Hi forum members,

Today I opened up a Digital weighing scale 200Kg Capacity with 20gm least count. i.e. giving a precision of 1 in 10000 that is in between 13 and 14 bits. The devices uses one ICL7135 delta sigma ADC. I noted following points.

1. The supply scheme is almost simillar one I showed in my earlier post.
2. The circuit board is distinctly divided into Analog and Digital section. with analog section comprimising of an Instrumentation amplifier built around two OP07 opAmp and the ADC. The digital section consists of AT89C52 microcontroller driving the 6 digit 7 segment LED display 4 button interface, serial interface and parallel printer interface.
3. The device is using a tranformer with two secondary windings a 0-9 and a 9-0-9. The 0-9 winding is used for generating +5V supply using 7805 and this is used for feeding Digital section. The other 9-0-9 supply is generating +5V and -5V using 7805 and 7905 these +5V and -5V us used for feeding the Analog section and exciting the Load Cell.
4. The PCB is two layer board with no complete slid ground planes but the tracks are quite thick and at some place partial planes are there in analog section. All the analog pins of ADC are gaurded by ground gaurd rings around them. The two grounds are joined at only one point. One strange thing I noted with the gaurd rings around ADC pins. On top side of the board the ADC's analog pins are gaurded by digital ground and the same pins are gaurded by Analog ground on bottom side. I couldn't understand if this is for any specific purpose?? I think some kind forum member will tell me what is the reason of such a strange thing.
5. There is intense filtering used in analog section. Yet I have not carefully gone through the filtering tomorrowly I will study it more carefully. But I could see several tantatlum and mylar capacitors in Analog section.

Thanks & Regards,
Prahlad Purohit