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I want to use uC LPC1769 (3.3V powered) for interfacing with 5V or 3.3V powered ADC.

Controller --> LPC1769

ADC --> AD7705

If we power ADC7705 with 5V, then Vref = 2.5V. But, if we power ADC7705 with 3.3V, then Vref = 1.25V.

Since my application is biomedical instrument where sensor used is photo diode followed by op-amp as I to V converter,high precision is needed. And I need wide range of analog input for optimum results.

CASE1: If we keep both (uC and ADC) 3.3 volt powered, Vref=1.25V and analog input range is reduced and hence resolution also.

CASE2: If we keep both (uC and ADC) 5 volt powered, Vref=2.5V and analog input range is higher than 3.3Volt case and hence resolution also better. This is preferable scenario.

What are possible configuration I can implement? Thanks in advance.

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    \$\begingroup\$ Lower reference voltage doesn't mean poor resolution. In fact, if you keep noise low or use oversampling, the smallest adc count will be equal to smaller voltage, which is considered better resolution. And since it keeps components count lower, i would go this way. \$\endgroup\$ – Gregory Kornblum Dec 8 '15 at 7:18
  • \$\begingroup\$ Photodiode current will be in nA/pA and then assigned to I to V converter for signal amplification. Does less ref voltage will cause problem? \$\endgroup\$ – Electroholic Dec 8 '15 at 8:43
  • \$\begingroup\$ What is the applications? It's not enough data to decide. \$\endgroup\$ – Gregory Kornblum Dec 8 '15 at 9:26
  • \$\begingroup\$ Application is Proteins Analyzer which is biomedical application so need precision analog measurements. \$\endgroup\$ – Electroholic Dec 8 '15 at 10:52
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To answer your question: I would use the 3V operation with the 1.225V reference so you can use less power with your controller. Here's my reasoning:

A/D resolution is usually specified in bits.

Let's denote the largest input allowed as FSR (full-scale-range). The FSR is often, but not always, determined directly by the reference voltage so often FSR and reference voltage are used interchangeably. It's the FSR we care about. If we define the A/D resolution as the smallest input difference that can be detected then the resolution is FSR/2^bits. For a 16 bit converter resolution is FSR/2^16. A 12-bit converter has a larger (poorer) resolution of FSR/2^12.

(Incidentally, the smallest input difference that can be detected is also obviously the smallest input that can be detected).

Therefore absolute resolution can be improved by using a converter with more bits, or by using a smaller FSR. However in my experience, choosing a small FSR A/D is rarely done. Instead, if optimal resolution is required, one scales (amplifies or divides) the input signal before the A/D so that the largest expected input signal is just less than FSR.

For example, if your input sensor will output a maximum of 300 mVolt after the I/V conversion, you would amplify the signal by 8 for an A/D with a 2.5 Volt FSR. (I've left a little headroom to avoid clipping). You could also divide a 12 Volt input signal by 5 and use the same A/D. In summary, the FSR doesn't need to matter if you simply scale your input signal before the A/D.

Note that the A/D converter you refer to in your question has an internal, programmable gain amplifier so you may be able to handle the scaling without any external parts.

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Analog range and resolution are mutually exclusive. You can have one or the other but not both. The resolution of an ADC is determined by its bit rate (16bit) and its reference voltage (1.25v or 2.5v). The resolution is calculated by dividing the reference voltage by the max ADC count. In your case 1.25 / 2^16 = 0.000019v or 2.5/ 2^16 = 0.000038v. The 1.25v reference gives you the best resolution and the 2.5v reference give you the best range.

The decision is whether you can fit the signal inside +-1.25v and get the better resolution or if the signal can only fit inside +-2.5v and you can live with the worse resolution.

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