Ecg Heart Rate Monitor Design

[keane2097]

Yes - the reason you have that +2.5V reference is that it is there to provide a signal ground; it makes the 0V look like -2.5V, the +2.5V ref look like 0V, and the +5V supply look like +2.5V.

On the spec sheet for the OPA2335 it even boasts of its "Single supply" operation, right at the top of the page. Most opamps can run this way, but it's an indication that was what the designer intended for this circuit.

Yeah I noticed that alright and I figured it was at least a good part of the reason I've been having problems. I'll try the setup you've recommended here anyway and I'll let you know how I get on.

Thanks a million for all your help.

 

[keane2097]

Ok I've now tired the setup Duffy recommended.

I've got pin 7 (positive supply pin) of the Instrumentation amp connected to pin 8 (positive supply pins) of all the op amps. These pins are all connected to a 5V supply.

Next I've connected pin 4 (negative supply pin) of the Instrumentation amp connected to pin 4 (negative supply pins) of the op amps and these pins are all connected to 0V from the same power supply.

Finally I have two 10k resistors connected in series between 0V and 5V. I'm taking the centre point of the two resistors (2.5V) to supply the non-inverting inputs of the 3 op amps A2, A3 and A4 with the 2.5V pedestal voltage.

Does this all sound ok?

When I turn on the power supply to the circuit when set up like this I'm immediately seeing the voltage drawn drop to almost nothing, and the power supply is telling me I have a short circuit somewhere.

Does anything in my power supply setup give a clue as to why this might be or have I simply got something wrong somewhere else in the circuit building?

 

[keane2097]

I figured out why I was getting the short circuit I mentioned in my last post - I tested the op amps by making a little voltage follower and one of them was blown. I replaced it and the short circuit disappeared.

I've now gotten rid of the voltage divider and I'm using a separate power supply to give me the 2.5V pedestal voltage. I've also added a voltage regulator at the input to stabilise the 5V supply...

I'll keep ye posted on my progress...

Thanks a lot for all the help so far...

 

[keane2097]

Yes - the reason you have that +2.5V reference is that it is there to provide a signal ground; it makes the 0V look like -2.5V, the +2.5V ref look like 0V, and the +5V supply look like +2.5V.

On the spec sheet for the OPA2335 it even boasts of its "Single supply" operation, right at the top of the page. Most opamps can run this way, but it's an indication that was what the designer intended for this circuit.

Just to clarify this point - when I probe the supply pins of the op amps relative to the ground from the 5V source I get a reading that 5V is going into the positive supplies of the op-amps and 0V into the negative supplies. Is this what I should be seeing or should I be seeing +/-2.5V?

 

[duffy]

Depends on where you put the negative lead of the probe. If you put it on 0V, it will read like that. If you put it on the +2.5V "pedestal voltage" (which is what the opamps consider "ground") it will read +/-2.5V.

 

[keane2097]

I've been testing out various parts of the circuit on their own since I still don't have the overall circuit performing correctly...

I've been having a look at the integrator part of the circuit and I've built it stand-alone to test it. This is the setup I have:

**broken link removed**

Now all the theory I've come across suggests this should behave as a low-pass filter, and indeed this is the behavior I see when I test the circuit, however in the overall schematic of the ECG Amplifier it seems to be suggested that it should behave as a high-pass filter to eliminate baseline wander...

Can someone please explain how the setups differ?

Thanks...

 

[keane2097]

I'm currently trying to debug my circuit by building the different blocks separately and making sure they are working as expected. Currently I'm looking at the O/P amplifier as shown:



**broken link removed**



The setup I'm using is supplying the op amp with +/- 2.5V and I've got the positive input grounded. The negative input takes in the signal...



If I have, say, a 10mV p-p sine wave as my input what should my output be? I'm guessing I should be getting a 2V p-p sine wave from the schematic, but I'm not sure how to do the calculations to confirm this?



Thanks