current to voltage with opamp

[Sharagim]

Hi,
I am working on DO (dissolved oxygen) circuit.
For DO probe polarization we should feed it with 700mv and I can see in this circuit the -700mv connected to "+IN" and I cant figure why.
Could you please clarify me about it ?

 

[barry]

That's kind of an incomplete question. You don't tell us what your input is or your power supplies. But that -700mV is simply an offset (which, curiously, is going to be multiplied by the gain of your second stage.)

 

[KlausST]

Hi,

the OPAMP circuit acts in a way that the probe sees that -700mV.
I think your probe needs this bias voltage. This voltage will not change until you overload the circuit. It is very constant.

The first OPAMP´s output voltage is: U(O) = I(probe) * 10^7 V/A.
The second stage is adjustable.

Both outputs refer to the -700mV. So if you use a voltmeter you can connect one line to -700mV the other to the OPAMP output...

One benifit of this schematic is that the capacitanc of the probe is negligible for frequency response (but it increases noise gain)

Klaus

 

[Sharagim]

thx,
is there any better idea for such a probe(less noise) ?
This Probe is polarographic which we should feed with 700mv