Hi All,
I'm designing a DTC 3-phase motor controller which needs to know the 3-phase voltages.
The motor voltages will be 0-400V PWM which will be scaled 131:1 for my 0-3.6 ADC input. The DTC may issue a new switching state every 5uS, and I plan to allow the signal 2.5uS to settle to its new value, which will leave me another 2.5uS to sample and convert the 3 voltages, before i need to issue another switch state.
I am using a voltage divider circuit to scale the voltages, and then input into a unity gain buffer op amp. As I don't have much time to take multiple samples (2.5uS) and average the new DC voltage (square wave high or low), I'd like to add in a filter cap.
I need these voltages to be extremely accurate, so i'm looking at a high precision opamp with ultra-low offset/drift/noise etc. I found the Maxim4239 which looks pretty good.
The slew rate of this opamp is 1.6V/uS, which means it will take 2.25uS to change from 0V to 3.6V (typical operation as PWM pulses switch between 0V and ~400VDC). This slew rate is however at conditions Vcc=5V, CL=100pF with Vout=2V step. This is a rail-rail opamp and i was thinking of running it at 3.6V with no resistor between the output and the ADC port (or filter cap charge resistor) [see attached picture], will this make a difference? Does the 2V step output make a difference? Could it perhaps be faster if its a 3.6V step?
So main question is, the opamp has an output short circuit current of 40mA (essentially what the filter cap will be without a precharge resistor) - what is the maximum filter cap I can put and still have the output stable after 2.5uS going from 0-3.6V???
Is it worth finding an opamp with a higher slew rate/short circuit current?
View attachment 64916
Thanks for your help in advance!
Stiive
I'm designing a DTC 3-phase motor controller which needs to know the 3-phase voltages.
The motor voltages will be 0-400V PWM which will be scaled 131:1 for my 0-3.6 ADC input. The DTC may issue a new switching state every 5uS, and I plan to allow the signal 2.5uS to settle to its new value, which will leave me another 2.5uS to sample and convert the 3 voltages, before i need to issue another switch state.
I am using a voltage divider circuit to scale the voltages, and then input into a unity gain buffer op amp. As I don't have much time to take multiple samples (2.5uS) and average the new DC voltage (square wave high or low), I'd like to add in a filter cap.
I need these voltages to be extremely accurate, so i'm looking at a high precision opamp with ultra-low offset/drift/noise etc. I found the Maxim4239 which looks pretty good.
The slew rate of this opamp is 1.6V/uS, which means it will take 2.25uS to change from 0V to 3.6V (typical operation as PWM pulses switch between 0V and ~400VDC). This slew rate is however at conditions Vcc=5V, CL=100pF with Vout=2V step. This is a rail-rail opamp and i was thinking of running it at 3.6V with no resistor between the output and the ADC port (or filter cap charge resistor) [see attached picture], will this make a difference? Does the 2V step output make a difference? Could it perhaps be faster if its a 3.6V step?
So main question is, the opamp has an output short circuit current of 40mA (essentially what the filter cap will be without a precharge resistor) - what is the maximum filter cap I can put and still have the output stable after 2.5uS going from 0-3.6V???
Is it worth finding an opamp with a higher slew rate/short circuit current?
View attachment 64916
Thanks for your help in advance!
Stiive