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GraemeJ

Noise and Bias Resistors

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An ion-selective electrode sensor, which gives a floating signal, is to be used with a USB6211 Daq. To reduce noise it would be desirable to use a differential connection but when the USB6211 is powered off, the problem below occurs.

The only solution (?) I can see is to use very high value bias resistors but this means that the noise suppression sheath of the sensor cable has a very high resistance to AI GND.

The sensor requires a very high input inpedance so the signal goes to a 1:1 gain amplifier with an input inpedance > 10^12 and the output is then connected to the Daq. However the amplifier, which is inside a Faraday Cage, is battery powered and permanently turned on, requires a minimum load of 250 K. The USB6211 has an impedance of 10 Gohm to AIGND, but when it is off the impedance is only 1200 ohm, and this becomes the load on the amplifier.

The intention is to put a 130 K resistor in each of the signal lines to the Daq and use differential bias resistors of 270 K. This would become a voltage divider and reduce the magnitude of the signal but that is OK. However, it would also result in the sheath of the co-ax cable from the sensor having a resistance to AIGND of 130 + 270 K.

I would appreciate any comments on a) the use of 270 K bias resistors, and b) the 400 K resistance between the sensor sheath and ground and its effect on noise suppression.

Even more, I would appreciate any suggestions for a better solution than the above.

Regards,

GraemeJ.

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Would it be possible to use a relay-based solution? The NC path would have your passive impedence and you could wire the USB6211 to NO and then flip the relay as part of your initialization.

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Thanks for the suggestion asbo. This would be easy to arrange. If bias resistors were in both the positive and negative leads of the amplifier output they would each have to be > = 125 K to satisfy the 250 K load requirement. This is only marginally greater than the range of 10 – 100 K that NI suggests for a differential connection in “Field Wiring and Noise Considerations for Analog Signals”.

For a differential connection, the same document also allows a single resistor in the negative lead of a DC- coupled signal “required to satisfy the bias current path requirement” if the signal source impedance is “sufficiently low”. I could do with some help here – I do not know how to determine either value.

A relay and single resistor solution would be great because the amplifier load is then the Daq impedance but, in addition, a lower value bias resistor, say 10 K, is the resistance between the sheath of the sensor and ground. Do you think that might be still a noise problem?

The only other issue is possible noise from the relay. I had in mind a DIL packaged MOS relay.

I would like to try and deal with (yet unknown) noise issues before the hardware is in the field (when it will be a pain to fix).

Regards,

GraemeJ.

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I agree, this is the time to handle all the design considerations. I can't give you any specific recommendations about relays, but NI has an article you might find useful: How to Choose the Right Relay.

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If you are looking for good relays, visit Omron relay web page. They have sorted relays as per the applications.

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