| ??? 02/18/09 13:49 Modified: 02/18/09 13:55 Read: times Msg Score: +1 +1 Good Answer/Helpful |
#162542 - Don't confuse "OPamp" with "OPamp circuit"! Responding to: ???'s previous message |
Ap, you confuse the input voltage of OPamp with the input voltage of OPamp circuit!
Look at this scheme: 
Apply 20V (15V+5V) to V2 and 15V to V1. Then, you will see at the non-inverting input of first OPamp a voltage of 3.7736V, due to the action of 43k/10k voltage divider. Because of the feedback the first OPamp controls its output voltage in such a way, that at the inverting input also 3.7736V can be seen. So, across the upper 43k resistor a voltage of 15V - 3.7736V = 11.2264V drops, resulting in a current of 261,08µV. This current flows also through the upper 10k resistor and causes a voltage drop of 2.6108V. As result the output voltage of first OPamp is 2.6108V more negative than the potential at inverting input (which is 3.7736V), yielding 1.1628V. So, the differential input voltage of OPamp circuit is 20V - 15V = 5V. But the differential input voltage of OPamp is 3.7736V - 3.7736V = 0V! In reality you will notice a slight deviation from zero, caused by the specified offset voltage. So, the differential input voltage of OPamp is within safe limits. Also, the maximum common mode input voltage range of LMP2234 of -0.2V...4.2V isn't violated either, because the common mode input voltage is 3.7736V. The second amplifier amplifies the output voltage of first OPamp and fabricates an output voltage of 1.1628V x 4.3 = 5.000V. Et voilà! Kai |
| Topic | Author | Date |
| Opamp question | 01/01/70 00:00 | |
| Please specify! | 01/01/70 00:00 | |
| Oops here its | 01/01/70 00:00 | |
| Mistake? | 01/01/70 00:00 | |
| transfer function | 01/01/70 00:00 | |
| Vout = V2 ? | 01/01/70 00:00 | |
| I 'm entirly confused... | 01/01/70 00:00 | |
| I explain it again | 01/01/70 00:00 | |
| So Vout = V2 - V1 | 01/01/70 00:00 | |
| I do have a common GND | 01/01/70 00:00 | |
| So Vout = V2 -15 ? | 01/01/70 00:00 | |
| trying to achieve | 01/01/70 00:00 | |
| Reference | 01/01/70 00:00 | |
| Reference | 01/01/70 00:00 | |
| Much clearer but.. | 01/01/70 00:00 | |
| Not clear | 01/01/70 00:00 | |
What VOut range do you want? | 01/01/70 00:00 | |
| I think this will do the trick... | 01/01/70 00:00 | |
| re: transfer function | 01/01/70 00:00 | |
| re: transfer function | 01/01/70 00:00 | |
| Ohm's law | 01/01/70 00:00 | |
| Oh the internal protection of opamp! | 01/01/70 00:00 | |
| Errr... | 01/01/70 00:00 | |
| Don't confuse "OPamp" with "OPamp circuit"! | 01/01/70 00:00 | |
| Don't confuse "OPamp" with "OPamp circuit"! | 01/01/70 00:00 | |
| If the feedback loop doesn't work... | 01/01/70 00:00 |