HMC346ALC3B application

Question

I have been trying to use the HMC346ALC3B voltage-variable attenuator as part of a laser intensity control circuit, and I am having some trouble with the external op-amp control circuit. 
 I have wired an external op-amp (AD8675) control circuit together with the HMC346ALC3B eval board, exactly as shown in the datasheet for the attenuator: https://www.analog.com/media/en/technical-documentation/data-sheets/hmc346alc3b.pdf 
 I am using an external DC power supply for the control voltage. However, I do not see continuously tunable attenuation, but rather 0 attenuation for 0 - 4V control signal and a sudden change to 20 dB attenuation above 4V. 
 Hoping to get some suggestions about the cause of this problem, or any kind of more detailed explanation / application note for the HMC346ALC3B which would allow me to design a new circuit. Currently it's not very clear to me from the datasheet alone what exactly the V1, V2 and I inputs should be.

zwang · Accepted Answer

1. When you wired an external op-amp with HMC346A as indicated in the datasheet, you only need to vary the voltage of V1 (from 0V to -5V). V2 will be complimentarily changed by the op-amp and I maintains the impedance match while attenuation is varied. Is this what you have been doing so far and still observe a sudden change of attenuation? 
 2. Since you have the HMC346A eval board, you can also check if the part is still functioning by not using the external op-amp first. You can find two power supplies and control V1 and V2 separately. Please refer to the figure "Relative Attenuation vs. Control Voltage @ 10 GHz" in the datasheet. You can try to check several combination points of V1 and V2, and to see if you can achieve the expected attenuation levels.

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HMC346ALC3B application

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