How to pick correct operational amplifier as unity gain buffer between TLC1543CN and LDR

Question

This is a follow up question on: Interfacing TLC1543CN adc with 8051

I have a LDR in voltage divider circuit. The resulting varying voltage is outputted to adc. Which in turn passes the converted signal to 8051. Lastly the LDR measurement is shown on a 16x2 lcd module.

My problem is that according to the adcs datasheet: http://www.ti.com/lit/ds/symlink/tlc1543.pdf source voltage resistance can be max 10k Ohm. However my LDRs resistance varies between 5k Ohm and 0.5M Ohm. My understanding is that the voltage outputted by voltage divisor circuitry needs to be therefore passed through unity gain buffer before passing it to adc.

So we come to my question: What kind of op amp would be sufficient for my needs? Preferably something that I can cheaply order from eBay.

Answer 1

Speed

An LDR is generally a slow device (many ms response time). 2Mhz bandwidth (=4Mhz sample frequency) is useless and unattainable. The ADC does not support it: its conversion time is 21us, which -if you could fully use it- impies a bandwidth of around 20kHz. But even that is extreme overkill for an LDR. Speedwise any opamp will do.

Supply voltage

When you have a positive power-supply only, you will have to choose a so called rail-to-rail opamp. Normal opamps often have trouble working on a single 5V supply.

Dynamic range

Using a unity gain buffer is fine, but you lose resolution because the ADC input range is not fully used...

Imagine a divider of the LDR and a 5k resistor to ground. In the dark case the output of the divider will be: U_in*5k/(500k+5k) = close to 0V (good). But when the LDR is lit, it's resistance will be 5k, and the divider-output is only half of the supply-voltage. So you use only half of the ADC-steps.

To alleviate that you could use more gain. You can use a non-inverting amplifier configuration with a gain of two instead. This will "scale up" the output to the full ADC range at the cost of two resistors. It doesn't influence the choice of opamp.