Post History

IrDA Receiver using discretes

I am designing discrete IrDA receiver using a TIA and a comparator. My initial circuit is given below. I won't be using a BPF after the TIA as given in the MAX3120 Functional block diagram. The output of comparator will be connected to the Microcontroller. For TIA the opamp used is OPA859-Q and the comparator is TLV7011. Positive reverse bias is provided for the photodiode.

![Image_alt_text](https://electrical.codidact.com/uploads/mfmc4k2qen31p3gsunc6pkedufot)

![Image_alt_text](https://electrical.codidact.com/uploads/cht7zp349ijf2duuehi6y8qf7rd3)

My current source IG1 is a bit stream of 1010 (for IrDA '0' is represented as a pulse and '1' is represented by no pulse) with a speed of 115.2 Kbps.

The typical current of photodiode is 2.53 µA. Min is 1.37 µA and max is 3.8 µA

Bit period = 8.68 µS and pulse width for '0' is (3/16) * 8.68 µs = 1.63 µS. My input waveform is given below.

**Input Signal**

![Image_alt_text](https://electrical.codidact.com/uploads/xzw1eh2ndsv3au8yco7h08zwth9g)

**The complete response is given below.**

![Image_alt_text](https://electrical.codidact.com/uploads/lgvriz9ijqqgfow1c9w30f6ivygm)

May I know your thoughts about this circuit? Is this logically correct?

In complete response you see the output of TIA is between 2.05 V and 1.8 V. When PD current is zero am expecting 3V(Vref(V2)) at the output and current is high I am expecting 2.75 V at the output. Don't know why and how this difference is coming.

I am planning to make some modifications in this circuit. My plan is to put a difference amplifier at the output of TIA to remove the DC offset currently set by the 3 V connected to TIA's non inverting input. This will help enhance the voltage difference between high and low photodiode currents. After that, I'll add a non-inverting comparator with a hysteresis of around 100 mV to 200 mV for more reliable signal detection.

**Kindly share your thoughts about the proposed modification in the existing circuit.**