![\"\"](\"https:\/\/zeugmatographix.org\/ocra\/wp-content\/uploads\/2021\/12\/GPA_Rev003_img-1-1024x354.png\")
<\/figure><\/li><\/ul><\/figure>\n\n\n\nSpecifications<\/u><\/strong><\/p>\n\n\n\n
Peak output currents \u00b110 A each channel<\/p>\n\n\n\n
Current settling time 7 \u00b5s @ 1 App<\/sub><\/p>\n\n\n\n
-3dB frequency bandwidth 80 kHz<\/p>\n\n\n\n
Max. temperature offset drift \u00b13 mA<\/p>\n\n\n\n
Max. current offset (linearity) \u00b12 mA<\/p>\n\n\n\n
Setup & Calibration<\/u><\/strong><\/p>\n\n\n\n
For safety reasons, the first PCB check and calibration of the GPA should be done with a current limited laboratory power supply! The following steps should be processed according to the order. If any of the steps do not give reliable results, you should NOT continue with the next task!<\/strong><\/p>\n\n\n\n
- Check the -15 V, +15 V and GND for shorts<\/li>
- Check the functionality of each channel via visual feedback
- Connect LEDs to the designated connectors (marked with symbols on each channel).<\/li>
- Connect the power supply (max. \u00b115 V) to the supply input.<\/li>
- All red LEDs should be on.<\/li>
- Short a 3.3 V pin with a Res# (Reset \u2013 normally open) pin shortly to reset the overcurrent protection circuit.<\/li>
- All green LEDs should be on – If not, check solder points at the logic ICs.<\/li><\/ul><\/li>
- Calibrate the ampere to voltage conversion with the \u201cOutput Gain\u201d trimmer (with our OCRA1 system\/software we are using a 1V=1A conversion).
- Apply a DC voltage of 100 mV on the gradient channel input and measure the current with a multimeter at the output. <\/li>
- Turn the “Output Gain” potentiometer till you measure exactly 100 mA. <\/li>
- Verify the output linearity by repeating these steps in a range of -500mA to 500mV (like 100mA steps).<\/li><\/ul><\/li><\/ul>\n\n\n\n
![\"\"](\"https:\/\/zeugmatographix.org\/ocra\/wp-content\/uploads\/2021\/12\/DC-Gain.png\")
<\/figure><\/li><\/ul>Left: Setup for the calibration of the DC output gain. (A) Ampere meter to verify the set gain. Right: Output gain potentiometer (red) for each gradient channel.<\/figcaption><\/figure>\n\n\n\n - Calibrate the AC behaviour (settling time and transient response).
- Connect the gradient amplifier to the gradient coil.<\/li><\/ul>
- Apply an AC voltage (square wave, 1kHz, 500 mVpp<\/sub>) on the gradient channel input and measure the voltage of the current sense with an oscilloscope. If you have a 1:1 conversion your mesured voltage on the oscilloscope should be 1\/10 of the input signal (50 mVpp<\/sub>). <\/li>
- Tune the fast response (“Ringing”) and slow response (“Slope”) with the potentiometers on the gradient channel so that you get the fastes reponse without oscillations.<\/li>
- For much higher or much lower output inductance of the coils in comparison to our gradient coils (approx. 4-8 \u00b5H) you probably have to change the C9 or C8 capacitor to achieve the best compensation.<\/li><\/ul><\/li><\/ul>\n\n\n\n
![\"\"](\"https:\/\/zeugmatographix.org\/ocra\/wp-content\/uploads\/2021\/12\/g1229-1024x396.png\")
<\/figure><\/li><\/ul>Left: Setup to calibrate the compensation. With our gradient coil, we can reach a settling time of 7\u00b5s (blue). For bad compensation, you will see the orange response. (red) Right: Tuning potentiometers for each gradient channel. Calibration of Ringing (R) and Slope (S).<\/figcaption><\/figure>\n\n\n\n - To limit each current output, use the \u201cCurrent Limit\u201d tuning potentiometer. Turn anticlockwise (clockwise) to increase (decrease) the output current limit.<\/li><\/ul>\n\n\n\n
GPA production files, BOM, parts list, fabrication plan and images can be found here:<\/p>\n\n\n\n
https:\/\/data.stimulate.ovgu.de\/f\/1174a1e5a0954a228a4b\/<\/a><\/p>\n\n\n\n
When downloading the files, you declare to cite us as the creator of the Tabletop GPA when using in your projects or publications (Ivan Fomin, Research Campus STIMULATE, <\/em>Otto-von-Guericke-University Magdeburg). You are not allowed to remove logos or references.<\/span><\/p>\n\n\n\n
If you have questions about the OCRA Tabletop GPA or interest in a preassembled PCB, Tabletop and OCRA components or even a whole OCRA Tabletop system contact me via\u00a0ivan.fomin@ovgu.de<\/a> (cc marcus.prier@ovgu.de<\/a>). <\/p>\n\n\n\n
Best Regards <\/p>\n\n\n\n
Ivan<\/p>\n","protected":false},"excerpt":{"rendered":"
\nHello OCRA and Tabletop Community, I would like to introduce the low-cost 4-channel 600 W Gradient Power Amplifier (GPA) for our OCRA Tabletop MRI system. Basically, the GPA converts a voltage input coming for example from the OCRA1 (see blog post from Marcus) into currents to supply gradient coils. The GPA can handle gradient sequences (pulsed…\n<\/div>\n
- To limit each current output, use the \u201cCurrent Limit\u201d tuning potentiometer. Turn anticlockwise (clockwise) to increase (decrease) the output current limit.<\/li><\/ul>\n\n\n\n
- Apply an AC voltage (square wave, 1kHz, 500 mVpp<\/sub>) on the gradient channel input and measure the voltage of the current sense with an oscilloscope. If you have a 1:1 conversion your mesured voltage on the oscilloscope should be 1\/10 of the input signal (50 mVpp<\/sub>). <\/li>
- Connect the gradient amplifier to the gradient coil.<\/li><\/ul>
- Calibrate the AC behaviour (settling time and transient response).