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Precision Data Converters
Precision Data Converters Forum
Negative pins condition when using the built-in function of augmented leads deriving in ADS1298
When using the built-in function of augmented leads deriving, the appropriate signal combinations are used for driving the negative lines of chanels 5,6 and 4 or 7. And there is no information in the datasheet about how the input pins of these lines should be connected. Does anybody know it? Should I keep these pins (IN4N, IN5N, IN6N, IN7N) not connected or grounded or in some another condition?
If you look through the schematic in the back of the ADS1298ECG-FE Users Guide, and consider the jumper defaults of the board, you will see that the IN(-) terminals are connected back through the WCT output of the ADS1298. This connection is also noted in the ADS1298 data sheet on page 55 in the description of the WCT1 register.
I already know it. Maybe my explanation is not clear. The idea is I want to use channels 5,6,7 as augmented leads. In this case I need to set the bits 5,6,7 (aVR_CH7, aVL_CH5, aVF_CH6) of register WCT1 to the "1". Thus, an appropriate reference signals will be routed to the corresponding negative lines (IN5N, IN6N, IN7N) inside of the IC, so that I don't need to drive the IN5N, IN6N and IN7N pins with any reference signals. But what should I do with these not connected pins? Keep not connected or connect to the ground or use some another way?
For the augmented lead channels, leave the negative (-) pins floating to avoid any summing problems. Use weak pull up resistors connected to AVDD for any unused analog input pins that are not used.
Thanks for your advice! But according to the figure 56 of the datasheet it would cause the summing of reference signal with AVDD. Am I right?
You are correct. I apologize. You will want to keep those inputs floating when using augmented leads. That is how we set the inputs during the parts characterization when we tested the augmented lead functionality. I will go in and edit the other post so future users understand the correct setup.
Thank you, Tony!
I'm glad that this post could be useful for other users.
And if you say these pins could be kept floating, then I assume that nothing special is placed at the input circuits of ADS1298. Thus the grounding of pins IN4N, IN5N, IN6N, IN7N is also correct.
I am not sure I understand you last comment. Pins IN5N, IN6N, and IN7N are the three used for the augmented leads in which an internal connection will be made from the appropriate WCT amplifiers to the input. Those three input pins must be left floating and not grounded.
Thanks a lot, Tony!
Now I have the comprehensive notion about this subject.
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