How is analog signal converted into digital in ble_app_hrs and with what accuracy?

[Ina]

Hi,

I have nrf51dk and analog EKG sensor. I know that ble_app_hrs allows to get R-R interval, but in my application i need to get also R-S and R-T intervals (or just R, S and T timestamps) and send it over BLE.

My questions are:

1. What kind of algorithm is used to convert analog signal from sensor into digital?
2. Is the resulting digital signal accurate enaugh to get R,S and T timestamps? (for example, if my analog signal would look like in the picture above, how would look digital signal retrieved from it - will all the meaning peaks be possible to catch?)
3. Is it possible to write such application by just modifying ble_app_hrs example?

Thank you in advance for any help with this issue.

[Øyvind Karlsen]

Hi,

The heart rate shown in the hrs example is a synthesized value. To get the values from your analog sensor you will have to use an analog-to-digital converter(ADC) fortunately there is one on the nRF51, have a look at the ADC HAL example from the SDK.

Once you have configured the ADC to get readings from your sensor you can replace the synthetic service with your actual measurements in the ble_app_hrs example.

From the product specification for the nRF51422 you can see the specifications for the ADC. The time to convert an 8 bit sample is stated as 20µs, this is your maximum sampling frequency, if this is lower than what is specified in your sensors datasheet then you should be fine. Note: The ADC can be configured to use more bits for higher accuracy(up to 10), however the sampling frequency will go down.

Best regards,

Øyvind