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Latest reply:
Nov 26, 2011 5:48 PM by
GregoireLeGros 
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Well thats useful, as I thought I migt have to use an op amp. How should I calculate the RC time constant ? Obviously I will not be considering the heartbeat rate ( about 70 bpm) but what element of the waveform should I use? Should it be the fastest rising/changing slope associated with the signal ? Here is a picture of a typical ECG trace
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I used this circuit right out of the Linear Technologies LT1168 datasheet. As you can see it is powered from a split rail power source presumably because it's essential to have a patient ground.
Here is the circuit description from the same datasheet
Nerve Impulse Amplifier
set a gain of ten. The potential on LT1112’s Pin 1 creates
The LT1168’s low current noise makes it ideal for EMG
monitors that have high source impedances. Demonstrating
the LT1168’s ability to amplify low level signals, the
circuit in Figure 8 takes advantage of the amplifier’s high
gain and low noise operation. This circuit amplifies the low
level nerve impulse signals received from a patient at
Pins 2 and 3. R
G and the parallel combination of R3 and R4a ground for the common mode signal. C1 was chosen to
maintain the stability of the patient ground. The LT1168’s
high CMRR ensures that the desired differential signal is
amplified and unwanted common mode signals are attenuated.
Since the DC portion of the signal is not important,
R6 and C2 make up a 0.3Hz highpass filter. The AC
signal at LT1112’s Pin 5 is amplified by a gain of 101 set
by R7/R8 +1. The parallel combination of C3 and R7 form
a lowpass filter that decreases this gain at frequencies
above 1kHz. The ability to operate at
±3V on 350μA ofsupply current makes the LT1168 ideal for battery-powered
applications. Total supply current for this application
is 1.05mA. Proper safeguards, such as isolation, must be
added to this circuit to protect the patient from possible
harm.
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OK, here is the "easy" fix. Instead of R6 and R8 returning to ground, they return to +1.5V. Since you are probably using batteries, you should be able to add a connection here. Now your output amplifier will be putting out +1.5V with no signal. You will be able to swing almost 3Vpp output into the ADC.
You are limited by +3V max output from the LT1112, and on the negative side 0V from the ADC. So biasing the LT1112 resistors to +1.5V (the midpoint) is the best place to be!! No extra parts. Just change some wiring or cut traces and add jumper on your board to the battery holder.
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GregoireLeGros,
My first thought is that something as simple as adding an old fashioned Summing Amplifier may accomplish both the level shifting and output scaling to fit your 3.3V input ADC.
You could set it up with two inputs to sum together. One would be your +2.5V/-2.5V output from your LT1168/LT1112 nerve impulse amplifier, and the other would be a -2.5V reference voltage you could set with a voltage divider off your -3V rail.
If you set the RF/Rin ratio to 0.5 then Vout would range from 0~2.5V instead of 0~5V. That voltage range should then be acceptable for the extra summing amp if you give it +3V/-3V rails as well, and for the 3.3V max input voltage of your ADC.
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Oh yes, I forgot. The data would be "flipped" due to the pesky inverting nature of the Summing Amplifier. Depending on how you handle the ADC value ranges, your display graph would probably end up mirrored across the horizontal axis. You should be able to correct this in software code easily enough though... Or alternatively have it graph from right to left instead and mount the LCD upside down just for the fun of it
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