| Kai Klaas 01/15/10 08:49 Read: 186 times Germany Msg Score: +1 +1 Good Answer/Helpful |
#172491 - The input multiplexer is damaged! Responding to: Gopal Amlekar's previous message |
Gopal said:
There is already a capacitor on that pin. It is 10uF and I tried lower values too like 100nF. Lowering the value only changed the discharge time and on ALE rising edge, voltage on that input pin went down fast instead of slowly decreasing. The only explanation for this is that the input multiplexer of ADC0809 is damaged! Take a fresh ADC0809. Take care, if you connect a too high capacitance across the input of ADC, then a brief overvoltage condition can result at this input during power down, when this capacitance is still charged while Vcc is already down!!! So, the time constant of the input filter should be way less than the time constant of power down. If you cannot guarantee this, you need an additional protection circuit, or better a Rail-to-Rail OPamp as buffer for the pot, which's output is directly connected to the ADC input. Kai Klaas |
| Topic | Author | Date |
| ADC0809 problem | Gopal Amlekar | 01/10/10 09:09 |
| Is the very same ADC chip in both cases? | Daniel Contarino | 01/10/10 09:51 |
| From time to time... | Kai Klaas | 01/10/10 14:22 |
| Datasheet read, same hardware | Gopal Amlekar | 01/11/10 02:53 |
| Define 1000000000% | Per Westermark | 01/11/10 06:48 |
| Thoughts... | Kai Klaas | 01/11/10 09:04 |
| Solved partially | Gopal Amlekar | 01/13/10 04:06 |
| So the cap did help? | Kai Klaas | 01/13/10 08:56 |
| RE: That cap did help? | Gopal Amlekar | 01/15/10 06:48 |
The input multiplexer is damaged!  | Kai Klaas | 01/15/10 08:49 |
| Discharge diode | Per Westermark | 01/15/10 10:14 |
| RE: The input multiplexer is damaged | Gopal Amlekar | 01/16/10 02:01 |
| Indeed, a very poor design!!! | Kai Klaas | 01/16/10 09:38 |
RE: Indeed, a very poor design!!! | Gopal Amlekar | 01/17/10 06:27 |