This is our first project in medical instrumentation. We are designing a educational medical measurement unit using ADS1298 device. We are using single ADS1298 device to capture 1 channel ECG (PGA inputs 1,2,3,4) and 4 channel EMG (PGA inputs 5,6,7,8). In our first version we saw both sections working together, however, the gain (set to 12) was inadequate for EMG and signal levels were very low and we were observing noise in the captured EMG signal. We wanted to include a pre-amp for EMG before feeding the signal to ADS1298 device. In this regard I had few questions (I am looking for answers for specific questions which I could not find in the E2E community postings):
The whole system is battery operated and works with single supply of +3.3V.
1. For EMG, each electrode (passive) will have 2 leads which we connect to ADS1298 PGA inputs, along with that there will be shield which is required for reference. Can we use the same RLD reference generated in the device for ECG for the EMG shield or it must be seperate shield signal which needs to be generated for each electrode pair?
2. We are planning to use a instrumentation amplifier such as INA128 or INA333 in pre-amp stage. The output of INA will be single ended. Since ADS1298 input can take differential or single ended, we wanted to use PGA inputs 5,6,7,8 in single ended mode. I will be connecting the output of INA128 to IN5P of ADS1298. What to do with IN5N? As per datasheet, "When the input is single-ended, the INN input is held at the common-mode voltage,preferably at mid-supply". Is this mid-supply referring to VDD/2 generated through resistor divider?
3. In single-ended mode, what will be the effect on max gain setting of the PGAs?
4. Since the device is non-isolated, where do we need to insert the isolation? Is this isolation required for battery operated devices?
Please clarify our doubts.
We are working on a reply and should be getting back with you shortly
Hardware and Software Platform Development
Precision Data Converters
I will do my best to try and answer your questions below -
1. The shield drive interest and questions have had more to do with ECG and the RLD. We are not intimately familiar with a lot of EMG applications where the requirements for the shield drive may be different. ECG shield drive for the RLD, we invert the RLD signal externally and route it as the shield for the RLD signals.
2. Depending on what you are measuring, you could either use the WCT or RLD signal set to mid supply. These two signals are configurable using internal registers which can be read about in the data sheet. Setting the negative supply to mid rail is ideal because it lets you maximize the code range you are using.
3. Depends on the gain stage from the INA and what you have the INN pin set to. You cannot exceed the full scale differential input voltage of +/-Vref / GAIN OF ADC. The analog input to the ADS1298 including common mode restrictions is explained in detail on pages 24 and 25.
4. You will need patient protection on the front end to limit current that can shock the body. Where and how you implement any patient protection will depend on the classification of the equipment and the safety standards your end equipment needs to pass.
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