Electronics > Projects, Designs, and Technical Stuff
Oscillation in PSU simulation
Zero999:
--- Quote from: ElectronSurf on April 17, 2020, 01:23:38 pm ---I wanted to use in-amp to measure the current accurately (0.1 mV/16 bit ADC) at the high side of circuit. unfortunately I couldn't find any seller who sells thin film resistors where I live, so the only option is to use a simple voltage divider or an in-amp/differential amplifier. is there a third option? can I compensate for the created loop instability somehow?
--- End quote ---
Why not simply use an ordinary op-amp for the current amplifier and an instrumentation amplifier to monitor the current?
--- Quote ---About the op amp slew rate; does JFET op amps speed makes them a good solution for lab power supply application?
This for example: https://www.analog.com/media/en/technical-documentation/data-sheets/OP249.pdf
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Whether it's J-FET or BJT makes no difference to the speed of the op-amp. J-FETs don't draw any current through their gate junctions, so J-FET op-amps have lower bias currents and a higher input impedance than BJT input devices. BJTs are essier to make with matched characteristics, than J-FETs so BJT input op-amps have lower input offset voltages, than J-FET op-amps, although modern J-FET devices are very good.
ElectronSurf:
--- Quote from: Zero999 on April 17, 2020, 01:51:07 pm ---Why not simply use an ordinary op-amp for the current amplifier and an instrumentation amplifier to monitor the current?
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Because I'm thinking of implanting an current/voltage set mode before enabling the output, using two different op amps one for measuring and displaying and another for current amplifier makes a little bit of difference between the user input and actual output.
I haven't had any luck at high side current sensing with an general purpose op amp without adding another op amp (two op amp in-amp) and even that introduced a lot of error. though this only is my thoughts/trial errors and I don't have any experience, just sharing it with you to come up with a solution...
Zero999:
--- Quote from: ElectronSurf on April 17, 2020, 02:04:04 pm ---
--- Quote from: Zero999 on April 17, 2020, 01:51:07 pm ---Why not simply use an ordinary op-amp for the current amplifier and an instrumentation amplifier to monitor the current?
--- End quote ---
Because I'm thinking of implanting an current/voltage set mode before enabling the output, using two different op amps one for measuring and displaying and another for current amplifier makes a little bit of difference between the user input and actual output.
--- End quote ---
I don't see how you'll be able to do this without using separate amplifiers. One of the control amplifiers will always give a false reading. When it's in current mode, the voltage amplifier will be saturated at the positive rail, thus be giving a false reading and the current amplifier will do the same, when it's in voltage mode. I suppose you could just ignore the reading from the amplifier which is pulling the ADC out of range and assume the current or voltage is equal to the value set, depending on the mode.
--- Quote ---I haven't had any luck at high side current sensing with an general purpose op amp without adding another op amp (two op amp in-amp) and even that introduced a lot of error. though this only is my thoughts/trial errors and I don't have any experience, just sharing it with you to come up with a solution...
--- End quote ---
What circuits have you tried?
ElectronSurf:
--- Quote from: Zero999 on April 17, 2020, 01:51:07 pm ---I don't see how you'll be able to do this without using separate amplifiers. One of the control amplifiers will always give a false reading. When it's in current mode, the voltage amplifier will be saturated at the positive rail, thus be giving a false reading and the current amplifier will do the same, when it's in voltage mode. I suppose you could just ignore the reading from the amplifier which is pulling the ADC out of range and assume the current or voltage is equal to the value set, depending on the mode.
--- End quote ---
But the values is not going to be readen from the control op amps, voltage using voltage divider at the output and current from current sensing in-amp output.
--- Quote ---What circuits have you tried?
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Differential amplifier with 1% resistors is not giving any meaningful output, two op amp in-amp gives a value close to actual current passing through the shunt resistor and with an "home-brew" in-amp I was able to get current reading with ~ ±1mA error.
Zero999:
--- Quote from: ElectronSurf on April 17, 2020, 06:54:23 pm ---
--- Quote from: Zero999 on April 17, 2020, 01:51:07 pm ---I don't see how you'll be able to do this without using separate amplifiers. One of the control amplifiers will always give a false reading. When it's in current mode, the voltage amplifier will be saturated at the positive rail, thus be giving a false reading and the current amplifier will do the same, when it's in voltage mode. I suppose you could just ignore the reading from the amplifier which is pulling the ADC out of range and assume the current or voltage is equal to the value set, depending on the mode.
--- End quote ---
But the values is not going to be readen from the control op amps, voltage using voltage divider at the output and current from current sensing in-amp output.
--- Quote ---What circuits have you tried?
--- End quote ---
Differential amplifier with 1% resistors is not giving any meaningful output, two op amp in-amp gives a value close to actual current passing through the shunt resistor and with an "home-brew" in-amp I was able to get current reading with ~ ±1mA error.
--- End quote ---
A 1mA error over what range?
I think I'm with you now. 1% resistors shouldn't be that bad. What op-amp and values did you use?
If you must put the instrumentation amplifier in the loop, then you'll need to add a low pass filter to the error amplifier, otherwise it'll oscillate. I'll post a schematic later, if no one beats me to it.
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