Reference electrode Sampling

[navneet]

Hello Everyone,

I had posted this question on another board, but unfortunately no one has been able to provide any help so far. Was hoping to get your views on this (I have always been active on LabVIEW board, so my reposting here ).

I have a question and was hoping to get this answered. Any references to proper literature will also be greatly appreciated.

I am using the USB 6212 DAQ and want to measure the potential difference between two reference electrodes (Cu|CuSO4). 

The requirements for the electrodes are that it should be connected through a high impedance amplifier to avoid leakage of any current.

I am using AICH0 under differential connection to connect both these electrodes. Am I correct in saying that under differential connection, the input impedance of the channels are very high (10 GOhm), and thus no external circuitry is needed. That means these electrodes can be directly connected to the differential inputs without the need for an external buffer (to boost the input impedance). 

Any advice will be greatly appreciated.

[johnsold]

The specifications for the 6212 show an input impedance of 100 GΩ in parallel with 100 pF with power applied.  It also specifies an input bias current of +/-100 pA. If your electrodes (and electrochemical system) can provide 100 pA without impairing performance, you may be OK.  If you will be switching channels, please look at the settling time data and be aware that the kinds of multiplexer circuitry used in many multichannel DAQ devices do not perform well with high impedance sources.

Lynn

[navneet]

Thanks Lynn. I did get a good reply in the other forum.

I tried the following.

For a pure resistive network consisting of 2 resistors (3.3 kOhm, and 220 kOhm) I tried to measure across the 220 kOhm (using ordinary wires):

a) the differential voltage across them - well measured.

b) use voltage buffers (TL071) on both the differential lines - well measured.

c) use INA111 to measure the voltage across the 220 kOhm referenced to AIGND - well measured.

I tried to simulate the reference electrode using high resistive load. All were measured well and infact the phase difference between the input and output for all 3 conditions above were very impressive (bound within -0.5mrad to 0.5 mrad). I am thinking because of this, it wont be necessary to apply any external signal conditioning to the signals from the elecrodes and feeding it directly into the card will be ok. Does this sound any firm? I will be trying the electrodes later during the week and see how it goes.

cheers,

[johnsold]

If your simulation of the electrode is accurate then you should be OK.

Lynn

[navneet]

Thanks Lynn.

Tried using the reference electrodes. Connecting directly to differential inputs OR using INA111 as a preamplifer and then connecting with respect to AIGND gives similar results.

Am impressed with USB6212.