Amplifier, buffer isolation

Acridinium ester—labeled chemiluminescent probes have been utilized to detect the specific protein-coding transcripts and to distinguish between transcripts that code for the 190-kDa protein and the two closely related 210-kDa proteins. The assay is called the hybridization protection assay (D3). In this assay, RNA isolated from the patient s white blood cells is first amplified by PCR. The amplified product is incubated with the chemiluminescent probe. The unhybridized probe is removed by selective hydrolysis in sodium tetraborate buffer, containing surfactant Triton X-100 at pH 8.5, in an incubation step at 60°C for 6 min. After the sample is cooled to room temperature, the chemiluminescence of the hybridized probe is measured in a luminometer. The procedure is reported to detect one leukemic cell in a population of a million or more normal cells. It is also rapid, requiring less than 30 min. Its reliability has been attested to by correlation with results obtained on karyotypic and Southern blot analysis (D3). 

The primary source of error is ground loop currents. This is caused by galvanic errors introduced by small potentials resulting from the ionic liquids and dissimilar metals that the electrode is in contact with in a bioreactor. Additional sources of error are interactions with other electrodes. We have frequently found that a pH electrode not connected to an isolation amplifier that floats the reference can show errors of 1-2 pH units. A simple test is to measure the pH of a buffered solution on-line and off-line to check the accuracy of a measurement. 

The process compatibility and the isolation of the ionics section of the chip from the electronics makes it relatively safe to increase the complexity of the chip. An eight channel structure which has a multiplexer and an A-to-D converter in addition to the buffers has been reported (54). Another example of a chip lay-out which contains CMOS circuitry is shown in Figure 6. This chip contains operational amplifiers, a pulse edge modulator and an RF transmitter (55). 

Transformer-coupled isolation amplifiers perform on the basis of inductive transmission of a carrier signal that is amplitude modulated by the biosignal. A synchronous demodulator on the output port reconstructs the signal before it is fed through a Bessel response low-pass filter to an output buffer. A power transformer, generally driven by a 400-900 kHz square wave, supphes isolated power to the amplifier. 

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