AD831 Bias Current

Hi again,

I am having some problems adjusting the bias current of the AD831. According to the datasheet a resistor must be connected between the BIAS pin and Vp. And for minimum bias current, connect it directly without resistor.

The problem is that the BIAS pin is connected directly to Vn (confirmed with a multimeter). If a shortcircuit Vn to Vp... nothing good happens , and if I put a small resistor (100 ohm) the overall current is increased obviously (10V difference in a 100ohm resistor -> high current).

Anyone know how is the bias current generation implemented in the chip? (and why is shortcircuited to Vn?) Or any experience reducing the bias current?

Thanks.

Julio

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  • Hi jdobler, we are finally replacing the part, we'll see...

    They have asked me though to check if the design was right, before mounting the new part. Let me ask you a few questions:

    1. The LO input was dc-coupled (0V) with a center-tapped transformer (being the center tap connected to ground). This didn't work, the signal was strangely deformed. I took out the transformer and replaced with a couple of capacitors (ac-coupled), and it started working. Strangely the DC of the signals was around -2.5V.

    2. The RF input was dc-coupled (0V) with a center-tapped transformer, and working. Is there any problem in having one dc-coupled and the other ac-coupled? (and is one preferred to the other?)

    3. Because I don't use the output amplifier, the AN&AP inputs are unconnected (which I think it shouldn't be a problem), and all the pins from the output amplifier are unconnected too (I wonder if I should connect VFB to OUT, and COM to gnd, even if I don't use them). The datasheet doesn't give a clue.

    Thanks for your support. I think some information is missing in the datasheet in this regard

Reply
  • Hi jdobler, we are finally replacing the part, we'll see...

    They have asked me though to check if the design was right, before mounting the new part. Let me ask you a few questions:

    1. The LO input was dc-coupled (0V) with a center-tapped transformer (being the center tap connected to ground). This didn't work, the signal was strangely deformed. I took out the transformer and replaced with a couple of capacitors (ac-coupled), and it started working. Strangely the DC of the signals was around -2.5V.

    2. The RF input was dc-coupled (0V) with a center-tapped transformer, and working. Is there any problem in having one dc-coupled and the other ac-coupled? (and is one preferred to the other?)

    3. Because I don't use the output amplifier, the AN&AP inputs are unconnected (which I think it shouldn't be a problem), and all the pins from the output amplifier are unconnected too (I wonder if I should connect VFB to OUT, and COM to gnd, even if I don't use them). The datasheet doesn't give a clue.

    Thanks for your support. I think some information is missing in the datasheet in this regard

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