Could you help me with understanding few points about an analog inputs in ADS1298?
1. As it's described in the datasheet, the input differential mode is preferable for use rather than single-mode. What exact advantages does that have? And what is disadvanteges of single-mode connection?
2. In the schematics for ADS1298 EVK the not installed resistors R58-R66 are used for driving the additional constant voltage to all the input lines. Does that mean the necessity of adding an offset voltage to analog inputs for proper operating of PGAs and ADCs?
3. In my application I want to calculate the ECG leads by PC program using recorded signals from all the electrodes. I'm trying to understand is there any difference between signals combinatting in digital and analog domains? I mean does that affect CMRR or another parameters?
1. Couple comments for this question. ADC range is +/-Vref and the best way to utilize the entire range is to use a differential input signal. Although, there are ways to use the range in a single ended mode through biasing the negative (-) input. A differential input signal allow you to keep a fixed common mode (CM) voltage which helps with noise reduction and CMR versus a single ended measurement which has a changing CM voltage. Depending on what you are trying to do with measuring ECG, a differential versus single ended measurement would fit.
2. Depending on your design, you may not need these resistors. Being an evaluation platform, we included pads for many not installed parts to allow customers to experiment. The idea of having the weak pull up and pull down resistors at the input are used to bias the input pins within the ADC's input range. Otherwise, if the input source (patient) is connected without using a RLD electrode, it may be at a different DC bias point from the ADC.
3. Could you elaborate more on this question. I am not sure what you are asking.
Hello, Tony. Thank you for answering questions. Comments for third one is following.
Usually ECG leads are made of combining of input signals using aoperations in analog domain. Another way is to digitize original signals and to combine them in digital domain. The second approach is much more flexible if different systems of leads are required. But I’m worry about CMR reducing - is it such effective in digital domain as in analog? It's kind of dillemma for me.
With regards, Alex.
Thanks for the additional information, I understand what you mean. We recommend using analog techniques to help improve the CMR. One way we have shown this is using the RLD is a closed loop configuration as shown in the following application note. Give this guy a read -
Thanks a lot!
This info is very important and useful for me.
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