Amplifiers and Interference Flashcards
What is an operational amplifer?
Designed to amplify i.e. increase or gain an electrical signal in a linear fashion without distortion.
They have a very high input resistance (impedance) more than 1MΩ in order not to draw current from the upstream circuit.
The inputs consist of one which maintains the sign of the input signal and one which switches the sign (inverts) the input signal. It then only amplifies the signal difference between the 2 arms to neutralise background signal.
What is Common mode rejection ratio (CMRR)?
Common mode rejection ratio is a feature of operational amplifiers.
Amplifiers which have CMMR, have 2 differential inputs: and inverting and a non inverting differential input.
A CMMR neutralises upstream circuit noise, i.e. signals common to both inputs, and only amplify input voltage difference (signal which is not common to both inputs).
What is meant by having a CMRR of 10 (to the power 4)?
Means a common mode signal of 10 000 mV is needed before 1 mV enters the op- amp for amplification
What would happen if you had a single input into an Op amplifier as part of an ECG circuit?
It may cause capacitance coupling (transfer of energy via capacitance) will amplify mains hum in which an ECG signal may become swamped and unreadable.
What are some of the important safety features of an ECG circuit?
No patient connection earthed. The patient circuit is ‘floating’
DC, non-earthed power source to op amp.
An opto-isolating circuit connected to a further (earthed) op amp circuit designed to produce a signal output waveform, which is completely isolated from the patient.
With regards to analogue to digital conversion what is the significance of the sampling rate?
Points on an analogue sine wave are sampled at a chosen frequency. A digital value is assigned to each sample, giving a ‘staircase’ waveform.
If the sampling rate is too low then the digitised wave form will be a poor representation of the analogue waveform.
What components may be required in a signal processing circuit?
Op-amp (amplify only wanted signals)
Filtering circuit (RC/LR low pass filter, RC high pass filter or band pass filters)
Differentiating/integrating circuits
Single input amplifier i.e. via a transducer from a Wheatstone bridge
Analogue to digital converters
When processing a biological signal (such as an invasive BP) what is required from the circuit?
Transduce pressure waveform
Amplify the appropriate signal
Filter distracting signals
Convert analogue to digital
Store the information and display it
