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input impedance

203 Views - Created 14/02/2019

14/02/2019

Posted by:
felipel

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Hello,

I'm trying to do some measurements of electrochemical potentials but sometimes get unstable values. Values will always be between -2 and 2 V. In general it is recommended to use meters with 'high input impedance', e.g at least 10 Mohm. Is there a way I can determine (or is it known) the input impedance for your adc differential cards? Maybe not directly related to the cards, but in case impedance is too low is there anyway I can increase it? Or would you not recommend to use these cards for such an application.

Many thanks for any hint.

14/02/2019

Posted by:
andrew

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United Kingdom

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Hello

According to the datasheet for the MCP3424 the differential input impedance is 2.25/PGA MΩ.  For an input voltage of -2 to +2V the PGA would be set to 1 so the impedance would be 2.25 MΩ.

To increase the input impedance you will need to put an operational amplifier in between your sensor and the ADC input. Op amps have an input impedance typically between 1 MΩ to 10 TΩ so you should be able to find one with an impedance of more than 10MΩ.

Before adding op amps to your circuit I would recommend checking the stability of your power supply for the sensors and the Raspberry Pi as noise or instability on the power supply could cause you to get unstable values from the ADC inputs.

AB Electronics UK Tech Support

15/02/2019

Posted by:
felipel

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Many thanks for your clear answer. I will also check the power supply for instability. I'm trying to find now an approriate opamp.

All the best.

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