The ECGUConn BME 290

Buffers

Buffers

• What is a buffer?– Non-inverting amplifier

with a gain of 1

• Why use a buffer?– Buffers provide a high

input impedance

C2 and C3

C2 and C3

• At high frequencies a capacitor acts as a:A. Short circuit

B. Open circuit

• At Low frequencies a capacitor acts as a:A. Short circuit

B. Open circuit

C2 and C3

• What do they do?– C2 and C3 act to remove any DC component

of the signal

• Why would you need to do that?– If the signal received from the body has a DC

offset it will be multiplied by the diff amp

Differential amplifier

Differential Amplifier

• V1 & V2 are in the microvolt range

• We need it to be in the milivolt range so Rf/R1 is set to ~1,000

• If C2 and C3 had not existed, it is possible that a 10mv offset could exist between V2 and V1

• This 10mv offset would then be multiplied by 1,000 (Gain) which would set to 9VDC

1st Order LPF

1st Order LPF

• The ECG is known to be a low frequency signal

• A LPF can be used to remove the high frequency noise in the signal

• How do you pick Fc?

Filter design using Fast Fourier TTransform (FFT)

• Problem– How to find a suitable

cutoff frequency for the ECG filter

• Solution– Use the FFT– The signal exists

below 40 Hz– Noise at 60 Hz?

Summing Amplifier

Summing Amplifier

• The ECG signal Goes below 0 VDC

• Microprocessor has an input range from 0 to 5 VDC

• Need to add a dc offset to the signal so that the signal stays within the input range of the microprocessor

Microprocessor

Microprocessor

• Provides Analog to Digital (A/D) conversion

• Transmits digital data serially to the Isolator

Digital Isolator

Digital Isolator

• Isolates the patient from the computer

RS232 Converter

RS232 Converter

• Converts Serial digital signal from TTL 0,5 VDC to RS232 format 10,-10 VDC