AD8336 Output signal level

I assembled the circuit that was referring to the Figure5 in the AN-934 in order to make the operation test of the AD8336.

In order to operate the AD8336 by itself, we have entered a 250mVp-p sine wave.

When I set GNEG=0.75V and GPOS=1.5V, output signal became square wave of ±5Vpp.

Could you please tell me what to do in order to make the output signal to 250mV of the sine wave?

add 12/2/2014

Circuit structure and resistance value are same as the Figure5 in the AN-934.

I am supplying the voltage directly to the GPOS and GNEG without AD736, AD8551 and ADP3339.

Please advice me about this issue.

Thanks.

  • 0
    •  Analog Employees 
    on Dec 2, 2014 11:45 PM over 6 years ago

    Hi,

    It looks like your output is getting saturated. If you would consult the datasheet of AD8336, on page 21, there is a section that discusses how to set the gain on the amplifier. Basically, if you have your gains in dB, it will be the sum of the gain of the preamp and the VGA. For the VGA, you have the equation below for the gain:

    In your case with a Vgneg of 0.75V and a Vgpos of 1.5V, you would have a total Vgain of 750mV. Substituting this in the above equation would give you a VGA gain of 41.9dB. Since the preamp is just configured in a gain of -1, we can just assume that this inverts your signal, but the total gain would effectively be 41.9dB. This high a gain would definitely saturate your input of 250mVpp.

    Since you want 250mVpp at your output, I'll assume you want a gain of 1 at the VGA. Try lowering Vgain to about -88mV (solved from the equation), and check whether you get your expected value.

    I hope this helps!

    Kris

  • Hi Kris,

    Figure 5 in AN-934 is a special case that shows how to connect the AD8336 to accommodate an offset gain control voltage by using the floating control feature. If you simply need to evaluate the AD8336, you can just wire the part as in figure 88 of the data sheet or obtain an evaluation board. Assuming you're using a split supply and your reference voltage is 0V, then the part will always be in saturation.

    If you still have problems after connecting your circuit as is fig 88, send us a copy of your actual circuit (not just a copy of the data sheet circuit) and voltage measurements at all the pins.

    Best,

    J


  • Oops, sorry Kris I misread the email, my response was really intended for the original non-identified author of the initial comment.

  • Hi Kris,

    Thank you for your advice.

    I tested under the following conditions, and got the desired results.

    Vgpos=0.662V

    Vgneg=0.75V

    VGAgain=-0.088V

    Could you please teach me control method when I make the AGC circuit as sample circuit in AN-934(Figure5)?

    (How to adjust the output level in this circuit?)

    Thanks,

  • Hello Krif,

    Do you have any updates about below?

    >Could you please teach me control method when I make the AGC circuit as sample circuit in AN-934(Figure5)?

    >(How to adjust the output level in this circuit?)

    Thanks,