ADS1198: Test signal

Silpa kc

Part Number: ADS1198

Hello Sir,

I have tried to generate the internal test signal for ADS1198 which is integrated with Stm32. For generating the test signal ,channel 1 is used, with gain=1,and sample rate as 250.All other channels was shutdown. The config 2 register is set as 0x30.The output wave that i am getting is...

i know the output of test patter should be a square pulse ,but i am getting like this.how to solve this?

My program flow for spi communication is given below:

ADS reset function(in that function set reset pin low to high, chipselect low)
ADS stop conversion
ADS stop read data continuously mode
Register read and write (which I am getting correctly)
Send start opcode
Send opcode for read data continuously mode.

Is there any mistake in the flow?

over 9 years ago

0 Brian Pisani over 9 years ago

TI__Mastermind 38745 points

Hello Silpa,

What are the units of the x-axis? The maximum positive code should be 0x7FFF = 32767. Perhaps you are not interpreting the data correctly.

Regards,
Brian Pisani

0 Silpa kc over 9 years ago in reply to Brian Pisani

Prodigy 135 points

Sir

I am plotting it with number of samples,not the time. The plotted y- value is taken by shifting the 2scomplement Hex value above the range of 32767. I programmed it in C.In this ,I combined the channel data (2 byte) then check for Msb bit for positive or negative value then shifting the value by adding 32767 to it correspondingly . Is it the proper way?If not can you please explain the method ?

0 Brian Pisani over 9 years ago in reply to Silpa kc

TI__Mastermind 38745 points

Hey Silpa,

That is not the correct way of converting a negative two's complement value to a casted negative value in your processor. Check out this blog by my colleague on the topic: e2e.ti.com/.../it-s-in-the-math-how-to-convert-adc-code-to-a-voltage-part-1

Regards,
Brian Pisani

0 Silpa kc over 9 years ago in reply to Brian Pisani

Prodigy 135 points

Sir

Thanks for the replay and the link ,I have gone through it and made changes to my program. I have generated test signal for my ADS1198 and plotted it as instructed .The config2 register setting is 30h. I am attaching the wave form.Is it correct (about the amplitude).

0 Brian Pisani over 9 years ago in reply to Silpa kc

TI__Mastermind 38745 points

Hello Silpa,

The signal should be centered around zero and the amplitude should be around 1 mV. For this device, 1 mV = 1e-3 V * (2^15-1)/2.4 V = 13.7 codes. The positive code should be roughly 0x000E and the negative code should be roughly 0xFFF2. These could vary slightly with offset error and noise. Are these the values you are reading from the device?

Brian

0 Silpa kc over 9 years ago in reply to Brian Pisani

Prodigy 135 points

Hai Sir,

My positive code is 0x000A and negative code is 0xFFEF.These pattern I am getting .Where i went wrong ?My graph is not centered around 0.what is the reason for this? Test signals are internally generated signals?Is there any complaint for the board?

0 Brian Pisani over 9 years ago in reply to Silpa kc

TI__Mastermind 38745 points

Silpa,

I would say that is close enough. 0x00A corresponds to 730 uV and 0xFFEF corresponds to roughly -1.2 mV. The ADS1198 will have some offset in it so it will not be perfectly centered around zero.

Brian

0 Silpa kc over 9 years ago in reply to Brian Pisani

Prodigy 135 points

Hai Sir

As I mentioned above i am able to plot the test signal,but when i tried to plot the ecg signal from the simulator, I am not getting the ecg plot correctly. So I just want to check whether my simulator went wrong or is it the problem with my SPI communication ,any data missing is occurring or not .So if possible can you please send me the raw ecg data for ADS1198 in HEX format .

Below I am sharing my hex value ,by enabling only channel3.

Link:https://drive.google.com/open?id=0B12gtKcFdNyVWVRXYlAwazl0cVE

0 Brian Pisani over 9 years ago in reply to Silpa kc

TI__Mastermind 38745 points

Hello Silpa,

There is no exact data output corresponding to ECG - everyone's ECG will be slightly different anyway. I think you are getting the correct binary data out of the ADC since the test signal data corresponds to what I expect. This rules out an SPI issue. Now the only possibilities are an issue in the analog portion of the design or a software problem. You are the only one who can debug the software, but I could take a look at your schematic to see if I can see any problems in the board design. Can you please post a PDF of the schematic portion including the ADS1198 here?

Brian

0 Silpa kc over 9 years ago in reply to Brian Pisani

Prodigy 135 points

Hai Sir,

Thanks for the reply.The example file i meant was about the data from the ecg simulator not the real ecg values .And as you mentioned above I am able to plot the test signals. If my code was wrong how will i get the correct values? Same method is used for ecg signal reading and plotting.I am attaching the schematic of the circuit , and DB15 connector circuit through which the simulator value is given as input and power supply section. I am not able to figure out the exact problem,I am confused that if i am able to plot the test signal ,why the ecg signals are not getting?

link: https://drive.google.com/open?id=0B12gtKcFdNyVM0w3UUptZXJUQnM
https://drive.google.com/open?id=0B12gtKcFdNyVZlppS2ZmNERXdWs

https://drive.google.com/open?id=0B12gtKcFdNyVWUFSM1pIbmdjelE

Regrads

Silpa

0 Brian Pisani over 9 years ago in reply to Silpa kc

TI__Mastermind 38745 points

Hello Silpa,

I do not have access to Google Drive from within my network. Can you click "Use rich formatting" then attach the file from your file system to your post?

Brian

0 Silpa kc over 9 years ago in reply to Brian Pisani

Prodigy 135 points

Hai Sir

Sorry for the late reply, I am attaching the files again please check it.Descriptions are given in the above response.

https://drive.google.com/open?id=0B12gtKcFdNyVZlppS2ZmNERXdWs

https://drive.google.com/open?id=0B12gtKcFdNyVM0w3UUptZXJUQnM

https://drive.google.com/open?id=0B12gtKcFdNyVWUFSM1pIbmdjelE

Regards

Silpa

0 Brian Pisani over 9 years ago in reply to Silpa kc

TI__Mastermind 38745 points

Silpa,

Please attach the actual files. I do not have access to Google Drive on my network. Click "Use rich formatting" and then click the paperclip icon, then navigate to the location of the files in your file system and attach the actual files to the post.

Brian

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ADS1198: Test signal