| ??? 04/16/11 23:02 Read: times |
#181914 - Some answers... Responding to: ???'s previous message |
Mehdi said:
2-why you apply gain in the last satge?
3-is MCP6274 is a good choise for this application? 4-why 4 stage gain?why not 2 stage gain? I guess you mean the circuit with OPA353? Well, in order to not overdrive the first stage of single supply OPamp circuit, the input signal must first be decreased by a factor of 4. So, after the subtraction of reference voltage, the signal must be increased again, by a factor of 8. Because at 100kHz an OPamp has already lost a big portion of gain reserve, I have distributed the gain of factor 8 over two stages, so that each has to provide a gain of only less than 3. Let's have an example: If you take a MCP6274 with a unity gain bandwidth of 2MHz, then you have no gain reserve at 2MHz. At 200kHz you have a gain reserve of factor 10, at 20kHz a gain resreve of factor 100, and so on. To make your OPamp work properly you must have a gain reserve of at least a factor of 10, better a factor of 100, at the highest frequency of interest (sine signals assumed!). So, if you plan to handle frequencies up to 100kHz, you can install only a gain of about factor 2 with your OPamp circuit containing the MCP6274. Then you will have a gain reserve of about factor 10 at 100kHz. Higher gains cannot be accomplished by the MCP6274 at 100kHz without ruining the signal integrity. So, how much gain can be installed at 100kHz depends on the bandwidth of OPamp. An OPamp with 10MHz unity gain bandwidth has a gain reserve of 10MHz/100kHz=100 at 100kHz. So, a gain of factor 10 can be installed at 100kHz. Using such an OPamp only one gain stage would be sufficient to provide a gain of 8 (see above). Things get worse, if your signal isn't a sine but has harmonics, like a square wave or else. In such a case you would need sufficient gain reserve even for the harmonics and your OPamp should be faster by at least a factor of about 10 than when only handling 100kHz sines. Mehdi said:
5-what about references like ad680 or ref43 ? What reference voltage generator is sufficient depends on your expected precision of circuit. I cannot say, which one you must use. The LM385-2.5V I have taken is just a standard chip being available everywhere. Mehdi said:
6-if dynamic range goes below 0.1v (8.6v to 8.7v)which gain is better to change? 2V/0,1V means a gain of 20. In this case I would take 10MHz-OPamps and put them into a two stage design, having the gain of factor 20 distributed evenly over the two stages (4x5 or so, e.g.). Mehdi, you see, everything depends on the bandwidth of your signal, the highest frequency of interest. Is it really 100kHz you have to amplify accurately? Tell us more about the nature and shape of your signal. Kai Klaas |
