ensure complete removal of the flux underneath the devices. However, in your PCB, the appropriate flux removal process may vary depending on the type of flux used.

    The LOG114EVM layout example incorporates only partial guard rings apparently due to the compromise to support different circuits on the EVM. Please check this post discussion and section 3-41 of the attached document for more information on a guard ring design:

    Guard Ring.pdf

    https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/274139/log114-photodiode-protection

    The LOG114EVM User’s Guide shows the layout plots on figure 15-17

    Thank you and Regards,

    Luis