The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Analog Design >> about ir receiver design problem
https://designers-guide.org/forum/YaBB.pl?num=1353681576

Message started by lhlbluesky_lhl on Nov 23^rd, 2012, 6:39am

Title: about ir receiver design problem
Post by lhlbluesky_lhl on Nov 23^rd, 2012, 6:39am

as the picture shows, a photodiode receives the IR photo current, and through R1, generating a signal voltage, then, through VGA-BPF-notch-amp-comp-integrator... among them, bpf is gm-c structure with center frequency f0=20khz, Q=9, notch filter is Twin-T structure, with center frequency f0=50khz, amp is an inverting amplifier (closed loop gain=40).
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 a saturated square wave (very easy to saturate), and noise and signal mixed together, signal can be drowned by 50khz noise. if VGA output is saturated, how to reject noise and 'pick up' the signal correctly?
besides, when there is no noise, and signal is small (nA order), the bpf output is basically ok, but if the signal is large (1uA or larger), the bpf output will oscillate a long time after photo signal ends, and causing wrong result. why? in simulation, i also found that, the ac and transient response is related to my reference voltage (bpf, notch, amplifier, etc), very strange.
i have tried some methods, but have no idea yet. i need your suggestions here.
thanks.

Title: Re: about ir receiver design problem
Post by loose-electron on Nov 23^rd, 2012, 12:23pm

Sounds to me like you need to do a
distributed filter & gain system where you
attenuate the interfering signal while amplifying
the desired signals.

Similar to a receiver front end.

Since this is an old design problem (IR devices have been
around for a while) you might want to research whats been done before.

Title: Re: about ir receiver design problem
Post by lhlbluesky_lhl on Nov 25^th, 2012, 1:59am

yes, i did as what you said, but have no effect. i wonder further suggestions here. thanks.

Title: Re: about ir receiver design problem
Post by lhlbluesky_lhl on Nov 26^th, 2012, 6:36am

i have post the schematic of amplifier, just as the picture shows. it is an inverting 50 times amplifying. when ac simulation, VI is the input, VR is reference voltage. if VR is an ideal vdc source (0.75v vdc), then ac response is ok. but if VR is generated by reference voltage circuit (as left shows), then the ac response is strange, just as the right upper shows, there are two pole-zero pairs in the response curve. i think it is induced by the reference voltage circuit, but the reference voltage circuit has a good ac loop response when simulation. i'm confused, what possible reasons here? thanks.

Title: Re: about ir receiver design problem
Post by Lex on Nov 27^th, 2012, 1:09am

It's probably resulting from the output resistance of the reference circuit. There is one thing though.. is the 2 pF capacitance a model of a bondpad?Then put some 100 nF on your bondpad, and it'll probably be fine.

Title: Re: about ir receiver design problem
Post by lhlbluesky_lhl on Nov 28^th, 2012, 4:12am

the 2 pF capacitance is not a model of a bondpad, just a capacitance load of latter stages.
any other advice here?