Difference between revisions of "DNA Melter Improvements"

Revision as of 07:05, 23 May 2011

This page catalogs improvements made to the apparatus for the 20.309 DNA Melting Lab Module.

Forced Cooling

Refer to Wikipedia for a basic explanation of the H-bridge motor controller. The TEC works like a motor, in the sense that if you apply voltage in one direction, one side gets hot and one side gets cold; if you apply voltage in the other direction, the hot side and the cold side are reversed. Thus, we can use an H-bridge to allow both forced heating and forced cooling of the sample block.

How it's controlled

Documentation of changes made to LabView VI

Monolithic Photodiode-Amplifier

Rather than rolling our own trans-impedance amplifiers, we can use the OPT101 integrated circuit, which contains both a photodiode and a million-gain amplifier.

Route the output of the photodiode-amplifier through a high-pass filter and then through an additional amplifier of gain 10, for an overall gain of 10^7.

Why is the high-pass filter necessary? Remember why the LED is driven at 5kHz, and how the lock-in amplifier reduces 1/f noise? That's wonderful, but the lock-in amplifier is implemented in software. It helps to have an additional filter in hardware, before the signal reaches the DAQ. The DAQ only has one analog-to-digital converter, which is shared by all the analog input channels. It has input limits of +/- 10V. If it receives inputs outside these limits on any one channel, it will get confused and give nonsensical readings on all channels. (If you get nonsensical temperature readings, but your RTD circuit is correct, check the output of your photodiode amplifier!) A simple high-pass filter helps avoid maxing out the DAQ with amplified low-frequency noise.

Physically Restrained Optical System

Because people have to carry their DNA melters from place to place, the optical system tends to get misaligned very easily, leading to noisy data. Here are some ways to make the system more stable:

• Use PART NAME to hold the heat sink in place on the breadboard.

• Bend down the leads of the TEC and tie them to the 'legs' of the heat sink with short pieces of wire. (By itself this doesn't help very much, but in combination with thermal grease, it makes a big difference.)

• Use cage rods to hold together the sample block, the photodiode, the filter, and the lenses of the fluorescence detector. To avoid conducting heat out of the sample block, add nylon standoffs to your cage rods.

• TODO: can I also attach the LED to the sample block?

• TODO: photos