Hi all,
I have a current meter design using a 2-Ohm sense resistor and 2 op-amps.
The op-amps each have a gain of 11 for a total overall gain of 121. I am basically using the design from EEVBLOG current meter.(http://www.eevblog.com/projects/ucurrent/)
My PCB is powered from 5V USB, this 5V input is first regulated by a 5V LDO (incase a higher supply voltage is supplied).
This 5V LDO drives a 4.0V low noise LDO LP2985 that powers the MAX4239 op-amps and a 3.3V LDO for some digital stuff.
The final voltage output from the MAX4239 drives an AD7741 Voltage-Frequency converter, the frequency output is counted by a PIC24F to measure the current. I'm using the V-F approach as I want an average current measurement not an instantaneous one.
The system works very well and seems to show great linearity but there is a few uA of noise(+- 3-5uA) on the reading. My question is how can I begin to analyse the noise of the system? I have at my disposal a digital scope with built in FFT and an RF spectrum analyser that can measure down to 2Hz.
What is the best way to find out where this few uA of noise is coming from?
I have a current meter design using a 2-Ohm sense resistor and 2 op-amps.
The op-amps each have a gain of 11 for a total overall gain of 121. I am basically using the design from EEVBLOG current meter.(http://www.eevblog.com/projects/ucurrent/)
My PCB is powered from 5V USB, this 5V input is first regulated by a 5V LDO (incase a higher supply voltage is supplied).
This 5V LDO drives a 4.0V low noise LDO LP2985 that powers the MAX4239 op-amps and a 3.3V LDO for some digital stuff.
The final voltage output from the MAX4239 drives an AD7741 Voltage-Frequency converter, the frequency output is counted by a PIC24F to measure the current. I'm using the V-F approach as I want an average current measurement not an instantaneous one.
The system works very well and seems to show great linearity but there is a few uA of noise(+- 3-5uA) on the reading. My question is how can I begin to analyse the noise of the system? I have at my disposal a digital scope with built in FFT and an RF spectrum analyser that can measure down to 2Hz.
What is the best way to find out where this few uA of noise is coming from?