input voltage problem

[andy257]

Hi all,

in the middle of a project where i need to use an ADC. My problem is the ADC spec says that the maximum input should not exceed 0.3V on the analogue input pin.

Now i plan to connect a microphone (those little pcb ones) and maybe later an ipod/cd player or something with an audio output. Trouble is i dont know what voltages these devices output. I know most audio equipment such as cd players etc output something like 1V pk - pk, but this is more than 0.3V and will blow my ADc chip up.

are there any circuits out there that can help me out?

Cheers

 

[Optikon]

Hi all,

in the middle of a project where i need to use an ADC. My problem is the ADC spec says that the maximum input should not exceed 0.3V on the analogue input pin.

Now i plan to connect a microphone (those little pcb ones) and maybe later an ipod/cd player or something with an audio output. Trouble is i dont know what voltages these devices output. I know most audio equipment such as cd players etc output something like 1V pk - pk, but this is more than 0.3V and will blow my ADc chip up.

are there any circuits out there that can help me out?

Cheers

OP amp circuits are usually used to correctly scale your signal to something that utilizes the full input range of the ADC. So place an appropriate stage between your sensor and ADC.

you sure the 0.3V maximum spec isnt for above or below the supply rails? I think a max input of 0.3V doesnt sound right. Make sure you are reading the spec right.

 

[Brian Hoskins]

Are you saying your ADC accepts an input of between 0 - 0.3V? Can you point me in the general direction of it's datasheet?

If your sensor output does not match the window of your ADC, then you'll need to do some signal conditioning on it. This will involve the use of Op-Amps to increase / decrease the size of the signal and to shift it's D.C. level if required as well. It's not too difficult to do, but I think you need to give yourself some background on it first. See if you can do some research on signal conditioning with Op-Amps and if after that you're still confused come back and I'll show you some circuits that we can talk about and alter to suit your application. In the meantime, if you point me in the direction of the ADC you're using I'd like to take a look at it.

Brian

 

[Brian Hoskins]

@Optikon

You beat me GRRRRRR!

Brian

 

[andy257]

Brian / Optikon,

you are right i have misread the datasheet, jesus i make this mistake so many times. It does indeed say Vdd+0.3V and GND -0.3V.

while i have you interested i am wondering about the input signal coupling. It says the signal must go from ground to Vdd, however looking at the datasheet on page 17 it shows an ac coupling capacitor and a switch leading to a bias of vdd/2.

Does this mean my audio signal still needs conditioning or can i go ahead and connect it up?

https://www.electro-tech-online.com/custompdfs/2008/01/ADCS7476.pdf

thanks

 

[Brian Hoskins]

Yes that's a little bit confusing isn't it. The datasheet does say your input can range from 0V - VDD so if you're powering it from a +5V supply then I guess your signal can be between 0-5V D.C. The diagram you're reffering to where VCC/2 is shown is actually a diagram of the internal operation of the IC - VDD/2 must be derived directly from the supply rail inside the chip itself.

I can't say I'm 100% certain on this point, but I think you can input a voltage that spans from 0V to VDD. Personally, I'd buy one in, set it up and have a look! Just connect a POT between the supply rails, connect the potential divider pin into the ADC and have a look what happens to the digital outputs when you set certain voltages.

In any case, did you say that your sensor output is 0V-1V? If so, we'll need to do some signal conditioning anyway because you'll want to use the full window of the ADC. If you don't know what the output of your sensor is, then you'll need to find out before you can proceed.

Brian

 

[Brian Hoskins]

PS: If you're going to experiment with the ADC like I've suggested, you'll need to pay attension to the timing diagrams to make sure you set the relevant control pins to the correct logic levels. Again, if you get stuck on this let me know and I'll see if I can help you.

Brian