IR receiver design problem

[lhlbluesky]

i'm designing an IR receiver recently, as my former post shows, it includes a bandpass filter with center frequency f0=20khz, 26khz and 38khz. in front end, i use a reverse-biasd photodiode series with a resistor to be the I-V converter,and a HPF (a capacitor with a resistor)follows, then entering the input of VGA (VGA: vth=0). now, my question is, when the photo current is very small, 1nA or smaller, the signal in VGA input is very small also(and not half-negative, half-positive, may be all positive. while in large photo current case, the input of VGA is half-negative, half-positive, that is, during one period, at one half period, input of VGA is smaller than zero, and at another half period, input of VGA is larger than zero), and so, not amplified rightly by VGA, so the final output is wrong also. besides, the input photo current may include 3uA 120hz noise and 1uA 50Khz noise, in such case, the small effective photo current (1nA or smaller) can not be recongnized rightly by VGA even more. therefore, i want to know, how to receive the very small photo current correctly when existing noise (above mentioned)?
thanks all.

 

[lhlbluesky]

if exsiting noise is much larger than signal (1nA for example), then the signal can be drowned by noise before arriving the input of VGA, then, the VGA cannot recongnize the signal rightly, so causing wrong results. am i right? if so, how to solve the problem when exsiting relative large noise?

 

[lhlbluesky]

in my case, first i select center frequency f0=20khz, noise @50khz has a pk-pk amplitude of 1uA (sine wave), signal amplitude is 1nA~100uA, then, in simulation, i find that, because 20khz and 50khz is very close to each other, therefore, the signal (signal adding 50khz noise) at VGA input is also a sine wave with pk-pk amplitude a little smaller, and the VGA output is wrong here. in normal case, the VGA input is zero, when there is signal injecting, the input of VGA is half-negative, half-positive during one period, so VGA has a effective output pulse responding to the signal input. however, when there is 1uA 50khz noise, the input of VGA is always half-negative, half-positive during one period, and causing wrong results.
in order to decrease the influence of the 50khz noise, i added some order LPF before VGA input, but because 20khz and 50khz is so close to each other, the noise attenuation @50khz is very small relative to signal @20khz (AC response), and with increasing of LPF order, signal amplitude @20khz decreases also.
so, how to suppress the 50khz noise to get a correct signal at VGA input(normally zero, when there is signal injecting, the input of VGA is half-negative, half-positive during one period)? the threshold of VGA is zero in my design.
thanks all.

 

[lhlbluesky]

what about band-reject filter before or about VGA? and any problems?

 

[lhlbluesky]

as the above picture shows, a photodiode receives the IR photo current, and through R1, generating a signal voltage, then, through VGA-notch-BPF-... now, there is a 50khz noise in photodiode, the noise amplitude can be 100mv or so, and the smallest signal amplitude can be 100uv or smaller (frequency=20khz). so, in this case, the noise frequency is very close to signal frequency, and noise amplitude is three order larger than signal amplitude. VGA has a very high gain (70dB or so), and VGA output swing is 200mv, so the VGA output is saturated square wave, and noise and signal mixed together. after notch filter (center frequency = 50khz), the output becomes strange, slowly rising for half period, slowly falling for half period, and there is a big glitch when edge transition, it is not a sine wave, why? and if VGA output is saturated, how to reject noise and 'pick up' signal correctly?
i have tried some methods, but not work.
thanks.