Recently I needed to actually “see” a current waveform in the 100 uA to 5 mA range with at least a couple MHz bandwidth. This extremely expensive probe would have been perfect, but instead I built something similar for about $30 using the amazing Analog Devices AD8428 amplifier.
Click “Read more” for details and a scope screenshot….
The first step was cutting the power trace and adding a resistor. I used two 1 ohm resistors in parallel.
At 5 mA, this makes only 2.5 mV. My scope’s supposed resolution is 1 mV, but the truth is there’s plenty of noise down in the 1 mV range. That’s pretty common for most scopes, even pretty spendy ones. So it’s just not feasible to measure this signal directly (not to mention using 2 probes and subtracting them in the scope).
That incredibly expensive Agilent probe probably has a couple really nice amplifiers inside…. so I went searching for an amplifier. After a bit of seaching, I found the AD8428. It has a fixed gain of 2000 and a bandwidth of 3.5 MHz. That’s a gain-bandwidth product of 7 GHz !!! It’s also an extremely well matched instrumentation amp with an amazimg CMRR of 140 dB. So it gets rid of the power supply voltage and outputs the amplified signal referenced to ground.
For more detail: Measuring microamps & milliamps at 3 MHz bandwidth
About The Author
Ibrar Ayyub
I am an experienced technical writer holding a Master's degree in computer science from BZU Multan, Pakistan University. With a background spanning various industries, particularly in home automation and engineering, I have honed my skills in crafting clear and concise content. Proficient in leveraging infographics and diagrams, I strive to simplify complex concepts for readers. My strength lies in thorough research and presenting information in a structured and logical format.
Follow Us:LinkedinTwitter