I am using ADS1298 EVM for my EEG project. Here are some questions about the CM voltage and Gain setting.
1) For my EEG measurement, I used the inputs as single-ened inputs, all the negtive inputs are shorted by jumpers, the positive inputs are connected to electrodes for recording data. My question is whether these negtive inputs are connected to the CM voltage 2.4V by default? There is no register bit to set the single-ended mode as SRB2 bit in ADS1299, I can only check my setting by floating all the positive electrodes and see the output value, the output value is 2.4V when G=1, is that correct?
2)When I using an external sqare wave(200mV,16Hz) to the positive input (CH1), the out wave in the virtual scope is as below.G=1.
When I set the G=2, the output turned to a DC voltage (1.2V), the amplitude must be doubled here, did I miss anything?
3) I also tried to use the internal test signal for Gain setting, the amplitude doesn't change.
Thank you for your help. Look forward to your reply.
(first picture is G=1)
Your message starts off asking about the ADS1298, but in Q1 you are asking about the ADS1299 - which board do you have? By the looks of your screen shot, you are saturating to the positive side. How do you have the power rails on the EVM configured - 0-3V or +/-2.5V?
Both 1298 and 1299 are using now. My question is about ADS1298. The power supply I choose is unipolar 0/3V AVDD, +3V for DVDD.
If you need any other information of my setting, let me know.
Looks like what you are seeing in your original post is a saturating input due to exceeding the reference voltage. This is why you see the input clip at 2.4V in a gain of one and clip at 1.2V when you increase the gain to two. Issue sounds like it lies in your setup of floating the negative input for the channels. If you are doing single ended measurements, you may want to tie the negative inputs to either the WCT or RLD line (set at the midpoint between the two supplies).
If you are doing EEG measurements, I would recommend ordering one of the new ADS1299EEG-FE boards recently released. This board works fluently with the new evaluation software for the ADS1299 allowing you access to the Rbias register bits thus allowing you to add bias to the inputs accordingly.
If you are doing the internal test signal, make sure you increase the number of samples/channel, especially if you are doing any kind of AC test signal. You need multiple samples to reproduce the square wave.
Thank you for your reply. I think you didn't understand my question. The samples are enough cover several preiod of test signal. I set CH1 G=1; CH2 G=2; CH3 G=4; ....CH7 G=12, the analysis shows that the Vpp still the same.
Did you notice the little box at the top of the screen (beside the 'Continuous button) that says 'Analysis Data is Input Referred'? When that box is checked, the displayed data has the gain removed. Un-check that box and you should see a difference in the amplitudes.
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