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Design >> Analog Design >> AD623 in single power - output saturates
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Message started by krmt on Dec 17^th, 2008, 9:10am

Title: AD623 in single power - output saturates
Post by krmt on Dec 17^th, 2008, 9:10am

Hi there,
new to this community, I already need to ask for some help:

I'm working with the AD623B but i get somewhat strange results. The problem seems to be that I operate the omamp in single supply mode. Everything works fine but the output wont get any higher than 1.16V, independent from the opamp's gain. Operating the opamp with dual supplies, the output is as expected.

I attached a multisim screenhot. (ps: multisim produces a simulation error for an output higher than 1.16V. Using a breadbord, the output saturates at the same value...)

I really appreciate any help.
thanks,
krmt

Title: Re: AD623 in single power - output saturates
Post by indysjl on Dec 17^th, 2008, 10:46am

Judging from your simulation test bench circuit, it appears that the circuit is working properly.

For a FB resistance of 100 you get a gain of 1000. You are applying a differential signal 1.16mV and seeing an output voltage of 1.16V. Vo/Vin = 1000, what you expect.

You are probably aware, but you will see problems if you try to use the amplifier for AC signals, as you have placed the signal ground at 0V. Your input and output signals cannot extend beyond the limits of the supply. To achieve max headroom choose your signal ground @ VDD/2 for single supply configurations.

For a dual supply (VDD and -VDD or VSS) the amplifier has no problem with a 0V signal ground.

Hope this helps.

Title: Re: AD623 in single power - output saturates
Post by krmt on Dec 17^th, 2008, 11:37am

Hi indysjl,
thanks for your prompt reply and hello to minneapolis (this is where I am right now, too)

Quote:

To achieve max headroom choose your signal ground @ VDD/2 for single supply configurations

What do you mean by that? if i disconnect the ground of the input, the simulation isn't working anymore...

You're right, the amplification seem to be ok. But this is only valid for the depicted case. If i increase the input, lets say to 2mV, the output should be 2V (being way within the power supply range), but it actually stays at 1.16V or produces a simulation error in multisim, respectively.

if you could explain the input signal issue a little deeper?

thanks again,
*

Title: Re: AD623 in single power - output saturates
Post by Mazz on Dec 23^rd, 2008, 12:03am

Try applying 2V at REF pin.
Mazz