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| Need help with bi directional constant current source (±100mA) |
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| SiliconWizard:
--- Quote from: OM222O on November 11, 2018, 11:16:52 pm ---I would rather use an integrated differential amplifier due to high common mode voltages and resistor matching becoming an issue. I was wrong and thought that the ADA4522 was a differential amplifier but it in fact is not! most common differential amplifiers have built in 1Meg resistors which is why I chose that value. I can't seem to find a formula that relates the resistor tolerance to the CMRR of an amplifier. would 0.05% be good enough? --- End quote --- As for the CMRR, you can start here: http://www.analog.com/media/en/reference-design-documentation/design-notes/dn1023f.pdf You will have a heck of a hard time finding an integrated instrumentation amplifier that is fast enough AND can take +/-24V. The opamp you selected seems to be a good fit actually. Are you really intending on driving a 3.2H inductor with a constant current source though? As I hinted above, 3.2H is HUGE. At those values, the dynamics of the current source may not matter that much anyway, as any real (not ideal) current source will take a long time to settle... --- Quote from: OM222O on November 11, 2018, 11:16:52 pm ---I think if I go with the 1K resistors, I would also need another op amp as a virtual ground as the low value resistors can pass mAs of current which creates an issue.Please watch the video linked below --- End quote --- What do you call "virtual ground"? The 1.024V reference you made out of the 2.048V one? In your original schematic, it's already buffered by an opamp follower (LTV9001) so that shouldn't be a problem whatsoever. |
| OM222O:
they are using pots to calibrate out errors. I need 12 of these in different parts of the robot and I don't want to fiddle around with pots to get it working. I think if I use 0.05% matched resistors with 15PPM/C temp co, I will have a maximum of 3% error at worst case scenario which is not really ideal ... if I don't have any other choice, I will go with a differential amp (like the LT1990) followed by the ADA4522 as unity gain. it seems like the best option in terms of accuracy so far. I'm just waiting for "duak" to explain more about the follower circuit. again maybe I'm calculating the common voltage wrong ... |
| OM222O:
--- Quote from: SiliconWizard on November 11, 2018, 11:53:21 pm --- --- Quote from: OM222O on November 11, 2018, 11:16:52 pm ---I would rather use an integrated differential amplifier due to high common mode voltages and resistor matching becoming an issue. I was wrong and thought that the ADA4522 was a differential amplifier but it in fact is not! most common differential amplifiers have built in 1Meg resistors which is why I chose that value. I can't seem to find a formula that relates the resistor tolerance to the CMRR of an amplifier. would 0.05% be good enough? --- End quote --- As for the CMRR, you can start here: http://www.analog.com/media/en/reference-design-documentation/design-notes/dn1023f.pdf You will have a heck of a hard time finding an integrated instrumentation amplifier that is fast enough AND can take +/-24V. The opamp you selected seems to be a good fit actually. Are you really intending on driving a 3.2H inductor with a constant current source though? As I hinted above, 3.2H is HUGE. At those values, the dynamics of the current source may not matter that much anyway, as any real (not ideal) current source will take a long time to settle... --- Quote from: OM222O on November 11, 2018, 11:16:52 pm ---I think if I go with the 1K resistors, I would also need another op amp as a virtual ground as the low value resistors can pass mAs of current which creates an issue.Please watch the video linked below --- End quote --- What do you call "virtual ground"? The 1.024V reference you made out of the 2.048V one? In your original schematic, it's already buffered by an opamp follower (LTV9001) so that shouldn't be a problem whatsoever. --- End quote --- as I mentioned it will be used to drive a voice coil actuator and yes, the inductance is really large. it's more like a transformer coil than an inductor and it cannot be swapped out for another one. The transient response is also important as it will be controlling parts of the robot that need to react fast to the environment. The cost is not a big issue as it's supported by university (that should be a given when I'm using a 16 bit DAC, high end voltage references,etc :-DD) by virtual ground I meant this point in the circuit: as you see the current changes from 1mA to 1.5mA when he removes the op amp and connects that point directly across the load resistor. then he increases the value of the 4 resistors to 100k to get rid of that error. using more op amps won't be an issue as the ADA4522 comes in a quad op amp package as well! 2 of the can be differential amplifiers and 2 can be buffers to keep the value of the resistors down for the better transient response. I'm just worried about the 24V common mode voltage. (It's actually 0V - 24V - 48V )I'm not sure if it's considered ground so we can dismiss it and consider the common mode voltage to be 1.024 or if it's 25.024 :-// but I think we're getting to a good solution here. I actually tried to use the analog devices error budget calculator but there seems to be an issue with it as it raises the error for the ADA4522 and says the supply difference exceeds 36 volts although it should be rated for 60V operation ... here's a link if you want to have a look: https://www.analog.com/en/design-center/interactive-design-tools/opamp-error-budget-calculator.html |
| duak:
OM, here's an awful sketch of a current booster. Ri handles the opamp's output current up to the point at which its voltage drop turns on one or the other transistors (~0.6 V / 100R = 6 mA). D1 or D2 turn on when the inductor voltage tries to go beyond the power supply rail voltages. If these weren't here, the inductor current would try to force its way thru the opamp. D3 and D4 are zener diodes that limit the power supply rail voltages when the energy in the inductor has to be dumped. Since this is an actuator in a mechanical system, there may be an energy source may be external to the inductor (such as an arm) in addition to the energy stored in the magnetic field. If there's a lot of energy, you might need some big zener diodes, or a more capable voltage limiter. Do you know how to calculate the worst case power dissipation in the transistors? It's not always obvious, but with a simple inductive load for the case of just holding, you can have maximum current with essentially zero voltage across it. The transistors would then have to drop the rail voltage and so dissipate 100 mA * 24 V = 2.4 W. If the actuator is moving, there will also be a back EMF impressed on the output voltage and further increase the power dissipation. Tons of fun, eh? Good luck! |
| Alex Nikitin:
Here is a suitable circuit. The opamp, transistors and diodes types choice is purely for the simulation, you should choose suitable for your application, including voltage and power dissipation limits. If you use appropriate devices the maximum output voltage can be boosted to 100V or more. As shown the circuit will produce +/-100mA output current for +/- 5V input, that also could be changed according to your needs. Cheers Alex |
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