High Input Impedance for Reference Electrodes - USB6212

[navneet]

Hello Everyone,

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 preamplifier (to boost the input impedance). 

Any advice will be greatly appreciated.

cheers,

[JVP]

Hi Navneet,

The 6212 may not require any pre-amplification. However, would you mind providing more information on the electrodes? It would be helpful if you were to perhaps link a datasheet for the electrodes. There is Field Wiring and Noise Considerations for Analog Signals tutorial that may be of relevance to your question.

Best Regards,

Jignesh

[navneet]

Hello Jignesh,

Thanks for your reply. These electrodes are built in lab. Before connecting the electrodes 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,

navneet

[Henrik_Volkers]

One thing is the input impedance, another the bias current of the input (correlated, but still two things;) )

If you need an externel signal conditioning I would not use a TL071 ... there are more sophisticated OPs now on the market ... LMC6001 to name one.

And depending on how fast your signals are, take a look at the input capacity...

[navneet]

Tried using the reference electrodes. I must say am impressed with USB 6212. Connecting them directly to differential inputs OR using INA111 as a preamplifier and then connecting with respect to AIGND gives similar results. Kudos to USB6212.