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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.

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  • \$\begingroup\$ First, the source resistance is the parallel combination of your LDR resistance and the other resistor in the divider. You probably still need a buffer, though. The choice of op-amp depends on your input voltage range, your available supply voltage(s), your accuracy requirements and the required bandwidth. \$\endgroup\$
    – John D
    Aug 10, 2016 at 21:15
  • \$\begingroup\$ Input voltage range is between 0V and 5V. I do not know about the accuracy but the bandwith should preferably be above 2.1M Hz. \$\endgroup\$ Aug 10, 2016 at 21:30
  • \$\begingroup\$ What is your LDR? Are you sure that the LDR has the bandwidth you need? And if your LDR output will be displayed on and LCD, why in the world do you think you need 2 MHz response? How fast can you read or update an LCD? \$\endgroup\$ Aug 10, 2016 at 21:41
  • \$\begingroup\$ I thought my adc would support it because the maximum clocking speed was specified as 2.1M Hz. I wanted an op amp which could support this speed in the future. However the conversion time is the factor here, not the clocking speed, so I do not need so high bandwidth. \$\endgroup\$ Aug 11, 2016 at 7:33

1 Answer 1

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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: Uin*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.

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  • \$\begingroup\$ Thanks for your comprehensive answer and especially for pointing out that I would lose resolution due to divider cutting half of the supply-voltage. I now know how to pick my op amp myself, but usually when I think I know everything and make the order I notice something missing when the item arrives. For example this project: I ordered LDR and realized that there is no analog input on 8051. Then I ordered this adc and afterwards noticed that the source resistance was too high. Would you like to suggest me a suitable op amp model for this project so that I would not need to order any more items \$\endgroup\$ Aug 11, 2016 at 7:58
  • \$\begingroup\$ Yes, hindsight is a well known fenomena, especially when interpreting data-sheets. Have a look [at Microchip's opamps](www.microchip.com/opamp) for a parametric selection chart. Also the the Fairchild FAN4174/4274 comes to mind (SOT23). \$\endgroup\$
    – peter
    Aug 11, 2016 at 8:36
  • \$\begingroup\$ Uh oh. Those are very small components. I can barely handle the precision required to solder header to my 16x2 lcd, but using components of that size seems impossible. Do I need some kind of mount? \$\endgroup\$ Aug 11, 2016 at 9:25
  • \$\begingroup\$ No need, Microchip still offers PDIP packages. The selector allows you to choose packages in its last column. \$\endgroup\$
    – peter
    Aug 11, 2016 at 9:40
  • \$\begingroup\$ Thanks! I did not notice that. You were really helpful. \$\endgroup\$ Aug 11, 2016 at 9:42

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