wtorek, 28 lutego 2012

Precise ammeter - the begginig.

I've started recently designing an ammeter that would be suitable for measurement of very very very low currents. Probably at the level of 1..100 nanoamperes. I need such device for measuring photo-current at work. Currently the project looks like that:

First IC is LMC6081. This is an precision CMOS Operational Amplifier, the main advantage of this op-amp is its very high input impedance (it needs only 10fA for biasing, geez, how do they measure it?!?) and low offset voltage. In this application it works as an I-to-V converter. Bandwidth of this op-amp is limited by C1.
Second op-amp is an low noise precision op-amp that will be used for amplifying the output voltage to measurable level (with 1Mohm measuring resistor R1 the output of the I/V converter is 100mV for 100nA). Second part of the OPA2227 is just an buffer to provide stable output.
PCB design is a real pain in the neck, at the current moment it look like so:

I don't have any idea how to optimize it more, especially to lower any leakage current that might change the measurement. I'm counting for some creative input of several people that I've asked to help me design this module. I hope to post some more info soon.

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