Hi all,
I am using CY8CKIT-059 kit to convert the current output of a photodiode to millivolts.
I have designed a transimpedance amplifier (TIA) in PSOC creator which takes photodiode's current as input and I am checking its output using multimeter from pin0[2]. The design is working fine when the current output is in order of microamperes but it doesn't sense anything when current output is in order of nano amperes (varying in 0 to 50 nanoAmperes).
Please suggest the changes in design required so that it can sense current which is in the order of nano amperes. I prefer to use PSOC creator's inbuilt components to solve this issue and don't want to use external components as they may add unrequired noise. The project design is attached for reference.
Thanks in advance.
Best Regards,
Rajat Bandejiya
Edit1: Circuit-->
Rajat,
Even if you were to achieve low-nA resolution, the circuit impedance would be 10M to 20M ohms as /odissey1 indicated.
Even in the best of circuits, the thermal noise at such very high impedance would induce a AC-like noise of +/- 57uV @ 20C. (See link below.) You can integrate most of the noise out by performing long-term averaging in SW or placing a cap large enough to average the signal. (Down-side: larger signal latencies).
You never indicated why you needed such low-current resolution. Many designs that use a photo-diode use it in a binary way. They create a circuit that measures the photo-to-current output against a trigger threshold.
Here's a suggested circuit based Andrea's suggestion:
The ADC_DelSig has control over the Vref, Input Range and the Buffer gain.
For example with the circuit above, using Vref = 1.024V, Input range = Vssa to 1.024V and Buffer gain = 1 you get 16bits of ADC resolution from 0 to 100nA. (Remember you're going to get a lot of thermal noise at this large impedance).
You can scale up the Input range and buffer gain in SW to achieve large current ranges.
Len
