mA to µA Sinusoidal Current Driver  - Page 1
Forum section: electronics / projects

author: ship_dit
author_rank: Newbie
timestamp: August 30, 2022, 06:25:01 pm
content: Optics person here! I am working on a sensitive optical magnetometer and need to generate sinusoidal nT fields at roughly 1KHz to validate the performance. I have a few turn electromagnetic coil that I have been driving with DC constant currently supply (simple enough). I was looking to switch to sinusoidal current control of the coil. Because the desired fields are so small I need to generate mA to µA through a low impedance <1ohm coil. There seem to be limited commercial options so I was considering using a unity gain op-amp buffer with a load resistor to control the current through the coil.Is this a reasonable circuit to use, or does anyone know of commercially available devices that would be able to do this?I did some initial testing with a high power LM675 op-amps that I had from a previous project but I accidentally destroyed the chips due to a soldering error, so I figured I would ask here before trying again.

author: Kleinstein
author_rank: Super Contributor
timestamp: August 30, 2022, 07:03:11 pm
content: The circuit is a bit tricky: depending on the exact properties of the coil and OP-amp there is a tendency of oscillation. The R-L combination can add too much phase shift to the OP-amp to be stable. One would normally add an extra capacitor and resistor so that there is an additional higher frequency feedback path. With the relatively low frequency of interest and likely small inductance this should be good enough.One may even get away with just a series resistor in the kohms range and function generator, as the coil impedance is quite small.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 30, 2022, 07:12:32 pm
content: If you need an AC you possibly may look at Howland current pump. Output current is very small so you may use any precision opamp (op27 for example). Your low-Ohm load doesn't need a high current amplifier. I guess any opamp can supply milliamperes into 1 R load.

author: SiliconWizard
author_rank: Super Contributor
timestamp: August 30, 2022, 07:49:54 pm
content: Yep, it will definitely tend to oscillate especially with such a load.Taming it in the usual way should do the trick here, though. (See attached.)Disconnect C1 and you'll see high-freq oscillations modulated by your 1kHz input.

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 30, 2022, 10:10:30 pm
content: Add a 33 ohm series resistor and drive it with a headphone amplifier.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 31, 2022, 01:22:19 am
content: Until the output current is smaller than the opamp maximum allowed output current, almost any opamp will work fine. You just want to use a low enough power supply voltage (kind of +-(3..5) VDC), it depends on an actual opamp.More info about driving loadsHow to Buffer an Op-Amp Output for Higher Current, Part 1.But those examples are more about higher currents.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 31, 2022, 03:14:19 pm
content: This is (one of) the simplest bidirectional precsion sources you can make. You dont have to use the amp chips specified- it will work with other opamps and inst amps.Feedback sets Io= Vin/Rsense . Note that the inductor current is suplied by the opamp. Rdamp is optional, it will help to reduce high frequency peaking. I've assumed an inductor size.No precison resistors are needed. R5 and R6 (optional) are simple over current protection for the inst amps inputs.