How to design the amplifier circuit?

[baileychic]

How to design the amplifier circuit?

I have to use INA126 and design an amplifier and for the final output I will use a UG buffer amplifier.

Client's data are these:

I have to apply 5 na current to electrode and record the voltage response
The electrode gives 0.005 to 1 mv depends on material

So, can I use MCP6001 for UG Amplifier?
For HPF what should I choose as cut-off frequency.

Refer attached PDF.

Two electrodes immersed in chemical solution has to be interfaced to the amplifier and further the output of the amplifier will be interfaced to PIC microcontroller for measurement.

 

[KlausST]

Hi,

Impossible to answer, because there are a lot of informations missing.

I have to apply 5 na current to electrode and record the voltage response

* at which electrode? Or both?
* if DC: positive current or negative current?
* if AC: frequency? waveform? What is this 5nA: RMS, amplitude, peak?

The electrode gives 0.005 to 1 mv depends on material

To be able to calculate the gain:
You need to know the peak-to-peak value if the input signal
And you need to know where the signal goes to (destination), or more precisely...
....you need to know it's peak-to-peak input voltage range .... and how much headroom you need to calculate with.

So, can I use MCP6001 for UG Amplifier?

To answer this you need to know above informations, plus the input characteristics (current, capacitance) of the destination.

For HPF what should I choose as cut-off frequency.

It needs to be lower than the excitation frequency. How much lower depends on the expected precision and other influences (noise? mains frequency?)

Schematic:
* the electrodes seem to be floating. This won't work. You need to ensure somehow that the voltage is within the INA's input voltage range. Additionally I strongly recommend to add some protection to the electrode nodes, else you risk the INA to get damaged by ESD or other externally (accidentally) applied currents.
* there is no information about supply voltage(s). You need to ensure that inputs and outputs of both amplifiers are within specified range.
* the buffer input need to be DC biased in either case to meet the PIC's decodable input voltage range.

***
Working with that low currents is not easy. It let's me assume that the electrodes are high impedance. This makes the whole circuit prone for noise and any other electrical influence. Be aware that there may be a lot of spikes, distortion, hum...that calls for increased signal voltage headroom.

All the circuit does not show nor mention any signal filtering (to attenuate all unwanted frequencies) this makes life at the digital side more difficult.

The PIC gets AC input of (until now) unknown frequency and waveform. How and what information do you want to get from this signal?
Carefully decided sampling frequency (don't forget a proper anti aliasing filter), carefully decided filters and measurement algorithms are essential.
Also you need to decide how often you want to calculate the result.

Many of the above open questions are "killer decisions". This means, if not decided correctly they can make the whole system to malfunction. ..or partly malfunction, in a way that it works properly in the laboratory or at one customer, but it fails at other customers.
A situation that surely has to be avoided.

Klaus