# ADC Differential inputs

I am trying to feed a 5V signal to AD7705 which is a 16 bit ADC and expects differential inputs. Do I need to divide my 5V signal using bridge and then feed it to the ADC or can I use it as a single ended ADC.

What confuses me is the need of Vref as 2.5V. I understood that the differential inputs don't need and Vref as the inputs are compared in respect to each other.

If I do need to split up my signal, then I assume a four resistor bridge of same values should give me center tapped output to feed into the differential inputs. What standard grade "R"values should be selected? The ADC has a buffer in built, thus input impedance is not much issue, I guess.

• A link to the datasheet would help us answer your question. – The Photon Mar 15 '15 at 16:30
• @ThePhoton Datasheet link now provided in the question – Anuj Purohit Mar 15 '15 at 16:39
• Achieving 16-bit accuracy in an ADC circuit requires extreme care in many areas of circuit and layout design. Based on the questions you're asking, I'd suggest that if achieving 16-bit accuracy in what you're working on is critical, you should consider hiring someone more experienced to do this project. If it's only a hobby project, at least ask us a question that lays out the "big picture" of what you're trying to do so we can help you get closer to the performance you're asking for. – The Photon Mar 15 '15 at 16:47
• I have a photodiode sensor which gives a output of 5V (the sensor double stage opamp with active low pass filter and buffer at last stage). The signal is currently processed via a 10 bit ADC which works fine. I want to "upgrade" the resolution to 16 bit. – Anuj Purohit Mar 15 '15 at 16:52

I am trying to feed a 5V signal to AD7705 which is a 16 bit ADC and expects differential inputs. Do I need to divide my 5V signal using bridge and then feed it to the ADC or can I use it as a single ended ADC.

Do not use a bridge. The result would be measuring how the bridge resistors vary with temperature, not measuring variations of the signal you want to measure.

Probably you can simply connect a fixed voltage to one of the inputs of the ADC and your signal to the other one. But watch out for whether the resulting common mode input could affect the conversion result.

Even better would be to simply switch to a single-ended ADC.

What confuses me is the need of Vref as 2.5V.

You still need to tell the ADC what voltage range you want to measure.

For example, the conversion result of a differential ADC might be given by

$$D = 2^n \frac{V_+ - V_-}{V_ref}$$

But check the datasheet for your IC because there's other possibilities (for example to allow for both positive and negative differential inputs).

What standard grade "R"values should be selected?

If you want 16-bit accuracy (or even 16-bit precision) you should probably not be using standard grade resistors anywhere in your signal path. But again, a bridge circuit won't convert your single-ended signal to differential.

• What "fixed" value voltage should be used if I expect the signal to be in range from 0 - 5V? Should the fixed voltage on AREF1+ or AREF1-? – Anuj Purohit Mar 15 '15 at 16:42
• I'd suggest ground, unless there's a good reason to use something else. You might use a ground line run from your sensor --- that would take advantage of the differential inputs to eliminate errors due to ground differences between the sensor and the converter circuits. – The Photon Mar 15 '15 at 16:44
• Then how does differential input differs from single ended? I am more than happy to ground one and put my signal on other input; but I doubt that it would work that way. – Anuj Purohit Mar 15 '15 at 16:46
• With a differential ADC, you have the option to directly connect a differential sensor (like a bridge configuration strain gauge, for example), or to use a reference voltage other than ground. But it doesn't sound like you need those capabilities, so in your case in fact there isn't any benefit to using a differential ADC. – The Photon Mar 15 '15 at 16:56