Marquette Hellige GmbH MicroSmart V 2.xx Page 23
Servicing Instructions 227 470 35 B - 97.12
Filter and interface module
This IC (DIGI_HEC2) essentially incorporates the filter functions and the serial interface.
Fundamentally, the transfer bandwidth is 0 - 250 Hz for a scanning frequency of 1 kHz and 500 Hz,
with the upper cut-off frequency determined by a sinc filter of the 3rd order. The lower cut-off
frequency can be set within the range of 0.039 - 79.6 Hz (4.08 - 0.002s) by selecting the time
constant. Selecting this time constant also causes the separation of the DC content, which may be
superimposed over the ECG signal as polarization voltage. An IIR filter algorithm is used. The
algorithm only captures the lower 12 bits of the 18 bit result. This means that any sudden changes
at the input are always represented as changes with amplitudes < 20 mV. Limiting the display range
to ± 10 mV and selecting a suitable value query prevents sudden changes over the entire display
range when exceeding the range limits. A saturation value is delivered at about 10 mV, until the
measuring signal returns to within the display range.
For the useful signal transfer (ECG signal) the lower 12 bits are transferred with the selected
scanning frequency. However, there is also the option of using the appropriate control words for
special function tests to transfer the upper 12 bits without DC separation. In this case 1 LSB
corresponds to 320 µV. Possible function checks include testing the signal path, measuring the
polarization voltage, measuring the electrode impedance and testing the serial data transfer, all by
activating these functions by using the appropriate control words.
Measurements of the polarization voltage are allowed by transferring the upper 12 bits of the
converter result (1 LSB = 320 µV).
During the serial data test, a test word transmitted by the CPU will be returned immediately
thereafter by the ASIC DIGI_HEC2.
To Next Page
Loading...