Monday, 10 April 2017

Terminating "High Speed" Photodiodes

Terminating a photodiode (PD) is vital to it's operation. I have been using high speed biased photodiodes from ThorLabs and I realized, to my dismay, why they weren't as sensitive as I had hoped! In this post I'll explain why, and how I fixed the problem.

Biasing a photodiode is critical if high speed operation is desired. The cathode of the PD is connected to a DC bias which is typically 5 to 12 volts. The anode is then usually connected to some sort of amplifier (ideally a transimpedance amplifier). In my case, I wanted to use a Mini Circuits wide-band, low noise amplifier which has a 50 ohm characteristic input impedance.

The assumption was that the photodiode would develop a small voltage across the input impedance which is then amplified by > 10 dB. I decided to crack open one of the amplifiers and realised why it didn't really work as I was hoping. This is a photo of what was inside:

I had fallen into the trap of assuming that the input impedance would be a real resistor! There is a DC blocking capacitor (which I need for my application as I don't want the DC bias of the photodiode), but no resistance for the PD current to develop a voltage across. This also means that the anode of the PD is connected in series with a capacitor, meaning that it's bias voltage has no DC path to ground, causing the bias to have no effect!

Obviously, a transimpedance amplifier would solve all of these woes, but I am still working on building one (keep a look out over the next few weeks). For now, a "feed through" 50 Ohm terminator connected between the biased PD and the amplifier should do the trick.

I couldn't find any such device for SMA. BNC versions also have a cutoff frequency that is pretty low (< 100 MHz) which is unacceptable to me. My "ghetto style" solution was to solder two PCB mount SMA connectors back to back and solder an 0603, thin film 50 Ohm resistor to ground. It looks like this:

I didn't have a 50 (or 49.9) Ohm resistor on hand, and I don't think this one is thin film either, but it works! Finally, I soldered some copper sheeting to seal in the RF and make it look great... I took some quick measurements which are fairly trustworthy up to 4 GHz (half way along the x-axis in the photo below). The S21 parameter (dB on the vertical) seem ok and there is about 3 to 4 dB of attenuation.

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