| ??? 02/10/05 01:39 Modified: 02/10/05 01:41 Read: times Msg Score: +1 +1 Good Answer/Helpful |
#87045 - You should see a change! Responding to: ???'s previous message |
Prahlad said:
I had connected 'Ra' exactly the same manner you showed in schematic but it seems to be not working to adjust the output signal of rogowski coil. I tried changing the value of 'Ra' but there is no change seen in output of integrator. Please let me know if I am missing something. Output voltage at output of integrator is inversely proportional to integration resistance, here 'Ra + R'. So, you must see a difference at output of integrator when changing 'Ra'! Maybe you overrange the integrator, by using a too small integration resistance? Try R = 100kOhm instead, or choose a Rogowski coil providing a smaller signal, means containing less windings. Or is it, because Rogowski coil isn't shielded and capacitive coupling of switching noise takes place? I think you are refering to the 100k trim pot in your schematic when you said adjust the output of TL051 to exacly 0.0mV am I right? Of course, of course. You turn the 100kOhm trimmer to adjust for zero volt output voltage. But think about what the circuitry makes with the intrinsic offset voltage of OPamp. Will you observe it unchanged at output of integrator? No, it's heaviliy amplified! The calibration routine I mentioned, namely How to adjust the offset voltaqe?
Connect the input of integrator directly to ground by the help of a very short wire. Any existing offset voltage of OPamp will be amplified by the factor (4M7 + 10k) / 10k = 471 then. Now adjust the output of TL051 to exactly 0.0mV. Remove the ground connection. That's it. should explain, how the intrinsic offset voltage is manipulated by the integrator circuit: Assume, offset voltage of Opamp is 2mV. Means, if non-inverting input is exactly zero, then 2mV can be measured at inverting input. Refering to calibration routine, this will result in a current of 2mV / 10kOhm = 200nA through 10k resistor. This current is delivered by output of OPamp and will cause a voltage drop of 200nA x 4.7MOhm = 0.94V across 4M7 resistor. As consequence, intrinsic offset voltage of OPamp appears at output of integrator as 0.94V + 2mV = 0.942V, which is 471 times the intrinsic offset voltage of OPamp! How works the offset adjust in this circuit? By the help of 100kOhm trimmer you simply apply a voltage to non-inverting input that equals the intrinsic offset voltage, but of opposite sign, in our example -2mV. By this you tear down the potential at inverting input to exactly zero volt, which will also cause zero volt at output of integrator, unless input bias current of OPamp does not introduce further voltage drop across 4M7 resistor. Estimate by yourself, how high this additional error can be with TL051. This little calculation should demonstrate, how critical this circuit is refering to offset voltage of OPamp. Only precision OPamp can be used here! To adjust the offset voltage do only use a multi turn trimmer made of cermet! Only the best is good enough!! Kai |
| Topic | Author | Date |
| My Rogowski Coils | 01/01/70 00:00 | |
| If it ain't broke | 01/01/70 00:00 | |
| Lack of confidence. | 01/01/70 00:00 | |
| Art of electronics | 01/01/70 00:00 | |
| No I haven't. | 01/01/70 00:00 | |
| OP AMP basic definitions | 01/01/70 00:00 | |
| Kai- Please Check your mailbox. | 01/01/70 00:00 | |
| Three things that scare me. | 01/01/70 00:00 | |
| More concrete schematic | 01/01/70 00:00 | |
| Re: Concrete Schematic. | 01/01/70 00:00 | |
| Rogowski coil | 01/01/70 00:00 | |
| Did I Miss Something -Kai. | 01/01/70 00:00 | |
| You should see a change! | 01/01/70 00:00 | |
| Yes I could see the change Thanks. | 01/01/70 00:00 | |
| Alu foil, asbestos shield | 01/01/70 00:00 | |
| Asbestos | 01/01/70 00:00 | |
There's no 'corner frequency' | 01/01/70 00:00 | |
| Integrating Capacitor. -Kai | 01/01/70 00:00 | |
| MKP, select for low offset | 01/01/70 00:00 | |
| How about these. | 01/01/70 00:00 | |
| Are ok | 01/01/70 00:00 |