Hi all, well, hope you can help me with this problem.
First all I'm electronic enginener with little experience on professional layout, except for very simple circuits as hobbist and few real job (simple designs), now I'm on an startup that require to design a pH meter with precision of 0.0X, as far I know pH probes have very high impedance (I've readed all documentation I found) so a very low current bias INA is requires to mesure the signal properly. Searching on google I founded
this design.
I decided to reply the reference circuit in Altium by few reasons:
1) I love doing this.
2) I'm not sure to understand all the complexities related with INA and I think buying the development tools and comparing my own layout and software I could learn that I need to feel that my design is well done.
3) This design will be the base of other designs that will come later and I need to be very sure of what I'm doing.
4) Practice my Altium skills.
Now the doubs:
1) Recomendations related to guard rings could be sometimes confusing and I'm not sure how implement it with AD0683.
The ring guard should keep traped all the current that could probably been liked sorrounding the input signal with the lower potencial pin to trap thous signals.
Next image show how I have my design in this moment, and the next one show how I think it should be implemented (red line in paint-lab). But the line is so tiny, that I think I could have problems in production and makes me think that the chip is not designed getting that in mind, as for example
INA116 that have dedicated pins for that porpouse.
Some
recomandations advice to use this layout for unity gain configuration, don't know why and I want to know.
2) In few technical notes I readed that power planes (4 layer PCB) are required for precise measurement, however, the high impedance of the probe makes me think that this don't apply to this design given the FR4 impedance could leak current on signal path, and maybe a good idea is to put a layer under the path of the signal at same potential as guard ring to recover all leaking currents, but with risk to get noise (I think), I don't wkow what to do about it.
I need some advise and if someone recomed a good book talking about this will be great.
Thanks in advance.
JP