Medical Electronics

[audioguru]

Ordinary opamps have a input offset voltage of up to 10mV. The gain of 1785 would amplify an input offset voltage of only 1.85mV causing the output to be saturated against the positive power supply voltage or to ground.

An instrumentation amplifier has very low input offset voltage.

If you used opamps that have very low input offset voltage then the circuit should work.

I don't know if patients are grounded.

 

[littletransistor]

Ordinary opamps have a input offset voltage of up to 10mV. The gain of 1785 would amplify an input offset voltage of only 1.85mV causing the output to be saturated against the positive power supply voltage or to ground.

An instrumentation amplifier has very low input offset voltage.

If you used opamps that have very low input offset voltage then the circuit should work.

I don't know if patients are grounded.

Oh, I see. Hmm... my opamps are all ST Electronics uA741. I gonna check the input offset voltages of these in the datasheet.

Meanwhile, I just ordered an Burr-Brown INA126 for myself a few days ago. So, for this particular all-in-one instrumentation amp, how to hook this thing up using single supply? Any guides? The datasheet didn't state so... so I'm still searching here.

 

[audioguru]

The uA741 opamp is too old and too noisy to be used in an ECG circuit.
It is spec'd with a 30V total power supply voltage. Some don't work when the power supply is only 10V.
Its max input offset voltage is 6mV at room temperature plus its input offset current.

 

[mneary]

There's no way the 741 will work in this circuit; its input offset is way too large. Wait for your INA126 to arrive.

For the INA126, just connect V- to Bat(-), V+ to Bat(+), and connect the common (patient) to a source of Vcc/2 (actually better to use a regulated voltage near the nominal Vcc/2) The output will be referenced to Vcc/2.

(If the voltage between input+ and input- is 0, then the output is the voltage you applied to common (patient).)

 

[littletransistor]

The uA741 opamp is too old and too noisy to be used in an ECG circuit.
It is spec'd with a 30V total power supply voltage. Some don't work when the power supply is only 10V.
Its max input offset voltage is 6mV at room temperature plus its input offset current.

I checked the datasheet already - the Vs stated is +/- 1.35V to +/- 18V. Is that the total power supply voltage? If so, I can continue using the 9V batteries, or even 1.5V x 2 batteries if this is permissable. The offset voltage stated in datasheet is also 250 microvolts max.

Meanwhile, I have seen a magazine section about homemade ECG system. Apparently the instrumention amp isn't homebrewed, but it's using the Analog Devices AD620, and it's pretty expensive. Not sure how effective (or precise) it is, but I'm pretty sure it'll work okay.

 

[mneary]

You're looking at a different 741 than we are. The National data sheet shows an offset voltage of 4 millivolts or more typical. These impressive specs are only valid at +/-15V supplies.

 

[audioguru]

The uA741 in the datasheet from ST Micro performs poorly.
Its input offset voltage is 5mV max. at room temperature.
It input offset current is 30nA max.
All of its spec's are when it has a 30V total supply voltage.

The datasheet has graphs:
1) Its gain is low when the supply voltage is low.
2) Its output voltage swing is low when the supply voltage is only 10V total.
3) Its input common-mode voltage range is very low when its supply is only 10V total.

 

[littletransistor]

The uA741 in the datasheet from ST Micro performs poorly.
Its input offset voltage is 5mV max. at room temperature.
It input offset current is 30nA max.
All of its spec's are when it has a 30V total supply voltage.

The datasheet has graphs:
1) Its gain is low when the supply voltage is low.
2) Its output voltage swing is low when the supply voltage is only 10V total.
3) Its input common-mode voltage range is very low when its supply is only 10V total.

Oops, sorry, I'm asking about my new Burr-Brown Instrumentation amp INA126 just now. The ST uA741 isn't up to the task pretty well I guess. But never mind, I think I need to find a better one.

 

[Oznog]

Hey there,

sorry for not replying earlier - i'm in the midst of developing a simple EKG system. So far it has been good, but there's still noise picked up. I've already 'installed' a low-pass filter to get rid of 60Hz hum, but there's still a small waveform 'riding' on my ECG signals. I guess I need a hi-pass filter.

Some ECG kits actually did their hi-passes using op-amps, but can I just do the conventional RC way?

Digital signal processing, in particular adaptive filters, are often necessary for obtaining a reliable, quality signal.
"using op-amps" is an "active filter", which greatly outperform passive filters made of just R, C, and L. You will want to use an active filter.

 

[audioguru]

A switched-capacitor Butterworth lowpass filter from Maxim-IC has a flat response in the passband then an extremely sharp slope.

 

[littletransistor]

Hi there,

I got my Burr-Brown INA126, and it's pretty simple a lot now with these collection of opamps!

