How can I apply a DC offset to a very small signal?

Development Boards Arduino
1

Hi!

I’ve developed an ECG acquisition circuit that will acquire an ECG signal using a high CMRR, and uses active lowpass and highpass filters to filter the signal to a desired frequency range (0.5 - 150 Hz).

My signal output is clear but it is usually between -0.2 and +0.2 Volts and sometimes as small as -0.1V and +0.1 Volts.

I’m trying to apply a DC offset to this signal so that I can read it into an Arduino Mega (which can only read in voltages from 0-5V!) using AnalogRead() ?

I’ve tried to use a positive clamper circuit but my signal is too small to overcome the bias voltage of a diode!

Any suggestions as to how to get my signal in the positive range, without causing issues with my upstream ECG circuit?

Thanks so much!

2

Use an op amp to add any DC bias you like.

For an informed response, post a clear schematic diagram of your circuit.

3

I’ll draft something up now.

What would the advanage be of using an op-amp (in a summer configuration, one Vin with my ECG signal, and one Vin with a DC signal?) and using a configuration such as a coupling capacitor between my ECG circuit output and a voltage divider providing 1V output?

4

With the op amp, there would be no loss of signal amplitude and the circuit would work for extremely low frequency or DC signals. But, try the coupling capacitor & divider offset, as it is much simpler.

5

I tested the circuit with a regular sin wave from a function generator and found that the coupling capacitor and DC offset caused some attentuation and distruption of the signal, I used a 10uF capacitor.

I also built a voltage divider and voltage follower to create a 0.5V DC signal and fed that into a summing amplifier with the sin wave signal as the second input which successfully brought the signal up.

My goal is to replace this sin wave signal with the output of my ECG signal.

Are there any implications of this on the ECG circuit upstream? My circuit schematic is here

http://imgur.com/a/cbOb6

the Op-amps are LM741

Vin is actually the output of the instrumentation amplifier which connects to leads which you connect to your right arm and left leg for inputs, and then your right leg to record the common-mode signal.