Ring Guard vs. Ground Plane - what difference does it make which one to use?
Hi all,
I'm planning on using an Si photodiode that has a built-in pre-amp (Hamamatsu s5591, see attached data sheet if you dare). This component uses an external feedback resistor Rf which page 4 of the datasheet says it requires a guard ring or a teflon standoff for the leads. I was planning on using a ground plane for the PCB, so I'm wondering: would a ring guard be necessary if I already have a ground plane? And if so, why?
Also, if anyone happens to know a good place to get teflon standoffs, I'd be much obliged on that topic, too. I can't seem to find anyone who sells less than a gazillion of them at a time.
thanks,
Mark
I'm planning on using an Si photodiode that has a built-in pre-amp (Hamamatsu s5591, see attached data sheet if you dare). This component uses an external feedback resistor Rf which page 4 of the datasheet says it requires a guard ring or a teflon standoff for the leads. I was planning on using a ground plane for the PCB, so I'm wondering: would a ring guard be necessary if I already have a ground plane? And if so, why?
Also, if anyone happens to know a good place to get teflon standoffs, I'd be much obliged on that topic, too. I can't seem to find anyone who sells less than a gazillion of them at a time.
thanks,
Mark
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Comments
You also need to make sure the board is cleaned properly to minimise leakage. The high input impedance of the amplifier needs to be maintained.
Leon
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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle
Post Edited (Leon) : 8/6/2009 9:27:55 PM GMT
Thanks, Leon,
but could somebody explain to me why a ring guard is better than using a ground plane everywhere on the PCB? I just don't understand it.
thanks,
Mark
Leon
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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle
But isn't the ring guard connected to ground? How would it be better than an entire plane connected to ground?
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Watching the world pass me by, one photon at a time.
The reason for the ring is to reduce leakage and minimize noise pickup.
Looking at the diagram of the S5591 in figure 8 of the data sheet, it looks more like a shield than a guard ring. Pins 5 & 11 are connected to the metal case. That should definitely be connected to your ground plane so as to provide a good Faraday shield around the sensitive circuit. It surrounds the whole circuit.
A guard ring is a different from a shield. They do suggest cutting short the pin 9 feedback lead if it will not be used for an external resistor. It would be prudent to connect the ground plane to pin 8 (non-inverting input) and run a little circle of isolated ground around pin 9 on both the top side and bottom side of the board. Isolating it from the rest of the ground plane except at pin 9 would avoid possible currents flowing in the ground plane and reference it directly to the non-inverting input. Emphasis, circled both top and bottom. The main thing is to avoid leakage from the power supplies and the output across to the sensitive input. If there is an external feedback resistor, it will almost certainly be one with leads, and you can bring the pin 9 lead up in thin air and solder one end of the resistor there and the other end to the output. Be sure the resistor body is squeaky clean. Maybe it will need shielding too. The 1 gigaOhm resistor inside the S5591 is probably composite, a split T configuration.
You can buy teflon spacers and use them as standoffs.
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Tracy Allen
www.emesystems.com
Post Edited (Tracy Allen) : 8/7/2009 4:28:00 AM GMT
thanks for your explanations on this. It's good to know that the ground plane by itself is not sufficient. I still need to do my homework on this, but at least you guys (and Leon, too) have helped prevent me from going too far astray.
much obliged,
Mark
- Contaminants on the resistors, pcb, and chips can produce major noise, drifting, and offset problems (leakage current).
- Humidity changes in the environment can cause drifting and noise problems (leakage current).
- Vibration can cause noise pickup because a tiny current is induced in a wire, pcb trace, or component by it's motion in the earth's magnetic field.
In general:
- Isolate the sensitive high impedance sections of the circuit from everything else using good layout, guard rings, and ground planes.
- Remove all traces of dirt, grease, flux, and other contaminants from the pcb and components using high purity chemicals (99.9% isopropyl and DI water)
- Mount the boards so that they are isolated from humidity changes, vibration, stray magnetic fields, and electrostatic fields.
That's amazing! Wow.... suddenly, the idea of using this component seems tougher than I thought. I knew it would be a learning experience, but.... maybe I'm biting off more than I can chew here. I was hoping the fact it has the pre-amp bundled inside its package would save me that kind of grief, but that feedback resistor suddenly leaves me feeling naked in the poison-ivy-infested wilderness all over again.
I'm thinking one way around the fussyness of the feedback resistor is just to leave it out, cut off the lead pins for feedback, and hope the dynamic response is good enough. Since I'm looking for light pulses, I'm not sure how that would work out. I'm guessing the high frequency components of the pulse would be un-gained, so the resulting output pulse would look like a smooth blob and not a clean square/spike. That's just my ignorant guess, though. And I'm not yet sure if that would matter or not to my application.
thanks, as always, for your insights,
Mark