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I have setup a circuit according to page 15 of AD811 Op-Amp datasheet . The design receives 0.5-1.2 vpp sine wave between 1Hz-50MHz from AD9850 DDS and it also has around +0.7 to +1 v offset voltage.

According to the schematic below, I have set VG- to Ground, VG+ to 0-1.3v ( the clipping voltage of AD834) and the signal goes to VIN. When I directly connect the signal with the DC offset voltage, everything goes well but no output is seen when I put a coupling capacitor = 100nF between the VIN and the signal to remove the DC part (Cin in the design).

As I read in AD834 datasheet , its output is set by (X1-X2)(Y1-Y2)x4mA and I have not seen limitations for AC signals around the ground.

In fact I want to have a pre-amplification phase with gain=x2 before this design and I need to send its signal without the DC part to this schematic to get rid of clipping to upper voltage region (VS+).

Why this design does not accept signals without offset?

enter image description here

EDITION: the pre-amp:

enter image description here

EDITION2: Inverting: enter image description here

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The AD534 produces a bias current in/out of all its inputs of 45\$\mu A\$ and this current will want to flow into a dc path. Typically, if there was a 1k ohm resistor from Y1 to ground, the offset current would create an input offset voltage of 45mV but, because you don't have a resistor to remove this current on your circuit, the offset voltage rises positively or negatively to some unholy value and your circuit refuses to work.

I think the AD534 is best driven from a device like an op-amp with a low output impedance (this of course is likely given your project). Maybe you can "trim" this offset out at the op-amp stage before the AD534?

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  • \$\begingroup\$ Thanks ! I am sure Analog Devices could be a more productive company if you worked for them! I just got a little confused. You mean I should add a resistor to Y1? I also added another AD811 before the design with low output impedance but it refuses to work with coupling capacitor ( It works without that) . Could you please refer a similar schematic ? I added a schematic of the possible op-amp before the design.How should I manage it to "trim" the offset? \$\endgroup\$ – Aug Oct 20 '13 at 15:35
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    \$\begingroup\$ Pin 3 of the op-amp circuit you added - you could have a 5k pot with the wiper going to pin 3 - the two outer ends of the pot you can connect, via (say) 22k resistors to plus and minus power rails. You now have a variable voltage that can inject onto the ac signal on pin 3. BTW, the op-amp circuit you added does need a resistor going to 0V to do precisely the same job at removing bias currents from the op-amp. The pot circuit I desribed should do that job for you. You might need to make the pot bigger value in case it attenuates the LF stuff too much (or make C1 a 10uF). \$\endgroup\$ – Andy aka Oct 20 '13 at 16:47
  • \$\begingroup\$ That works! you are a magician! I just added that pot with/without op-amp and works just fine but now I realized that selecting a non-inverting op-amp was a wrong choice because it simply starts oscillating. I added the same pot design to Vin- pin of AD811 in inverting mode but it does not remove the offset voltage . How should I connect for non-inverting design? \$\endgroup\$ – Aug Oct 21 '13 at 7:49
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    \$\begingroup\$ @Aug it won't work the same in an inverting amplifier - it probably needs very much smaller resistances for it to work - post a diagram of that section if you can dude. \$\endgroup\$ – Andy aka Oct 21 '13 at 8:03
  • \$\begingroup\$ I added it as Edition2 to the question. I really really appreciate your help! \$\endgroup\$ – Aug Oct 21 '13 at 8:18

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