| ??? 11/02/09 16:36 Modified: 11/02/09 16:39 Read: times |
#170356 - Answers... Responding to: ???'s previous message |
Juan said:
1) What is the function of the third 10k resistor next to the 10nF capacitor near the sensor. Y understand that the others 10k down it are used to create a virtual earth to 4,5V. Yes, the two other 10k resistors generate a virtual ground of about 4.5V. The third 10k resistor references the input of first OPamp also to this virtual ground. Will say, the input is biased at 4.5V. Juan said:
2) I agree with you that the comparator is not suitable. I couldn´t find the tlc393. Instead, I bought the tlc374. I think it may be the same according to its datasheet (including the LinCMOS technology). Yes, should also work. Juan said:
3) What is the function of the pair of 4,7k resistors at each input of the comapartor. Are their use to avoid oscilations on the tlc393? To protect the inputs if Vcc is suddenly removed and the caps still hold some charge. It's good to have them when experimenting with the circuit. If you have a big supply voltage decoupling cap at the supply pins and the supply voltage cannot decrease quickly, you can omit these two resistors. Juan said:
4) Could you explain why you said "The threshold voltage generation with the diode at TLC393 is tolerant against supply voltage changes. " I can´t see know the diode and the 1M resistor work to do that. Without any signal the "+" input of TLC393 is at about 4.5V. As the TLC393 shall not skip in this situation you should provide a reference voltage at the "-" input which is higher than 4.5V. But if you do that with a simple voltage divider and your supply voltage can drift, because using a 9V battery for instance, then the sensitivity of the comparator will change when the supply voltage changes: Assume you have a threshold of 9V / 1.5 = 6V, when the supply voltage is 9V, then the sensitivity is 6V - 4.5V = 1.5V. But if the supply voltage is only 7V, then the threshold will be 7V / 1.5V = 4.7V and the sensitivity will be 4.7V - 3.5V = 1.2V. With the circuit shown, the sensitivity is always nearly the same, namely one forward voltage drop of a diode. Juan said:
5) I have calculated a total gain of 1055,6. Each stage´s gain is 5,7. But why the first one has different resistor values than the others if all of them have the same gain. Has it got something to see with the offset error at the output of the first opamp? Or perhaps something with the S/R at the input of the hgain chain? It's only to keep the noise low. Juan said:
6) You have a point there: Yes I am using batteries. But no, I amn´t using a 9V one. Instead am I am using 4 AA batteries and a step-up converter that is working fine. I have two outputs: one of 12V and the other of 15V You didn´t say anything about the LM324 power. I suppose it is 9V. But I am sure that I can use 12V. Am I wrong? 12V is ok. Kai Klaas |
| Topic | Author | Date |
| Ultrasonic Measuremnt | 01/01/70 00:00 | |
| Don't you need a load for D2? | 01/01/70 00:00 | |
| You wont see much across D2 | 01/01/70 00:00 | |
| LM339 input bias current | 01/01/70 00:00 | |
| Bias Voltage + diode? | 01/01/70 00:00 | |
| Threshold voltage | 01/01/70 00:00 | |
| I would give this a try... | 01/01/70 00:00 | |
| ? | 01/01/70 00:00 | |
| Greinacher voltage doubler | 01/01/70 00:00 | |
| You have a good memory!!! | 01/01/70 00:00 | |
| Why use 9V | 01/01/70 00:00 | |
| Availability.. | 01/01/70 00:00 | |
| True but.. | 01/01/70 00:00 | |
Absolutely! | 01/01/70 00:00 | |
| Answers... | 01/01/70 00:00 |