How to choose Vref (reference voltage) for flash ADC.

[Osawa_Odessa]

Hi,
I want to ask a question about how to choose Vref (reference voltage) for flash ADC.
For example, I want to design a 2-bit flash ADC like this:

And the input voltage like this:

I wonder how can I choose reference voltage to get better precision.
Thanks for help.

 

[Jinzpaul4u]

Hello there,

how much your output voltage?

Vref would provide maximum adc voltage that you can set.If you need more precision go with low Vref and a well dedicated hardware for glitch reduction.

Best regards,

 

[Osawa_Odessa]

Thanks Jinzpaul4u!
I am not sure your question.
My input signal is about 0.6V. But in order to make transistors operating in saturation mode, I need to boost my signal to Voffset. Vtotal = Vin + Voffset. Now I am not sure what reference voltage I need to use. My interested signal is a small signal "sinusoidal signal" about 0.6V pp. I am thinking about two reference voltages but I have no idea about it. Could you suggest some ideas? I don't know the key words to search for two reference voltages. I think I have heard about it somewhere but I can't find it now.

 

[Jinzpaul4u]

Hi there,

these links might be helpful for you.

http://www.maximintegrated.com/app-notes/index.mvp/id/810

**broken link removed**

http://www.allaboutcircuits.com/vol_4/chpt_13/4.html

**broken link removed**

 I don't know the key words to search for two reference voltages

Do you mean positive and negative Vref voltage.??

Best regards,

 

[Osawa_Odessa]

Hello Jinzpaul4u!
I have read these articles but they don't say specifically about how to choose reference voltage (Vref). You know in my case the interested signal is a very small sinusoidal signal, much smaller than offset voltage Voffset. Therefore, I think that the using only one reference voltage is not good in this case. However, I don't know how to use two reference voltages and don't know the key words to search for.

Do you mean positive and negative Vref voltage.??

I think, no. For example, my signal is from 4V to 5V then I think I don't necessary using 5V reference voltage. I get I can using two reference voltages. The first is Vref1= 3V and the second one is Vref2 = 5V. It is my guess. I don't know much about this and want to find some material about this method.

 

[spudboy488]

Let's say your offset is 2.5VDC. For your signal of 0.6Vpp, your expected measurement range is 2.5V +/- 0.3V. Give a little margin and you could use 3V (2.5V + 0.3V + Margin) as the positive reference. For your 2 bit resolution, a 3V reference will give you a resolution of 0.75V per bit when using ground as the -VRef (which your schematic shows). Now, to get better resolution, you can set the -VRef to 2V (2.5V - 0.3V - Margin). What this does is reduces the voltage range that your comparators will use to determine the measured voltage from 3V to 1V. Now your resolution is 0.25V.

You chose your references based on the signal you are trying to measure. In your case, the 0.6Vpp sine wave. You also know that the AC sine wave is riding on a DC offset so the effective voltage that you need to measure is OffsetV +/- 0.3V. You don't need to measure anything outside of that range so limit your measurement system to the range of voltage you are interested in. Thinks of it this way, when you measure a 50 Ohm resistor, which range would give you the most precise measurement? The 100 Ohm range or the 100KOhm range?