Technical Description - ADInstruments pH Amp Owner's Manual

Figure A–1
Block diagram of the
pH Amp
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Technical Description

As with other PowerLab front-ends, all internal functions of the
pH Amp are controlled from the PowerLab by sending information on
a special communications connection called the I
bus. This connection supplies power to the ADInstruments front-end
as well. ADInstruments front-ends are connected to the analog inputs
of the PowerLab via BNC-to-BNC cables, through which the amplified
and filtered signal is sent to the PowerLab. The overall operation of
the pH Amp can be better understood by referring to Figure A–1.
The pH input amplifier of the pH Amp is an electrometer type
amplifier with extremely high input impedance. The input also has
provision for connecting a reference electrode to the analog ground of
the amplifier. The input amplifier gain can be set by the software to 1
or 10, depending on the required sensitivity. The output of the
electrometer amplifier is fed to a 2 pole, 10 Hz low-pass filter which is
software selectable. The output is then fed to the PowerLab for further
amplification and filtering.
The temperature amplifier is effectively a separate ADInstruments
front-end. It consists of a bridge circuit that is factory configured for
use with RTD temperature probes. The pH Amp comes supplied with
an RTD temperature probe. The output from the temperature
amplifier has been factory set with the supplied RTD probe to produce
0 – 5 V over 0 – 100 °C.
Reference input
Electrode input
Electrode online
indicator
Temperature
online indicator
RTD input
2
Low-pass filter
2
I C interface
pH Amp Owner's Guide
C (eye-squared-sea)
Electrode
signal output
2
I C Output
2
I C Input
Temperature
signal output