Re: [volt-nuts] 731A output impedance
The simplest way to drop the output impedance without adding much circuitry is to just change the series R to 100 ohms or so - that would still give pretty good isolation from capacitive loading. If the R is dropped to zero, the DC performance will be best, but you'll have to worry about the amount of capacitive loading. If the lines are short - say a couple of meters or less of open wire, it would probably be OK, but that much coaxial cable may make it oscillate. The suggestion to get the feedback right from the output terminal,or even with external sensing at the load would be best for DC accuracy, but would have the same problems as above. You can also take the DC feedback from the output directly, and the AC feedback from the amplifier output, while the series resistor isolates the two. This would give good DC accuracy and AC stability, but would alter the dynamic response and LF noise shape somewhat. If you add an amplifier, you'll of course have to consider its offset and noise contribution, and it will have the same stability issues to resolve. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] 731A output impedance
Hi Ed, I am a little confused with regard to your paragraphs 3 4. Could you elaborate on those two paragraphs ? thank you, BillWB6BNQ ed breya wrote: The simplest way to drop the output impedance without adding much circuitry is to just change the series R to 100 ohms or so - that would still give pretty good isolation from capacitive loading. If the R is dropped to zero, the DC performance will be best, but you'll have to worry about the amount of capacitive loading. If the lines are short - say a couple of meters or less of open wire, it would probably be OK, but that much coaxial cable may make it oscillate. The suggestion to get the feedback right from the output terminal,or even with external sensing at the load would be best for DC accuracy, but would have the same problems as above. You can also take the DC feedback from the output directly, and the AC feedback from the amplifier output, while the series resistor isolates the two. This would give good DC accuracy and AC stability, but would alter the dynamic response and LF noise shape somewhat. If you add an amplifier, you'll of course have to consider its offset and noise contribution, and it will have the same stability issues to resolve. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] 731A output impedance
I'm not sure how much elaboration is needed, but here's some: If you take all of the feedback from the output terminal, that's better for DC accuracy by eliminating the voltage drop of the series resistor, while still providing some overload protection to the opamp. But, it also decreases phase margin so that it will be more prone to oscillate with capacitive load. If the series R becomes zero, the voltage drop and the extra loss of phase margin are eliminated, but the inability to drive large capacitive loads remains - it is a limitation of the amplifier. Usually a small amount of series R can help a lot with capacitive loading stability, but even when small it can drop enough DCV to be a problem. A common way to solve both problems is to sense the DC right at the output to eliminate the drop in the series R as above, but to increase stability by taking some AC ahead of the resistor - usually at the output of the amplifier. If the amplifier has an integrating feedback capacitor, it's usually already connected that way, so only the resistive part of the feedback needs to go to the terminal. If there is no feedback capacitance, then a small amount can be added from the amplifier output to the effective inverting input. I don't know what the output stage of the 731A looks like, but it must be an inverting (integrator) amplifier or a buffer, if using an opamp. In either case there should be a way to modify the feedback network. However, whatever is changed or added may affect the overall frequency response and noise. Ed ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] 731A output impedance
On 11/27/2012 07:37 PM, ed breya wrote: I'm not sure how much elaboration is needed, but here's some: If you take all of the feedback from the output terminal, that's better for DC accuracy by eliminating the voltage drop of the series resistor, while still providing some overload protection to the opamp. But, it also decreases phase margin so that it will be more prone to oscillate with capacitive load. If the series R becomes zero, the voltage drop and the extra loss of phase margin are eliminated, but the inability to drive large capacitive loads remains - it is a limitation of the amplifier. Usually a small amount of series R can help a lot with capacitive loading stability, but even when small it can drop enough DCV to be a problem. A common way to solve both problems is to sense the DC right at the output to eliminate the drop in the series R as above, but to increase stability by taking some AC ahead of the resistor - usually at the output of the amplifier. Figure 9 of the TI data sheet shows exactly what you are suggesting. If the amplifier has an integrating feedback capacitor, it's usually already connected that way, so only the resistive part of the feedback needs to go to the terminal. If there is no feedback capacitance, then a small amount can be added from the amplifier output to the effective inverting input. I don't know what the output stage of the 731A looks like, but it must be an inverting (integrator) amplifier or a buffer, if using an opamp. In either case there should be a way to modify the feedback network. However, whatever is changed or added may affect the overall frequency response and noise. Excellent points Ed. The output stage is a non-inverting amplifier with a small gain (about 1.3). The compensation of the lm301A is OK but I think it could be improved to better tolerate load capacitance. I have not looked at what would be required to move the op-amp sense point to the 731A output and leave the 1K inside the loop. I would prefer to not butcher the board. Clearly shorting the 1K is pretty easy! The lm301 is protected against shorts to ground. Thanks to all who added to this thread. -- = Bob Smither, PhD Circuit Concepts, Inc. I've come to realize that protecting freedom of choice in our everyday lives is essential to maintaining a healthy civil society. -- George McGovern smit...@c-c-i.comhttp://www.C-C-I.Com281-331-2744 = attachment: smither.vcf___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
