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An IMPORTANT NOTICE at the end of this TI reference design addresses authorized use, intellectual property matters and

other important disclaimers and information.

TINA-TI is a trademark of Texas Instruments

WEBENCH is a registered trademark of Texas Instruments

TIDUB75-November 2015 Software Pacemaker Detection Reference Design 1

Brian Pisani

TI Precision Designs: Verified Design

Software Pacemaker Detection Reference Design

TI Precision Designs Circuit Description

TI Precision Designs are analog solutions created by

TI’s analog experts. Verified Designs offer the theory,

component selection, simulation, complete PCB

schematic & layout, bill of materials, and measured

performance of useful circuits. Circuit modifications

that help to meet alternate design goals are also

discussed.

This circuit is designed to condition and digitize an

electrocardiogram signal output from the integrated

PACE_OUT buffer on the ADS129x to detect artifacts

of a pacemaker. This circuit includes an op amp

which serves as a signal conditioner and input driver

for a fast-sampling SAR ADC. The ADC

communicates using an SPI compatible interface.

This document also discusses developing a detection

algorithm and other digital signal processing

considerations.

Design Resources

TIPD197 All Design files

TINA-TI™ SPICE Simulator

ADS7042 Product Folder

OPA320 Product Folder

ADS1298 Product Folder

Ask The Analog Experts

WEBENCH® Design Center

TI Precision Designs Library

ADS7042

Block

Bias

Anti-alias

DSP/FPGA/

MCU

OPA320

PACE_OUTx

VCAP2

ADS129x

TIPD197

www.ti.com

TIDUB75-November 2015 Software Pacemaker Detection Reference Design 3

2 Theory of Operation

According to AAMI EC11, medical instrumentation must be capable of displaying pacemaker pulses with

amplitudes between 2 and 250 mV, durations between 0.5 and 2 ms, and a rise time of less than 100 μs.

These parameters will be used as the basis for defining the signal that this solution aims to detect. Figure

2 shows a circuit that may be used to detect a pacemaker pulse.

DSP/FPGA/

MCU

Mid-supply

SAR ADC

ECG

Lead

Differential

Amplifier

Differential to

single-ended

converter

Non-inverting

Gain Stage/

ADC Front

End

Coupling

Anti-aliasing

REF

Figure 2: Pacemaker detection circuit

The transfer function measured as the output digital code of the ADC is shown in Equation ( 1 ). The

quantity G

total

represents the total gain from all of the amplifier stages.

Ref

CodeOutput

total

Lea d



( 1 )

2.1 Understanding a Pacemaker Pulse

A pacemaker artifact will appear as a narrow pulse in an ECG waveform. The amplitude at which the

pacemaker signal appears in the ECG signal depends on the lead.

The characteristic of a pacemaker pulse which separates it from other biopotential signals is its fast rise

time and narrow width. In general, these characteristics will be leveraged in detection, but also they

provide constraints on the design. Figure 3 shows and example Lead II ECG waveform with a ventricular

pacer present.

Figure 3: Example ECG signal with pacemaker present

Such a narrow pulse would intuitively suggest a wide bandwidth. In this design, the narrowest pulse

targeted for detection measures 0.5 ms. A bandwidth of 4 kHz is sufficient to resolve the pulse. The

Nyquist inequality dictates that systems must sample more than twice as fast as the bandwidth. In practice

the signals should be well oversampled to produce a better reconstruction.

-2

-1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Voltage

(mV)

Time (s)

www.ti.com

4 Software Pacemaker Detection Reference Design TIDUB75-November 2015

In addition to the constraint created by the speed of the pulse, it also has the potential to be small in

magnitude. The input referred noise must be less than 2 mV if the pulse is to be identified. Ideally it should

be significantly smaller than 2 mV to prevent a false detection. How much margin is needed exactly will

depend largely on the detection algorithm.

2.2 Signal Conditioning

An ECG signal needs to be conditioned to be made ideal for pacemaker detection. The signal may need

gain, electrode offset needs to be removed, and the signal should be biased at the mid-supply voltage of

the ADC to provide the signal with the maximum possible range within the ADC’s conversion range.

2.2.1 ECG Front End

A typical ECG lead is comprised of the difference between potentials at two electrodes. For instance Lead

I is defined as LA – RA. This means the front end of any ECG sensor must be differential. The front end

must also comply with medical regulatory standards which limit the amount of current that can flow in our

out of electronic medical equipment. A differential amplifier with a high impedance input is a natural choice

for ECG front end since it meets both requirements.

Figure 4 shows a differential amplifier as an ECG front end. Gain can be provided by selecting R

and R

using Equation ( 2 ).

ECG

Lead

Figure 4: Fully differential amplifier as ECG front end

Equation ( 2 ) describes the transfer function of this amplifier scheme. The output can conveniently be

routed to a typical ECG acquisition channel as well as a pacemaker detection channel.















Leado

( 2 )

2.2.2 Differential to Single-Ended Conversion

It is convenient for pacemaker detection for the lead which is being probed for a pacemaker signal to be

single ended and referenced to a known potential. An amplifier can be used to take the output from the

fully differential amplifier and refer it to some voltage which is constant with respect to the board supplies.

Figure 5 shows a differential to single ended converter whose output is referred to mid-supply. The resistor

values will define the gain according to Equation ( 3 ).

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