Another Guard Ring Post - Page 1
Forum section: electronics / projects

author: LoveLaika
author_rank: Frequent Contributor
timestamp: September 24, 2020, 02:04:34 am
content: I was told to try and submit this on a separate thread. Now, here I am doing so.Previously, I was trying to repair a transimpedance amplifier (TIA) for an STM device, but in doing so, it turned into a replacement job. Thus, I'm trying to build one from scratch. This TIA will turn a pico-amp current into a voltage, which is then passed to another preamplifier off board. That is out of my scope. I am just building the TIA. For reference, my reference TIA schematic is below, labeled as 'ref Schematic'. That is what I'm basing my TIA off of. "TIP" is the connection where current is coming from the STM, "It" is the output voltage, and "Io" is a reference voltage. +Vcc and -Vcc are supply voltages measured with respect to "Io", so for the purposes of the TIA, "Io" is effectively ground, the reference potential.Using some sample current measurements provided, I used LTspice and used an ideal op-amp model to compare output voltages amongst different op-amp models, focusing on low input bias currents (and making sure that the op-amp will fit the supply voltages). Thus, I came up with the following schematic, 'full schematic'. Based on my calculations, the AD795 is the op-amp I'm using having a low input bias current. In SPICE, when I compared the output voltage to the ideal output using the same test current, there was only a few nano-volts of difference between the two voltages. That's how I've been testing the op-amps: comparing the output voltages to the ideal op-amp model and seeing which one comes close. There are some other op-amps that I also tested in SPICE, and they seem to work as well, and luckily, they also come in the same SOIC-8 package, so I can make different configurations and test them. I also considered op-amps like the AD549 in the TO-99 package which also seems to work very well, but the price was a bit too high. (although, I'm working on making a board using the TO-99 footprint in case I need it)So, following my circuit, I laid out everything, and then I tried to follow the guidelines for the datasheet. The AD795's datasheet mentions that I should put a guard ring (tied to the common potential at the non-inverting input for the TIA configuration) around my input to prevent leakage currents. It would seem that this is a common method when dealing with pico-amp currents. So, I got to work and came up with the layout in 'Guard V3 Copper'. It's a 2-layer board with the op-amp in the center. The top and bottom ground pours are tied to 'Io'. For my guard ring, I created a no-copper-fill zone and used vias and single-pad nets to lay out my ring around the appropriate net, "NEGIN". The resulting 3D view can be seen in "Guard V3 top". It only shows the top of the board, but the ring and the no-fill zone is duplicated on the bottom side of the board. Using traces, I connected my ring to the non-inverting input of my op-amp as well as to the ground pour to establish that the ring is tied to the reference "Io".I'm not an RF expert by any means, and this is my first time implementing a guard ring of any sorts with an op-amp. My ideal simulations show that the AD795 works well, but regarding my board layout, does this look correct? No soldermask is on the guard ring, and the no-fill zone ensures that there's no surrounding copper within the ring, so there's less capacitance. I'm worried that there's something I'm drastically missing with my design that would cause all of this to fall flat.

author: Tj138waterboy
author_rank: Regular Contributor
timestamp: November 01, 2020, 02:41:45 pm
content: Have also been looking for proper layout and solder mask/ copper keepout settings. Any luck getting an answer?

author: bson
author_rank: Unknown
timestamp: November 02, 2020, 06:37:39 pm
content: It's not a guard trace, it's a shield. And why make it separate from the ground pour right next to it? This makes no sense. There will never be anything sacrificial on it. There are tons of vias... why? Is this going in a shielded metal can, if so what do you expect to need to shield from?Put a choke on the supply near its board ingress point.Do you really want 10000x gain in a single stage?