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TIDUB75-November 2015 Software Pacemaker Detection Reference Design 5
Copyright © 2015, Texas Instruments Incorporated
Mid-supply
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v
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R
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R
1
R
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R
2
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Figure 5: Differential to single-ended converter
2.2.3 AC Coupling
Even after the output has been referred to mid-supply, it may still have dc content. In ECGs, dc offset can
range up to a few hundred millivolts. It’s important to remove it so that the input to the pacemaker
detection ADC is centered at mid-supply giving it the most range within the rails of the converter.
This can easily be done by placing a capacitor in series with the input and biasing to mid-supply with a
large shunt resistor as shown in Figure 6.
Figure 6: AC coupling circuit
This circuit forms a high-pass filter. The cutoff filter should be placed as low as possible if the designer
intends to preserve the QRS complex of the ECG waveform. The constraint placed on the cutoff frequency
can be relaxed if the QRS complex is not needed on the pacemaker channel. Equation ( 4 ) describes the
half-power frequency of the circuit.
2.2.4 Non-Inverting Gain Stage/ADC Front End
Another amplifier is needed to drive the ADC’s sampling circuitry. This amplifier must have sufficient
bandwidth to successfully resolve the high bandwidth pacemaker pulse as well as charge the SAR ADC’s
sample and hold circuitry. Since an anti-aliasing filter will be placed at the output of the amplifier, the
bandwidth should be at least 4 times as large as the anti-aliasing filter’s cutoff frequency. This will
minimize harmonic distortion and improve overall stability of the circuit.