But, after I connected the wires (right arm, left arm, leg = ground) the thing still refused to pick up electric signals. But touching the contacts hard actually make the lines in the 'oscilloscope' (I used some voice recording software) to jump - and it almost resembles pressure detector instead of the ones which takes in faint electrical signals. Not sure why, but I really need a full oscilloscope to actually see whether it has the waves or not. Currently, my computer's "line in" isn't pretty good at all! Any problems in my amp (see pic 1) ?

the 2nd picture shows the Amateur Scientist's excerpt of simple ECG system - using either AD620 or 624 will do and it gets signals quite nice too in the oscilloscope! But not sure whether did he hooked it up to the computer line in or not.

 

[audioguru]

It looks like your INA126 circuit is missing many things:
1) No power supply. Will it be single or dual polarity?
2) If you use a single supply, then the Ref pin 5 must be biased at about half the supply voltage and a filter capacitor must be added.
3) No gain adjust resistor.

The ECG circuit I attached uses many more parts.

The output pin of the INA126 must have a DC voltage on it that is about half of the supply voltage when it has a single supply. Make this new circuit then measure yours.

 

[littletransistor]

Hi there,

I did rebuild the circuit you posted with the attachment - I got a feeling in my gut that this circuit is working, but I'm not sure why my casual wires isn't picking up any electrical signals from my body upon contacting it. The only thing the 'oscilloscope' jump is when I repeatedly touch the wires, but holding them in one spot I got a straight line.

Must I get the electrodes, or must I need to add some more things into the circuit?

edit: also, i have a gain of above 1000 in the circuit. the electric signals of body is -0.4 to 1.0mV. So it should be at best, -0.4 to 1V too.

 

[audioguru]

I did rebuild the circuit you posted with the attachment

Please attach the schematic of which circuit you made. Mark the DC voltage of the power supply, the DC output of the instrumentation amp and the DC output of any opamps you used.

 

[littletransistor]

Please attach the schematic of which circuit you made. Mark the DC voltage of the power supply, the DC output of the instrumentation amp and the DC output of any opamps you used.

Okay. Here's the schematic. Some of the new details are added to yours. Btw, the Rgain is 56 ohms, so I pretty can say that it's pretty sufficient for amplification of the small electrical signal. The DC voltage is 9V. But the DC output... how am I going to measure it?

And I read that these simple ECGs needed leg driver as well - but the Amatuer scientist edition only uses one instrumentation and one filter and the author could get it done.

 

[audioguru]

Pin 4 of the operational amp is connected to the negative terminal of the 9V battery and to the shield of the output shielded cable to the scope or analyser.
The DC voltage at the output pin 6 should be from +3.75V to +5.25V with respect to pin 4.

The Biased Ref must connect to the body (a leg) and the inputs must also connect to the body (arms). Do not connect the body to pin 4 of the instrumentation amp.
Your filter capacitor for the reference must be pretty big, try 100uF.

The circuit does not have a lowpass filter to reduce mains hum.

 

[littletransistor]

Pin 4 of the operational amp is connected to the negative terminal of the 9V battery and to the shield of the output shielded cable to the scope or analyser.
The DC voltage at the output pin 6 should be from +3.75V to +5.25V with respect to pin 4.

The Biased Ref must connect to the body (a leg) and the inputs must also connect to the body (arms). Do not connect the body to pin 4 of the instrumentation amp.
Your filter capacitor for the reference must be pretty big, try 100uF.

The circuit does not have a lowpass filter to reduce mains hum.

I see. Unfortunately the DC voltage is at least +7V when I tested it. I don't know whether the output should be grounded, but I'll test more later. Btw, can I use a multimeter to test my body voltages first?

I did using a simple ECG using a filter, and only one 741 with the feedback, and it worked, but there's the weak amplification and too much noise. But I already have an instrumentation - should be easy right? And all I got is no output except when I tapped the inputs.

 

[audioguru]

Maybe connections to the body need some conductive grease.

Test the input offset voltage of your circuit by connecting both inputs to the Ref then measure the output voltage. It should be plus or minus about 0.75V from half the supply voltage.

I don't know if medical circuits are earthed. They might be double-insulated for safety.

 

[littletransistor]

Maybe connections to the body need some conductive grease.

Test the input offset voltage of your circuit by connecting both inputs to the Ref then measure the output voltage. It should be plus or minus about 0.75V from half the supply voltage.

I don't know if medical circuits are earthed. They might be double-insulated for safety.

Yeah, I have these conductive greases. Looks like when I rip out the Rgain, that thing still doesn't budge - I suspect the thing isn't amplifying at all, regardless of the presence of the resistor.

Here's my breadboard attachment - I'm sure there are errors inside, but I couldn't find any so far.

 

[audioguru]

Your circuit is missing the 100uF capacitor from pin 5 to pin 4. The inputs are not balanced without it and then they pickup and amplify a lot of mains hum.

Also, you don't show a wire called Ref from pin 5 that connects to the patient's leg. Without it then the inputs are not biased.

Measure the output DC voltage again. It should be close to half the supply voltage. What is the supply voltage?