EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: LoveLaika on September 23, 2020, 02:44:06 am
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I'm trying to design a transimpedance amplifier, and one of the recommended design guidelines suggest to use a guard ring. Now, i gave it a shot and came up with this below. I laid out a no fill zone in my copper plane, and I ran a guard ring with vias around my high impedance input, at the inverting node of my op-amp. The ring is tied to the non-inverting input, which is itself tied to the ground plane through traces extending from the ring and connecting to the ground plane. You can see the results below. Is this the proper way to implement a guard ring? Datasheet and guides say that the ring should be tied to the reference potential, and i did that with the op-amp input and tracks to the ground plane, but is that correct?
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Simply exposing GND is not a guard ring. In your case it simply does not make any positive difference. From leakage to what does it guard, to GND input signal is surrounded with anyway? And what is that supposed to do on the bottom side where is surrounds no signal?
In this case exposing GND would help only if there was some other voltage nearby but there is none. Only a short section where it separates -VCC it makes some sense.
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My guard ring is supposed to protect the op-amp inverting input node from leakage current. I've posted my schematic below. Following the guidelines of the AD795, my op-amp, Figure 34 on the datasheet has a guard ring that's tied to the non-inverting terminal, which is connected to ground. That's what I'm trying to accomplish in my schematic. For all intents and purposes, in my schematic, Io is the reference/bias voltage to which all other voltages are referenced to. That is what i refer to as ground.
I've also included some other links of guard ring notes that I tried to incorporate. You can see in link 2 (figure 6) the same idea. Link 3 is an op-ap reference that I also drew from. I included it on the bottom side as well in order to help strengthen the ring by covering both sides of the PCB. If what I have now doesn't do anything, what am I doing wrong, and what can I do to prevent leakage currents, if not by following what the datasheet says?
Link 1: https://www.analog.com/media/en/technical-documentation/data-sheets/ad795.pdf (https://www.analog.com/media/en/technical-documentation/data-sheets/ad795.pdf)
Link 2: https://www.analog.com/en/technical-articles/layout-for-precision-op-amps.html (https://www.analog.com/en/technical-articles/layout-for-precision-op-amps.html)
Link 3: https://www.analog.com/media/en/technical-documentation/user-guides/ADA4530-1R-EBZ_UG-865.pdf (https://www.analog.com/media/en/technical-documentation/user-guides/ADA4530-1R-EBZ_UG-865.pdf)