the best unipolar single ended to differential circuit

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hi guys,

I'm wondering about the best ways to convert my unipolar single ended sensor output to differential input of ADC. I don't need any signal amplification or just a mirrored signal and its original signal is enough.

I can think of two ways:

either to use a single op amp inverter with just 2 resistors.

or to use dual op amps design mentioned in:
http://www.edn.com/article/CA633...
and
http://www.edn.com/article/CA302...
where one use 6 resistors and another one with 4 resistors.

I really don't know the advantage of these two dual op amp design in term of signal accuracy. I got a feeling that the first choice is better for me.

my parameters are signal accuracy, component number, PCB size, power consumption.

anyone has an idea?

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I'm wondering about the need for a differential signal. Are you worried about transmission noise or only have a differential ADC, or want to use the gain modes? It seems to me that if you have a single-ended sensor that it's easiest to connect it to a single-ended ADC, with a voltage follower or somesuch to correct output impedance.

If noise is an issue and everything's on a single PCB then you have control over many of the parameters and should probably attempt to minimize the issue there before going differential. If it can be avoided that will be the best for all of the criteria you mentioned.

Otherwise if you have to do it I would probably go with the dual-op amp method, although skimming the articles it doesn't look like it is necessarily two components - just on differential amplifier in a package. That would be best in terms of signal accuracy - the resistors within single IC's are usually very well matched and that will be your primary concern.

Using a single op-amp that just inverts the signal - I assume you have positive and negative supplies? Also, that will invert the signal about ground, a gain of -1. If you hook that signal into a differential ADC then the signal will be twice what the original was. To get the same signal only differential, if you have positive and negative supplies you'd still have to have two op-amps - one with a gain of -.5 to invert the signal and cut it in half, and then another with a gain of -1 to take THAT signal and make it positive. By that time you've run the the signal serially through two discrete op-amps with different resistor ratios, different bias currents, etc etc. It will not be a good system.

Bottom line: if you have to go differential use the two op-amp system mentioned in either of the articles. But for best results in all the areas you mentioned, determine if differential is absolutely necessary.

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hi,

The reason that I need to convert is is that my ADC is full differential input, and it's a really good low power ADC. so I have to convert single ended to differential.

My sensor is 5v single supply sensor, so is my ADC. so i need a 2.5v reference point to invert sensor's output. I just realized that i need dual op amp anyway, because my sensor output needs to be buffered before ADC, so then the only difference I can make is number of resistors and their power consumption.

I can't find any differential amps with power consumption less than 1mA. The op amps i use is about 50uA each.

I think i will go for 4 resistors one. My sensor is not output fast change, max 400Hz, so i guess 4 resistor one is fine for DC accuracy.

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yes, i think i will change a ADC to single ended. Thanks.

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If you are using the differential ADC with a single ended input why can't you just tie the diff - input to ground, and feed your signal into the diff + input?

JC

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that ADC is true differential, so it's +input plus -input / 2 should always be Vref/2.