[SOLVED] Opam input capacitance for amplifying the DC voltage with +/- range

[milind.a.kulkarni]

I have a opam CA413 operating in buffer configuration......the voltage rating of input is -413mv to +413mv range from the sensor...the sensor have 400Mohm resistance....what is input capacitance that i need to use such that my sensor will not get overloaded....please suggest me the type of capacitance that i need to use also like disk/box etc....Thanks in advance

 

[goldsmith]

Dear milind.a.kulkarni
Hi
I think you wanna amplify the out put signal of your sensor , if yes, why you don't use AD620. it is for these applications.
Best Regards
Goldsmith

 

[milind.a.kulkarni]

Hi Goldsmith,

The input baise current for AD620 is 1.0 nA max where as in CA314 it is 10 pA max.....100 time less than the chip that you mentioned....aslo my question here is what is capacitor that I need to use so that sensor will not get overloaded.....as I mentioned sensor resistance is 400Mega Ohm....

Thanks for your answer of AD620 but My question is slightly different....

with regards,

Milind

 

[FvM]

I'm not aware of an OP CA413. Do you mean CA3140? I don't however understand where you see the need to place an input capacitor for your amplifier. I guess, you're intending to make a pH electrometer amplifier.

 

[milind.a.kulkarni]

Yes FvM,

You are right....my probmlem is to store the charge or voltage produced by the electodes....So I was looking for the some infomation of input interfacing for such a sensor charactersitics...
Did you had tried this kind of the application....

regards,

Milind

 

[FvM]

I fear, I don't exactly understand the problem. The 400 MOhm specification suggests a voltage source with (high) internal resistance, e.g. a pH electrode, not a pure capacitive source impedance.

In contrast to a pH electrometer, which is basically a high impedance DC amplifier, a capacitive source would always require some kind of AC signal processing. So the difference matters.

 

[milind.a.kulkarni]

Actually FvM the problem is like this ....Ph electrodes generated comparitively high voltage ....but you can not draw the current from them...that why the concept of opamp with PICO amper rating had come....but one need get this voltage to be feed to capacitor as any variation in the solution of the ph to be measured casues varition on the output voltage of the electrode ...so output voltage of electrodes varies.....putting the capacitor there helps in two ways....one is you can filter this variation and can get the some sort of stable voltage from electrodes and second is in case of dip in voltage the capacitor can feed the voltage too....but what I don't know is how to choose these values.....

I will be very thankful for any guidance from you...

Best regards,

Milind

 

[milind.a.kulkarni]

Dear FvM.....

I had attempted to use 100nf box capacitor.....it is giving me results but the problem what I am getting is it is taking to much time to charge....so output voltage is increasing and then it takes some time and then settel down....also when i chage in input concetration....it takes much more time to dischage too...any guidance to solve this problem....

thanking you

 

[FvM]

The requirement of low amplifier input current is primarly imposed by the intended DC accuracy. A capacitor can't help in this regard. There's no purpose of "collecting charge" unless you are able to disconnect the amplifier.

Regarding input capacitance, you'll notice, that in fast pH measurement applications, it's even usual to bootstrap the cable screen to remove it's influence on the response time. An example is given in Figure 5 of the TI OPA129 datasheet. OPA 129 is a typical recent "electrometer" OP, as it would be used in a high performance pH sensor amplifier.

If filtering of the pH sensor output is intended for some reason, it would be preferably applied to the amplified signal after the input stage. You can choose higher order active filters according to your specification without affecting the high impedance sensor signal.

 

[milind.a.kulkarni]

Hi FvM,

I could able to get the circuit working but I have a problem that voltage is saturating at the opamp output.....will you suggest that should i put a load for the opamp output ? or opamp can be used with out load in this kind of applications ?

Regards,

Milind

 

[FvM]

Regular OPs can be perfectly operated without a load. I guess, there's a a problem with your circuit.