Measuring microamps & milliamps at 3 MHz bandwidth

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 spend ones.  So it’s just not feasible to measure this signal directly (not to mention using 2 probes and subtracting them in the scope).
Measuring microamps & milliamps at 3 MHz bandwidthThat incredibly expensive Agilent probe probably has a couple really nice amplifiers inside…. so I went searching for an amplifier.  After a bit of searching, 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 amazing CMRR of 140 db.  So it gets rid of the power supply voltage and outputs the amplified signal referenced to ground.
The AD8428 is perfect.  It’s so very easy!  Of course, such amazing performance costs money: about $20.  Here’s that expensive little amplifier, and a 5V to +/15V power supply (about $10) to power it. The one trick with measuring such tiny voltages is twisting the 2 sense wires together.  Honestly, I didn’t try it running them separately, but since this thing is getting voltages in only the micro voltage range for the lower measurements, I didn’t want to risk picking up noise.  I also put a 100 ohm resistor on the output, just in case I accidentally short the output or do something clumsy that might blow that little $20 part.

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