Hardware Pace Using Slope
Detection
Tony Calabria
10/22/2012
The Concept
Subhead text here
2
ECG + Pacemaker Signal
Slope Detection
• Frequency components of ECG lie in 0.05 – 150Hz. Frequency of
PACE components reside in >1KHz.
• Monitor for Slope of Pace Signal while ignoring ECG signal slope.
• Need to measure the rate of change of voltage to determine slope:
Differentiator Circuit
– Output signal should not respond to ECG
– Pace Signal should only pass
– Looking for a specific dV/dT
• Pace Spec required to meet:
– 2mV Amplitude Signal
– 100us Period Signal
Alert/Trigger
• Need to trigger an Alert bit when Pace is present
• Using a window comparator, a pulse can be created once the output of
the Differentiator Circuit shows a change outside the preset window.
• Threshold limits set externally depending on amplitude of Differentiator
Output at minimum Pace signal spec requirements (2mV, 100us).
• SR Latch can be used to latch and hold the value of the Comparator
output indicating a Pace Signal has occurred.
• Mandatory that an ECG Signal does not create a response by the
comparator outputs.
Differentiator Circuit
• Looking for specific slope: dV/dT
change
• Duration of slope produces
output amplitude
• R used to set gain
fc 
1
2  R  C
• Frequencies above fc, the circuit
is acting as ordinary inverting
amplifier
Source: http://circuitalley.phpnet.us/circuit3.html
6
Window Comparator
• The Differentiator output signal will idle within the window when PACE
is not present
• When PACE appears, the output of the differentiator circuit will toggle,
forcing the voltage outside the limits of the window comparator
– The output to pulse low.
– That low pulse must be latched and held until read back by a uC or GPIO
7
SR Latch
• SR Latch output high once Window Comparator outputs low pulse
• Reset line is pulled high
• Latch stays until reset by user.
• Would need to reset with every QRS waveform if wanting to monitor for
pace with each heartbeat
8
Proposed Circuit
9
Circuit Stability
Subhead text here
10
Differentiator Circuit Analysis
C1 1u
R2 0
C2 10n
C2 used for Op amp Stability
and High Freq gain control
R4 392k
Vo
VCC
Vfb
Vref
V1 2.5
L1 1T
C5 10p
V2 2.5
C4 1T
+
-
+
Vref
+
VCC
C3 10p
VG1
U1 OPA348
11
Differentiator Circuit Stability
T 200.00
Rate of Closure =
20dB/Dec
Vo
100.00
Gain (dB)
Vfb
beta
0.00
-100.00
-200.00
200.00
Vo
Phase [deg]
100.00
Vfb
0.00
beta
-100.00
1.00m
100.00
Frequency (Hz)
10.00M
12
Time Domain Analysis
VCC
Vref
C2 10n
2mV Amplitude
100us Period
V2 2.5
C1 1u
-
+
+
OPA348
ECGp
R4 392k
R2 0
-
Differentiator out
U1 OPA348
+
+
VECG_block
+
VCC
+
VCC
R1 1M
ECG+PACE
Vpace Pos
V1 2.5
Vref
Model to replace
internal ADS1298
Pace Amplifier
13
Time Domain Analysis Cont.
T
2.51
Differentiator out
Set threshold point for
Window Comparator
2.44
2.00m
Pace input pulse
ECG+PACE
-1.00m
2.50
VECG_block
2.50
180.00m
190.00m
Time (s)
200.00m
14
Window Comparator Threshold
VCC
VCC
VCC
~ 2.77V
R6 10k
+
R16 100k
R5 8.06k
Vref
+
-
U3 TLV3401
V1 2.5
V2 2.5
V_PACE_OUT
To SR Latch
Differentiator out
VCC
+
U2 TLV3401
+
-
R8 10k
~ 2.48V
R7 10.2k
VCC
15
Pace Circuit Design with Values
16
Testing
Subhead text here
17
Pace Card Design
• Designed to mate with
ADS1298ECG-FE board
• Top side populated for PACE
OUT 2 and bottom side for
PACE OUT 1
• Requires one 5V supply
• SR Latch Outputs routed to
ADS1298 GPIOs
• Analysis required for circuit
stability
Unstable Design
Differentiator out
Comparator out
SR Latch
19
Stable Design
Differentiator out
Comparator out
SR Latch
21
Pace Circuit Design with Values
22
Pace Circuit Layout
23