I am looking for some suggestions/guidance with setting up a pre-amplifier for electrode potentials (20mV to 100mV signal range, the band-width (BW) 10kHz to 100kHz). The excitation pair represents ac current excitation (10-20mA) and the measurements pair represents potential from remaining electrodes. The electrodes have high impedance (@1kHz) of 1 MOhm. The potential between electrode are connected to instrumentation amplifier (the gain should be between 10-100) and the output connected to RMS converter. The RMS output of each channels are connected to a DAC of dsPIC microcontroller.
For additional information, both the +IN and -IN will be multiplexed.
a) I don't know if there will be some kind of switching noise or cross-talk due to the multiplexer and if I should amplify the signals before I route them to the multiplexer to increase the SNR signal/reduce switching noise?
b) What would be the multiplexing frequency recommended for this application? How to calculate them?
c) How to reduce any capacitances at the input path
Sorry for the delay in getting back to you. I would for starters suggest you take a look at our general switch application note as it has some good information about using switches with amplifiers, see link below. Note, our lowest capacitance switches are the ADG12xx family and these minimize glitch noise (charge injection) when switching.
Thank you for the reply. I changed the switching circuit using 4:1 instead of 16:1 multiplexer which has identical circuits for all electrodes and easier to fabricate. The output selection of the 4:1 multiplexers are controlled by micro-controller. The new circuit is shown in attached file. I'm trying to construct a circuit which can measure conductivity of aqueous solution (conductivity range:100uS/cm to 3mS/cm) in a pipe with 16 electrodes. For that purpose, there will be current excitation pair which inject ac current (max 15mA) with frequency range from 10kHz to 250kHz. In the attached circuit the excitation pair is E1 and E2. Therefore the 4:1 multiplexer at E1 is switched to S1A (I+) and 4:1 multiplexer at E2 is switched to S2A (I-). The measurement electrodes are at E3 and E4 with both 4:1 multiplexer are switched to S3A. These electrodes measured surface potential (which is in pair) caused by the excitation current injected into the aqueous solution.
I followed your advice by searching the most suitable switches/multiplexers and I ended with two devices: ADG5204 and ADG1204. I seek for your expertise, based on my application which device is most suitable to be used in the circuit.
Our ADG5204 is a newer part and is a good part with low charge injection and capacitance values. Also it is latch up proof so I would say go with this one. But note it has an Ron in the region of 160 typical. It sounds like low capacitance is a priority in your application and the amplifiers have a high input impedance.