HOWTO: Designing and building a Bioamplifier & Acquisition System : Instrumentation Amplifier & DRL
In this second post i will talking about the design and the configuration of the instrumentation amplifier.
A basic instrumentation amplifier is here : http://www.national.com/ms/LB/LB-1.pdf
National, being one of the earlier semiconductor companies to work in analog have put it well in that application note.
A Biological Signals have a few qualities (mostly irritating), that make them difficult to be picked up and amplified. In biological signals, a signal amplitude of anything around 100uV is considered aking to striking gold. Normally we deal with signals that are in the range of few tens of microvolts. and the current that is noticed is generally in nA. The second most important feature is the observed frequency of the signals, they tend to be within 100hz.
At Such low values of voltage and current, a simple set up which is supposed to pick up these signals at high gain, will instead pick up the 50Hz power signal that is radiated by wires carrying electricity that powers your lab or work place (in non shielded rooms), And when connected to your body, your body will act as a antenna for the radiated power signals and then this becomes a part of the input given to the amplifier.
Generally a good quality instrumentation amplifier can handle the 50Hz noise because of inherent ability of the amplifier to reject the common mode voltage while dealing with everyday signals. However, at these low voltages and because the “ground” of the two objects in question. i.e. the patient and the object are different, most medical devices opt for “Active Cancellation”, this technique is referred to as a using a DRL circuit (Driven Right Leg) , the DRL circuit measures the common-mode volatage and sends back a phase shifted signal which cancels the incoming 50hz signal, thus providing a signal clear of any powerline interference.
Now, Let us Look at the design for Channel 1.
The Datasheets one would need are for the:
AD8221 and OP07
The Reference Schematic is:
<Click to View an Enlarged Image>
I’ll be referring to components by the labels in the schematic.
The Front End of the channel first has,
Two resistors and capacitors to make sure that there is no short between the device and the electrode, this was you can ensure that you might not inadvertently electrocute your subject, this is just basic protection better isolation devices using optically coupled isolators are usually used in approved medical instruments.
The diodes are there to prevent any excess voltage from affecting the instrumentation amplifier, since eeg’s have a range of hundreds of uV, this suits us fine.
The ping out for AD8221 Looks like:
The connections are self explanatory.
The DRL circuit is built using the LM358, the LM358 takes the common mode voltage input from the circuit and uses that to generate a signal that will actively cancel the 50Hz noise.
The OP-07 filters the signal, gets the DC offset generated and that is used as reference voltage (Vref) to cancel any baseline drifting and wandering.
This Concludes Part I of the Series. The next post will be on the Filtering process, the gain stage and how to use an analog multiplexer.
